STELLAR ABLATION OF PLANETARY ATMOSPHERES

STELLAR ABLATION OF PLANETARY ATMOSPHERES

T. E. Moore1 and J. L. Horwitz2

Received 21 December 2005; revised 22 October 2006; accepted 3 January 2007; published 9 August 2007.

[1] We review observations and theories of the solarablation of planetary atmospheres, focusing on theterrestrial case where a large magnetosphere holds off thesolar wind, so that there is little direct atmospheric impact,but also couples the solar wind electromagnetically to theauroral zones. We consider the photothermal escape flowsknown as the polar wind or refilling flows, the enhancedmass flux escape flows that result from localized solar windenergy dissipation in the auroral zones, and the resultantenhanced neutral atom escape flows. We term these lattertwo escape flows the ‘‘auroral wind.’’ We reviewobservations and theories of the heating and accelerationof auroral winds, including energy inputs from precipitatingparticles, electromagnetic energy flux atmagnetohydrodynamicand plasma wave frequencies, and acceleration by parallel

electric fields and by convection pickup processes also knownas ‘‘centrifugal acceleration.’’ We consider also the globalcirculation of ionospheric plasmas within the magnetosphere,their participation in magnetospheric disturbances as absorbersof momentum and energy, and their ultimate loss from themagnetosphere into the downstream solar wind, loadingreconnection processes that occur at high altitudes near themagnetospheric boundaries. We consider the role of planetarymagnetization and the accumulating evidence of stellar ablationof extrasolar planetary atmospheres. Finally, we suggest anddiscuss future needs for both the theory and observation of theplanetary ionospheres and their role in solarwind interactions, toachieve the generality required for a predictive science of thecoupling of stellar and planetary atmospheres over the full rangeof possible conditions.

Citation: Moore, T. E., and J. L. Horwitz (2007), Stellar ablation of planetary atmospheres, Rev. Geophys., 45, RG3002, doi:10.1029/

2005RG000194.

1. INTRODUCTION

[2] This review has a focus upon the terrestrial iono-

sphere and upper atmosphere, and their expansion into

geospace and loss from the Earth, owing to energy inputs

from the Sun, both electromagnetic and mechanical. The

term ‘‘geospace’’ is used here in the sense suggested by the

NSF Geospace Environment Modeling (GEM) Program, to

encompass the coupled atmosphere, ionosphere, magneto-

sphere, and heliosphere. Over the past 40 years, the Sun-

Earth system has been the subject of observations and

theoretical analyses of increasing sophistication, far beyond

what has been possible for the other planets of our solar

system, or what will be possible for planets of other astro-

spheres in the foreseeable future. Geospace studies are

directly applicable to other planets, solar and extrasolar,

and to the Earth during early solar system formation, or

geomagnetic reversals. These studies will contribute to our

preparedness for space exploration, as well as our under-

standing of our home planet in space. In addition, the results

of geospace studies are essential to advancing the frontiers

of space weather prediction.

[3] Some nomenclature explanations are in order. We

refer to the region dominated by terrestrial material

(‘‘geogenic’’) as the ‘‘geosphere,’’ and its outer interface,

beyond which lies mainly solar material, as the ‘‘geopause,’’

following Moore and Delcourt [1995]. Solar energy depos-

ited in the geosphere, particularly the third (gas) and fourth

(plasma) geospheres, produces expansion of the gas and

plasma, initially producing what we term as ‘‘upflow’’

against gravitational confinement. With sufficient energy,

upflow becomes ‘‘outflow,’’ escaping gravity, but the plasma

is still trapped in the magnetosphere. When geospheric

plasma finds its way to the outer magnetospheric boundary

and escapes downstream in the solar wind, we call it

‘‘ablation’’ because material has then been completely

removed from the Earth and can never return, given the

supersonic expansion of the solar wind to fill the heliosphere.

[4] Planets and their early atmospheres are thought to

condense out of nebulae in parallel with their central stars

[Jeans, 1902]. By the time stellar ignition occurs, however,

condensation competes with the process of nebular and

atmospheric ablation, powered by an enhanced early stellar

wind thought to prevail during the T-Tauri phase for stars in

the size class of our Sun [Balick, 1987]. Volatiles may be

acquired by planets through impacts of long-period comets

[Chyba et al., 1990], but ablation continues and may

eventually dominate for planets sufficiently close to their

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1NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.2Department of Physics, University of Texas at Arlington, Arlington,

Texas, USA.

Copyright 2007 by the American Geophysical Union.

8755-1209/07/2005RG000194$15.00

Reviews of Geophysics, 45, RG3002 / 2007

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Paper number 2005RG000194

RG3002

http://dx.doi.org/10.1029/2005RG000194

central star [Vidal-Madjar et al., 2004]. The terrestrial

planets of our solar system all bear evidence of long-term

atmospheric evolution to which solar photon or solar wind

ablation or erosion must have contributed. Mercury barely

has an exosphere [Hunten et al., 1988]. Venus’ atmosphere

has been desiccated of water [Kasting, 1988]. Mars has lost

most of an atmosphere that increasingly appears to have

contained substantial water vapor [Squyres and Kasting,

1994]. Earth is unique among these planets in the amount of

water that it acquired and retains. Intriguingly, of these

rocky inner planets, the Earth has by far the strongest

planetary magnetic field [Stevenson et al., 1983].

[5] The presence of a magnetic field has a strong influ-

ence on the interaction between a planet and the solar wind

[Stern and Ness, 1982], and thus the global distribution of

energy dissipated into atmospheric gases by the solar wind.

A planet without an intrinsic magnetic field might seem

more exposed to atmospheric ablation, with direct impact of

the solar wind on the entire upper atmosphere. However, as

we will discuss further below, the net loss may be compa-

rable with or without a magnetic field, because its presence

concentrates electro-mechanical energy dissipation at the

planetary end of flux tubes that link with the boundary layer

between the solar wind and its magnetic field and the

planetary magnetic field and its enclosed plasma. This is

the region known as the auroral zone, where strong electri-

cal currents link the solar wind to the ionosphere, and

charged particles are accelerated and in turn generate the

aurora [Ergun et al., 1998].

[6] This high-latitude ionosphere-magnetosphere region

is one of the richest and most interesting regions within the

space plasma universe that is accessible to direct measure-

ments. The many energetic processes occurring there con-

tribute to the expansion and escape of the ionosphere

beyond the outer regions of geospace. Figure 1 shows a

schematic illustration of the range of processes within the

ionosphere and lower magnetosphere that propel these out-

flows upward and outward.

[7] As depicted by Figure 1, ionospheric outflows

emerge from the high-latitude ionosphere�lower magneto-

sphere generation regions on magnetic field lines that guide

these flows into magnetospheric regions such as the tail

lobes and the plasma sheet, as shown in Figure 2. If these

ionospheric plasmas remained on polar lobe field lines,

which are ultimately connected to the interplanetary mag-

netic field, they would be lost from the magnetosphere.

However, these plasma outflows join the convective circu-

lation of plasma in the magnetosphere as they expand out of

the ionosphere proper. Hence these emerging ionospheric

plasmas flow across the magnetic field into the closed field

line region of the plasma sheet, which carries the current

responsible for the stretched magnetotail. Ionospheric flows

from the nightside auroral regions are injected directly into

the plasma sheet, and since the plasma sheet itself is the

Figure 1. Noon-midnight cross section of ionosphere�lower magnetosphere summarizing phenomenainvolved in the heating and escape of ionospheric plasmas. After Moore et al. [1999a] with kindpermission of Springer Science and Business Media.

RG3002 Moore and Horwitz: ABLATION OF ATMOSPHERES

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primary source of plasmas for the quasi-trapped hot plasmas

of the inner magnetosphere, these ionospheric plasmas

ultimately populate the ring current in considerable quantities

as well. An ongoing controversy for the magnetospheric

community, particularly since the provocative article of

Chappell et al. [1987], has been the question of how much

of the plasma density and/or pressure within various regions

of the magnetosphere originates in the ionosphere, as

opposed to the other principal plasma source, the solar

wind.

[8] Figure 1 portrays several of the driving processes

and features of ionospheric upflow and outflows in a noon-

midnight cross section of the high-latitude ionosphere�lower

magnetosphere region. Various types of ion conics and

beams [e.g., Øieroset et al., 1999], rings [Moore et al.,

1986], electron beams and conics [e.g., Menietti and

Weimer, 1998], counterstreaming and modulated field-

aligned electrons [e.g., Carlson et al., 1998; McFadden et

al., 1998], and other highly non-Maxwellian particle distri-

bution functions have been consistently observed, often

in conjunction with a variety of wave and strong field

disturbances [e.g., Hirahara et al., 1998; Norqvist et al.,

1998; Vaivads et al., 1999]. The ion conics and rings are

often the results of perpendicular heating or acceleration of

ionospheric ions by various types of waves, including lower

hybrid, broadband, and ion cyclotron waves, whereas the

beam ion distributions are often caused by parallel electric

fields, either in quasi-DC or wave or impulsive forms. Also,

in addition to the light (H+, He+) ions and heavier, often

dominant O+, ions, at times, molecular ions (NO+, N2+, O2

+)

are observed in the 1000- to 4000-km regions over the

auroral zones and the cusp, as well as over subauroral ion

drifts [e.g., Peterson et al., 1994; Wilson and Craven, 1998,

1999].

[9] The overall process of generating ionospheric out-

flows often occurs in a multistage sequence, particularly in

terms of the escape of normally gravitationally bound

heavier ions such as O+. One set of processes operates

within and just above the principal ionospheric production

region, at altitudes from 300 to 1000 km, to either produce

ionization or propel initial upflows, or both. Frictional

or Joule heating (FH in Figure 1), caused by the cross field

E � B drift of ionospheric ions through the neutral

atmosphere at these altitudes, enhances the ion pressure

and produces an initial upflow [e.g., Wahlund et al., 1992;

Korosmezey et al., 1992; Ho et al., 1997]. Soft electron

precipitation produces both ionization and collisional heat-

ing of thermal electrons, which increases the local ambipo-

lar electric field and lifts the ions upward [e.g., Wahlund et

al., 1992; Seo et al., 1997; Su et al., 1999]. The wave-

Figure 2. Global circulation of plasmas in Earth’s magnetosphere, in the noon-midnight meridian. AfterHultqvist et al. [1999] with kind permission of Springer Science and Business Media.


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particle driven ion heating, parallel electric field, and other

processes occurring at altitudes from 1000 km upward

[Andre et al., 1998] then supply the additional energy

needed, particularly for O+ to escape the Earth’s gravita-

tional well (about 10 eV). Wu et al. [1999] has systemat-

ically investigated the effects of the different stage effects

on overall ion outflows, focusing on soft electron precipi-

tation effects in the lower (<1000 km) ionospheric region,

and ion cyclotron wave–based ion heating above 1000 km

altitude.

[10] During the 1960s, ionospheric expansion into upper

geospace was viewed in terms of the so-called (classical)

polar wind [Dessler and Michel, 1966; Axford, 1968; Banks

and Holzer, 1968, 1969]. This classical polar wind outflow

was predicted as a light ion (H+ and He+) outflow along

open magnetic field lines threading the polar cap, in which

the principal upward driving forces are the plasma pressure

gradients and the ambipolar electric field sustained by

charge balance requirements involving the light electrons

and the heavy, gravitationally bound majority ion species,

O+. The first definitive measurements of this classical light

ion polar wind outflow were obtained by a polar orbiting

satellite above 1000 km altitude in the early 1970s [Hoffman

et al., 1974]. However, as a result of satellite measurements

of ions during the 1970s, the importance of outflows driven

by the stronger auroral processes, including the wave-

particle interactions and parallel electric fields mentioned

above, became apparent. Furthermore, the term ‘‘polar

wind’’ has become somewhat imprecise in community

usage. Some authors include outflows driven by auroral

processes within the polar wind purview, while others tend

to confine use of the term mostly to the ‘‘classical,’’ thermal,

nonauroral processes and their natural extensions within the

paradigm arising from the analyses of Banks and Holzer as

suggested by Axford.

[11] In view of the above, we will thus organize the first

portion of this review into the physics considerations on two

relatively distinct classes of ion expansion into geospace.

We introduce the term ‘‘photothermal outflows’’ to describe

those outflows driven chiefly by photon energy from the

Sun. We introduce the term ‘‘auroral wind’’ for those

outflows driven by solar wind mechanical energy. Essen-

tially, the ‘‘photothermal outflows’’ will be congruent with

what many refer to as the ‘‘classical polar wind,’’ while the

‘‘induced outflows’’ will be mostly consistent with ‘‘auroral

wind.’’ The remainder of the review will describe the

morphologies and larger-scale physics involved in high-

latitude ‘‘fountain’’ and ‘‘plume’’ flows, the relationship of

the ionospheric expansion in terms of (lower) boundary

conditions set on the dynamics of the magnetosphere,

desirable directions for future theory and measurements

investigations, and discussion and conclusions.

[12] Progress in compiling and understanding the char-

acteristics and significance of ionospheric expansion into

geospace has advanced in the past decade or so chiefly

through ongoing in situ measurements by sophisticated

particle instrumentation on such polar-orbiting spacecraft

as Akebono, Polar, Freja, FAST, and Cluster, augmented by

promising remote energetic neutral atom imaging results

from the IMAGE spacecraft, in parallel with strides in

simulation techniques and applications utilizing fluid and

kinetic and combined fluid-kinetic computer codes over one

or more dimensions and different timescales. The aim of

this review is to present this progress within the organiza-

tional context described above. The last article within

Reviews of Geophysics that covered the topical purview of

the present review appears to be one by Ganguli [1996],

which had a focus primarily on the polar wind. Therefore

much of the detailed emphasis in the present review will be

on progress on understanding ionospheric expansion into

geospace since 1996, and with more emphasis on the

auroral wind, as defined above.

2. GEOSPACE MODELING IMPLICATIONS

[13] The Heliophysics research community aspires to

incorporate its understanding of space plasma physics into

theoretical models and global simulations [Winglee, 1998;

Lyon et al., 2004; Siscoe et al., 2002b; Gombosi et al.,

2003; Raeder et al., 1997; Ashour-Abdalla et al., 1997].

The preference is clearly for simulations that run faster than

real time on available computer hardware, leading to

limitations that are gradually relaxed as hardware capabil-

ities develop. Criticism of global models tends to focus on

the lack of kinetic or microphysical dissipative processes in

critical current carrying regions best exemplified by the

reconnection diffusion zone. Practical computer codes are

subject to numerical effects that give rise to diffusive

processes, independent of the true microphysics. A promi-

nent objective of the code developers is to answer these

criticisms through reduction of the numerical effects to a

level that is negligible compared with the true physical

dissipative effects or their implementation in code. This is

an important goal and deserves all the attention it receives.

[14] However, there is another assumption that has tradi-

tionally been made to obtain practical global simulation

codes. This is an assumption that the dissipative medium of

terrestrial (ionospheric) plasmas is confined exclusively

within a thin layer of scale height less than 100 km, close

to the Earth. This thin layer is generally taken to lie

completely outside the global simulation space and is

lumped into the boundary condition. From the perspective

of electro-dynamic coupling, this is equivalent to assuming

that all solar wind-driven current systems close outside the

simulation space, or that the magnetosphere is exclusively

an electro-dynamic dynamo and transmission system with

substantial dissipative or inertial elements limited to the

ionospheric thin layer, beyond the system boundaries.

Exceptions may include the bow shock region where shock

heating accompanies compression, and the magnetosheath,

where the shocked solar wind is reaccelerated downstream.

Not surprisingly, in view of this assumption, an outstanding

problem of global simulations is their inability to produce

sufficient plasma pressure in the inner magnetosphere hot

plasma, or ring current region.


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[15] However, the ‘‘thin shell’’ assumption leads to a

number of problems. One fundamental problem is perhaps

that codes based on this assumption can never include the

plasmasphere because its ionospheric origin violates the

assumption of a thin shell ionosphere. The plasmasphere

is an important region of the inner magnetosphere that

was discovered early in the space age [Carpenter, 1963;

Carpenter et al., 1993]. Recent advances in imaging of

space plasmas have shown that the plasmasphere is a

dynamic participant in magnetospheric plasma circulation,

producing global-scale drainage plumes that supply plasma

to the dayside subsolar magnetopause region [Sandel et al.,

2001; Goldstein et al., 2002, 2003a], and to the polar cap

topside ionosphere [Foster et al., 2002]. It is clear from

these recent developments that the ionosphere, via the

plasmasphere, is at times an important source of plasma

mass density to the dayside magnetosphere. Given this new

validation of global ionospheric circulation at high altitudes,

a credible global simulation must evidently include these

plasmas to get the bulk characteristics of the magnetosphere

correct. The minimum requirement to include them is that

plasma be allowed to flow across the boundary between the

thin shell ionosphere into the magnetosphere at large.

[16] As important as the photothermal polar wind out-

flows may be, the auroral outflows at latitudes higher than

that of the plasmapause have greater potential impact upon

magnetospheric dynamics, given sufficient available energy,

owing to their much higher escape flux limits and more

massive ions. As solar wind mechanical energy is much

more variable than the solar photon flux, so is the auroral

wind much more strongly modulated than is the polar wind.

The area from which auroral wind is emitted is substantially

smaller than the area outside the plasmasphere that emits

light ion polar wind, but it is substantially more productive

of mass flux than the polar wind regions, when the solar

wind interaction is strong. Owing to recent observational

and theoretical advances, it is now becoming feasible to

relax the assumption of a thin shell ionosphere in global

magnetospheric simulations either by allowing flow of a

single plasma fluid into the system from the ionosphere, or

by introducing a separate ionospheric fluid (or fluids). We

can distinguish the outflows beyond the thin shell into

‘‘photothermal’’ and ‘‘auroral wind’’ categories. We further

consider and summarize the global outflow structures that

result from interactions between structured outflows and

magnetospheric circulation. Then we consider the role of

planetary magnetization, and the evidence that extrasolar

planets also experience atmospheric ablation by their stellar

photon flux and plasma winds. Finally, we summarize the

outstanding problems, new observations required, and the-

oretical developments that will allow us to incorporate our

new knowledge into our global simulations to further

improve their predictive capability and generality.

3. PHOTOTHERMAL OUTFLOWS

[17] Our Sun is at a stage of its evolution during which

the electromagnetic energy flux of its photon emission

(�1� 103 J/s-m2) greatly exceeds the mechanical energy flux

of its stellar wind (�3 � 1017 eV/s-m2 or 5 � 10�2 J/s-m2).

Thus the predominant interaction between the Sun and Earth

is the deposition of the photon energy. However, the vast

majority of that is unabsorbed until it reaches the ground.

The UV component of the photon flux is absorbed in the

upper atmosphere and creates the ionosphere, maintaining its

temperature at a few thousand K in sunlight, by removing

electrons from atmospheric atoms to create a partially

ionized plasma. This plasma is substantially warmer than

the parent gases from which it is created and hence contains

a larger fraction of particles with enough energy to escape

from the Earth’s gravity. Photothermal escape of neutral gas

(that is, neutral escape which is driven by the photon flux

absorbed by the upper atmosphere) is for Earth limited

mainly to the lightest species, H and to some degree He.

The same is true of the plasma species H+ and He+, the

escape of which is called ‘‘polar wind,’’ following the

nomenclature of Axford [1968] and Banks and Holzer

[1968, 1969].

3.1. Jeans Escape

[18] ‘‘Jeans’’ escape [Jeans, 1902] refers to the escape of

an atmosphere from a gravitating body, owing simply to the

thermal motions of the gas atoms or molecules. Calculations

of the expected escape as a function of the temperature of

the gas have been made [Chamberlain and Hunten, 1987].

Loss of the more energetic particles into space constitutes a

heat loss from the atmosphere, which must clearly be

balanced by other energy sources, such as incoming radia-

tion or upward heat conduction from below (normally

negligible or even negative).

[19] Analogous escape of ions is also expected from an

ionosphere or partially ionized atmosphere. The simplest

possible ionosphere is formed when ultraviolet radiation

from a nearby star partially ionizes an upper atmosphere,

giving rise to electron-ion pairs, and creating a plasma that

is far from thermodynamic equilibrium, reflecting the pres-

ence of a significant flux of photons with spectrum that

extends above the electron binding energy of the gaseous

atoms. Where the photoelectron energy is partly thermalized

by collisions with ions, the ion gas may be hotter than the

neutral gas, and hence have a larger than expected kinetic

escape rate, under this simple model. The enhancement of

mass flux is attributable to the energy flux of photons

absorbed by the gas in the processes of ionization and

thermalization.

3.2. Polar Wind and Plasmasphere

[20] In practice, ionization that occurs in the presence of a

magnetic field produces a more complex situation than that

described by Jeans escape. Moreover, the presence of the

solar wind in addition to the solar photon flux complicates

the situation substantially. Initial exploration of the Earth’s

magnetosphere revealed the presence of an extended iono-

sphere within the inner magnetosphere, and of plasma

circulation throughout the outer magnetosphere. Soon, the

implications for ionospheric escape flows were appreciated


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and a theory of polar outflows was outlined and subse-

quently developed [Dessler and Michel, 1966; Axford,

1968; Banks and Holzer, 1968, 1969]. This theory, as

further developed and reviewed by others [Raitt et al.,

1978; Schunk, 1988; Gombosi et al., 1985; Ganguli,

1996; Moore et al., 1999a], has been confirmed in most

respects [Abe et al., 1996; Su et al., 1998b; Drakou et al.,

1997; Huddleston et al., 2005], the main exception being

that the initial theory made no account of auroral zone

energetic phenomena that affect the nature of the iono-

spheric outflows. In recent studies of the polar wind, global

circulation has been considered [Demars and Schunk,

2002], and complex structures have been shown to result

that cannot be resolved well using statistical surveys of

spacecraft data.

[21] Because plasma electrons and ions are linked by

electrostatic forces, they are not free to separate and are

required to maintain very nearly equal densities by an

ambipolar electric field that swiftly forms if they begin to

separate. Since electrons are so much faster and more

mobile than ions, and are negligibly bound by gravity, the

topside ionosphere forms an approximately radially directed

ambipolar electric field that holds down the electrons,

exerting a balancing upward force on the ions. The strength

of the field depends on the mass of the dominant ion

species. When the field is strong enough to hold O+ in

equilibrium, it is strong enough to accelerate lighter ions

upward strongly, and this is an important correction to the

theory of the light ion polar wind. Such ambipolar electric

fields are important wherever differential transport of elec-

trons and ions tend to make them separate because of either

gravity or inertia. Moreover, the ambipolar field links

electrons and ions such that heating of either species adds

energy to the system and enhances escape of both species as

an ambipolar plasma, though gravitational stratification by

mass will occur.

[22] Within high-latitude magnetospheric flows, as par-

tially depicted in Figure 2, plasmas undergo a circulation

cycle whose timescale ranges from diurnal to much shorter

than diurnal, depending on the strength of the solar wind

interaction. As specified by Alfven’s [1942] theorem, mag-

netized plasmas move on large scales as elastic ‘‘flux tubes’’

linked along their length by magnetic field lines of force.

During the course of this circulation cycle, a plasma flux

tube is first stretched from �10 RE to >100 RE in half-length

as it is convected antisunward, or it may be disconnected

from the conjugate hemisphere and reconnected into the

solar wind plasma during part of the cycle, permitting direct

plasma exchange between the two media. Later, after

reconnection in the tail (if disconnection has occurred),

the flux tube relaxes back to its starting point. During each

circulation cycle, the flux tube volume changes by a factor

on the order of 104.

[23] During the stretch part of the cycle, or when the flux

tube is connected to the downstream solar wind (which

evacuates rather than filling the flux tube), ionospheric

plasma expands freely into the flux tube as if there were a

zero-pressure upper boundary condition, introducing light

ion plasma to the flux tube at a rate limited by the available

source of plasma. The net result is a very low density,

supersonic flux of cold light ions through the polar caps and

into the magnetospheric lobes. This outflow corresponds

closely to the polar wind and has been observed from

ionospheric topside heights [Brinton et al., 1971; Hoffman

et al., 1974; Hoffman and Dodson, 1980] up through

altitudes around 1 RE [Abe et al., 1993a, 1993b, 1996],

around 3–4 RE [Nagai et al., 1984; Olsen et al., 1986; Su et

al., 1998b], and at altitudes up to 9.5 RE [Moore et al.,

1997; Su et al., 1998b; Liemohn et al., 2005], extending into

the lobes of the magnetosphere. However, as has been noted

by many of these authors, the cold polar wind is often

accompanied by comparable or larger fluxes of O+ ions,

even under conditions of low solar activity. The high-

altitude, cold supersonic outflows into the magnetotail,

including O+ as well as light ions, have recently been

studied as the ‘‘lobal wind’’ [Liemohn et al., 2005].

[24] The polar wind has recently been considered as a

three-dimensional system, in the context of global iono-

spheric convection, with Joule heating and auroral electron

precipitation effects on the ambipolar electric field [Demars

and Schunk, 2002; Coley et al., 2003]. A complex three-

dimensional picture emerges of light ion outflow variations,

especially in velocity. Moreover, heavy ion ‘‘breathing’’

motions inflate and deflate the topside ionosphere with

mixed upflows and downflows, in response to these pro-

cesses. Particularly in the case of the heavy O+, what goes

up in regions of Joule heating or precipitation for the most

part falls back into the ionosphere in the polar cap, or

subauroral regions that are downstream of the aurora in the

ionospheric circulation pattern. This leads to complex

patterns of counterstreaming in which the light ions are

flowing upward in the same region where O+ downflows are

occurring. The authors cite good agreement with observa-

tional studies of O+ downflow [Chandler et al., 1991;

Chandler, 1995].

[25] During the relaxation part of the flux tube convec-

tion cycle, stretched flux tubes contract through the mag-

netotail toward the Earth. Disconnected flux tubes also

reconnect there and circulate back toward the dayside

subsolar region. However, these flux tubes do not pass

through the inner magnetosphere, except during conditions

of strong magnetospheric circulation. When the interplane-

tary magnetic field (IMF) tends northward, convection

stagnates in the inner magnetosphere, while the polar wind

escapes downstream as flux tubes are peeled off the

magnetospheric lobes by high-latitude reconnection. This

is illustrated in Figure 3 (left), from a global simulation that

tracks ionospheric fluids in addition to the solar wind fluid

[Winglee, 1998]. The surface plotted in Figure 3 bounds the

region dominated by polar and auroral wind plasmas

and excludes the region dominated by solar plasmas.

Polar wind outflow at lower, stagnant latitudes, closer to

the Earth, continues until large densities are accumulated

(up to >1000 cm�3), forming the region of extended iono-


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sphere known as the plasmasphere. In Figure 3 (left),

intermediate latitudes are dominated by solar wind plasmas

that circulate inward from the low-latitude boundary layers in

this case.

[26] For normal (BZ � 0, where Z points northward

normal to the ecliptic plane) or southward IMF, as shown

in Figure 3 (middle and right), magnetospheric circulation is

excited by a combination of dayside and nightside recon-

nection that grows increasingly stronger with more south-

ward IMF, penetrating deeper into the inner magnetosphere

and eroding the outer parts of the dense plasmasphere (see

also section 3.3). Polar wind outflows are convected into

the plasma sheet and then sunward, replacing the denser

plasmaspheric material being removed toward the subsolar

magnetosphere. The surface separating solar and geogenic

plasmas, the geopause [Moore and Delcourt, 1995],

changes in shape and grows in size with increasing mag-

netospheric convection. In contrast, the classically defined

plasmapause, which bounds the high-density light ion

region, is eroded to smaller size during the strong circula-

tion driven by southward IMF.

3.3. Plasmasphere Filling, Erosion, and Refilling

[27] The basic paradigm is that the outer portion of the

dense plasmasphere is eroded away by enhanced magneto-

spheric circulation and then refills via upward ionospheric

plasma flow during the recovery phase of magnetospheric

storms. This conceptual framework dates from the seminal

papers by Nishida [1966] and Brice [1967]. In this picture, a

period of enhanced magnetospheric convection during a

storm’s main phase causes the boundary between closed

(Earth-encircling) flux tube trajectories and circulating flux

tube trajectories to shrink. The plasma on the flux tubes that

are on open trajectories (in which the flux tubes link with

the interplanetary magnetic field at some point during the

cycle) may then vent to the interplanetary medium, resulting

in low densities. In steady state conditions, this boundary

Figure 3. A simulated configuration of ionospheric plasmas in the magnetosphere [Winglee, 1998]. Thehighlighted surface represents the limits of the region within which the plasma is predominantly ofionosphere origin, regardless of its absolute density, that is, the geopause of Moore and Delcourt [1995].

Figure 4. Images of the plasmasphere in scattered He+ resonant sunlight [Goldstein et al., 2003a],illustrating the observed and modeled plasmasphere as it is eroded by a period of enhancedmagnetospheric convection, from left to right. Sun is to the right.


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coincides with the density gradient known as the plasma-

pause [Carpenter, 1963]. At the onset of the storm recovery

phase, when the intensity of magnetospheric convection is

reduced, there is an outer region of initially low density

between the quiet and active closed/open flux tube trajec-

tory boundaries. This outer region of newly closed or Earth-

encircling trajectories is subject to plasma upflow from the

ionosphere below on the same field lines. The reality of this

description was confirmed when the IMAGE mission made

possible global imaging of the plasmasphere, showing that

it expands during quiet times and is then eroded by

convection, producing pronounced sunward extending

plumes, as well as other dynamic features, as illustrated in

Figure 4.

[28] A particularly influential theoretical paper on how

the refilling could proceed was that of Banks et al. [1971].

They considered a single-stream hydrodynamic model treat-

ment in which equal supersonic polar wind streams from

conjugate ionospheres flowed onto initially empty flux

tubes and interacted at the magnetic equator. The interaction

in that treatment led to a ‘‘slab’’ of thermalized denser

plasma at the equator, whose edges were shock fronts which

propagated back down the field lines toward the ionosphere.

When these shock front slab edges reach the flow critical

points near the topside ionosphere, the overall flow character

changes to subsonic flow throughout. The flow proceeds

through this second, subsonic flow stage until the flux tube

density distribution attains diffusive equilibrium or is inter-

rupted by a new storm or gust of enhanced magnetospheric

convection.

[29] One of the principal issues with this scenario concerns

the single-stream restriction and the interaction between the

opposing ionospheric flows. In the above picture, the

modeling requirement that the ion plasma be described as

a single stream everywhere, with a single bulk velocity (as

well as temperature and density), necessarily forces a

stationary slab around the equator as a consequence of the

interaction. However, as Banks et al. [1971] acknowledged,

the densities of the interacting streams would be sufficiently

low that such a strong interaction should not occur purely on

the basis of Coulomb collisions. Other processes, such as

wave-particle interactions, would be required in order to

sustain such an interaction. This issue led to treatment by

two-stream models [e.g., Rasmussen and Schunk, 1988].

Here the streams originating from the Northern and Southern

Hemispheres are treated as identifiably separate populations.

Owing to the low collision frequencies involved, the

streams typically would pass through each other largely

undisturbed as interpenetrating streams, until each would

interact with the conjugate ionosphere. Each stream would

then reflect from that conjugate ionosphere, but would now

create a shock front that would propagate back up the field

line toward the equator. The reverse shock in this frame-

work was presumably a consequence of the fact that above

the topside ionospheres of each hemisphere, the incident

stream and its reflection were required by the treatment to

be considered as a single, largely stationary stream. The

low-altitude shocks stem from the same physics as the

equatorial thermalized plasma slab formation in the equa-

torial region for the single-stream formulation.

[30] An alternative to these single-stream and two-stream

hydrodynamic approaches is to use a kinetic treatment, in

which the ions are treated as particles. Wilson et al. [1992]

considered a refilling flux tube simulation in which ions

were injected above the conjugate ionospheres onto an

initially empty flux tube. The electrons were treated as a

Boltzmann neutralizing fluid. No shocks were formed in the

ensuing development, but rather a more gradual isotropiza-

tion of the counterstreaming polar wind flows, caused

primarily by small-angle Coulomb collisions, until eventu-

ally a diffusive equilibrium condition was achieved. Under

the same kinetic framework, the effect of equatorially

concentrated ion heating perpendicular to the magnetic field

by ion cyclotron waves was considered by Lin et al. [1992,

1994]. These processes, in combination with a presumption

of a heated, field-aligned or isotropic electron component

around the equator, produced an electric potential peak at

the magnetic equator of order 1 V, an electric field directed

away from the magnetic equator. Since the refilling flows

from each conjugate ionospheric source region had (polar

wind) energies below this potential level, this led to a

‘‘hemispheric decoupling’’ in the refilling. That is, the ion

streams in each hemisphere reflected off this potential layer

back toward the ionospheric end, to be either isotropized via

collisions, mirrored, or lost to the atmosphere below. These

simulation results were in accord with observations about

the magnetic equator on L � 4.5 flux tubes by Dynamics

Explorer 1 (DE-1) [Olsen et al., 1987].

[31] Other kinetic aspects of refilling that have been

studied with similar models include velocity dispersion

[Miller et al., 1993], other ion heating by waves [Singh,

1996], and further examination of reflection of incoming

ionospheric polar wind streams by potentials created

through resulting differential ion-electron anisotropies

[Liemohn et al., 1999]. However, a serious limitation with

these kinetic-focused studies is that the altitudinal boundary

conditions typically involved presumed plasma injection

into the simulation regions at topside altitudes (generally

1000�2000 km). Thus there has been no true self-consistent

coupling of the ionospheric plasma and energy sources and

losses and chemistry to be synergistic with the intriguing

kinetic aspects of these simulations.

[32] There are a number of intriguing features of the outer

plasmasphere and trough regions that have been observed

by spacecraft but are not presently understood, and probably

require incorporation of high-altitude kinetic processes and

self-consistent coupling of ionospheric processes to model

and understand quantitatively. For example, in the Horwitz

et al. [1984] DE-1 examination of plasmaspheric refilling

during the recovery of a magnetospheric storm, it was found

that O+ densities were greatly elevated and actually

appeared to be temporarily the dominant ion species at

mid magnetic latitude refilling plasmasphere. Horwitz et al.

[1984] also showed that the O+ in this event was flowing


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along the field lines out of the northern ionosphere. It is

possible that such brief strong impulsive O+ upward flows

may represent the incipient formation of the heavy ion

density enhancements characteristically observed in the

outer plasmasphere in the vicinity of steep plasmapause

density gradients and associated with enhanced ion and

electron temperatures at both ionospheric and plasmaspheric

altitudes [e.g., Horwitz et al., 1986; Roberts et al., 1987;

Horwitz et al., 1990].

[33] Another intriguing feature is the apparent distribution

of trapped light ions and heavy ion density enhancements on

outer plasmaspheric field lines. Olsen et al. [1987] showed

that perpendicularly peaked fluxes of light ions (H+ and He+)

were predominantly observed by DE-1 within about 10� ofthe magnetic equator on L = 3 � 4.6 flux tubes. Craven

[1993], on the other hand, showed that DE-1 measured heavy

N+ ion fluxes were detectable only at midlatitudes. Although

O+ fluxes were not specifically represented, N+ is typically a

‘‘proxy’’ for O+ with N+/O+ density ratios typically around

0.10 in the plasmasphere [Craven et al., 1995]. The combi-

nation of these and other observations may suggest that

the heavy ion density enhancements occur at midlatitudes

and the light ion ‘‘pancake’’ or trapped distributions occur

around the equator, on field lines threading the ring

current-plasmasphere overlap region near the plasmapause,

possibly also related to SAR arc types of phenomena [e.g.,

Horwitz et al., 1986, 1990].

[34] Other species-dependent behavior on outer plasma-

sphere�trough flux tubes has been observed, particularly

for He+, which is especially significant in view of the

remote large-scale plasmasphere He+ observations by the

magnetospheric IMAGE mission via the 30.4-nm EUV

scattering line of He+, and others bearing on this issue,

including the refilling process, for example by Yoshikawa et

al. [2003]. Chandler and Chappell [1986] examined the

field-aligned flow velocities of He+ and H+ from DE-1

observations in the plasmasphere, and found instances when

the H+ and He+ were counterstreaming toward opposite

hemispheres. Anderson and Fuselier [1994] examined low-

energy H+ and He+ in the presence of electromagnetic ion

cyclotron waves, in the dayside morning trough region with

AMPTE observations. They found that ‘‘X’’ distributions

(or bidirectional conics) of He+ were observed near the

magnetic equator with energies around 35 eV on average,

while colocated H+ perpendicularly peaked had temper-

atures of 5 eV. Anderson and Fuselier [1994] argued that

these He+ ‘‘X’’ distributions were gyroresonantly heated by

the EMIC waves at off-equatorial latitudes.

[35] Still further intriguing features reported by Olsen

and Boardsen [1992] were not only interesting density

minima near the magnetic equator, but also latitudinally

asymmetrical densities around the magnetic equator, on outer

plasmasphere field lines. If such latitudinal asymmetries in

the densities do exist, it is possible they could involve the

above-noted electrostatic hemispheric decoupling [Lin et al.,

1994] of asymmetric Northern and Southern Hemisphere

ionospheric flows.

[36] The IMAGE spacecraft, launched in 2000, opened a

new window into understanding the large-scale dynamic

response of the plasmasphere to magnetospheric electric

field changes. For example, Goldstein et al. [2003b] exam-

ined observations by the IMAGE EUV imager that showed

erosion of the nightside plasmasphere occurring in two

bursts during one interval in July 2000, and indicated that

these erosion bursts were correlated with solar wind south-

ward turnings of about 30 min prior (after correcting for

transit time from observing satellite). Foster et al. [2002]

examined total electron content (TEC) measurements from

GPS satellites in conjunction with Millstone Hill incoherent

scatter radar observations of storm-associated ionospheric

electron densities during IMAGE EUV observations of

plasmasphere erosion and formation of elongated plasma-

spheric tails formed by convection of the main plasma-

sphere, including the dusk bulge region. Foster et al. [2002]

found that enhanced electron densities in the incoherent

scatter radar and TEC measurements corresponded to

regions of these plasmaspheric plumes mapped down along

magnetic field lines into the ionosphere.

4. AURORAL WIND

[37] The photon flux of the sun is uniformly distributed

over the dayside of the Earth, and the mechanical energy

flux of the solar wind is similarly distributed over the

boundary of the magnetosphere. However, that mechanical

energy is by no means uniformly transmitted to the upper

atmosphere. Ring-like concentrations of energy input to the

atmosphere exist in the regions of bright auroras, known as

the auroral ovals. Indeed, the auroral ovals can usefully be

thought of as the footprint of the magnetospheric boundary

layer in the ionosphere, defined by the conduction of

electrical currents connecting the boundary layer with the

ionosphere. The largest energy inputs to the atmosphere

occur in these twin halos roughly centered on each magnetic

pole. These mark the cleft between magnetic field lines that

connect directly between hemispheres and those which have

been recently reconnected to the solar wind or have been

swept back into the magnetotail by earlier connection to the

solar wind, and may still be so connected. Plasmas

connected to different parts of the auroral ovals are sub-

stantially separated in space and time and may have quite

different histories, so the ovals are not monolithic. They

may be characterized by quite different behavior at any

given instant, their dynamics revealing to some degree the

different dynamics going on upstream, downstream, at the

north or south polar regions, or on the dawn or dusk side of

the Earth, in the solar wind and boundary layers. Neverthe-

less, the energy fluxes delivered to the ionosphere are

substantial. A small fraction of those energy fluxes, in the

form of thermal energy of the electrons and ions, is

sufficient to greatly enhance the escape flux of plasma into

space, even for the heavier ions O+, N+, and at times also

NO+, O2+, and N2

+ [Wilson and Craven, 1999]. As noted

above, this heavy ion outflow (with accelerated light on ion

outflows) will herein be referred to as the ‘‘auroral wind.’’


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In this section we consider the detailed causes and con-

sequences of the auroral wind.

4.1. Overview

[38] While photothermal outflows originate from the

entire sunlit globe, only the lighter ion species have signif-

icant escape fluxes at Earth, since only they have thermal

energies exceeding their escape energy. The gravitational

escape energy for the C-N-O group is of order several eV,

and a negligible fraction of these ions can escape for typical

ionospheric temperatures. Thus the photothermal polar wind

contains relatively small amounts of these heavy ions,

owing to gravitational confinement.

[39] The auroral zones are defined as regions in which

solar wind energy dissipation is frequent and substantial,

often leading to auroral light emissions. This is equivalent

to saying that the auroral zones are magnetically conjugate

with the boundary layers and associated region-1 field-

aligned current system, along and inside the magnetopause,

or with the hot plasmas created inside the magnetosphere,

and their associated region-2 field-aligned current system.

For our purposes, the auroral zones represent a localized

source energy deposited in the topside ionosphere. This

energy comes in various forms, with the net effect of

enhancing ionospheric expansion, outflow and escape, or

in short, ablation.

[40] The altitude distribution of solar wind energy dissi-

pation is also important and relevant. The theory of astro-

physical winds [Leer and Holzer, 1980] states that energy

added below the supersonic outflow transition (critical

height) increases the escaping mass flux, while energy

added above the critical height increases the velocity of

outflow without increasing the mass flux. We thus anticipate

this same effect in the auroral wind, with energy added low

down producing enhanced mass flux, while energy added

higher up will mainly accelerate the flux determined lower

down. However, some level of high-altitude acceleration

may be required to sustain escape, turning upflow into

outflow.

[41] Two types of low-altitude heating have been identi-

fied as relevant to ionospheric upflows [Banks and Holzer,

1968, 1969; Wahlund et al., 1992]. First, ion heating by

frictional collisions with neutral gas is an important heating

mechanism in the topside F layer. It is often referred to as

‘‘Joule heating’’ even though that term is usually reserved

for the heating of current conduction electrons by electric

fields. In the ionosphere, horizontal currents are carried

largely by ions, owing to their demagnetization by colli-

sions with neutrals, so this may be appropriate for that

medium. Second, electron heating by the impact of precip-

itating energetic auroral electrons is a substantial source of

energy for the ionospheric electron gas. Owing to electro-

static forces, heating of the electron gas is just as effective in

producing expansion and upflow as heating of the ion gas,

but clearly has a different signature when observed, for

example, by upward looking ionospheric radars.

[42] Another important concept for ionospheric outflows

is that of limiting flux [Hanson and Patterson, 1963].

Though originally based on the subsonic flow regime of

plasmaspheric filling, in steady state, this concept describes

a real limitation on the average rate at which ions of a

specific species can flow upward out of the ionosphere.

Higher transient fluxes are possible but cannot be sustained,

so a longer term limit is ultimately enforced. The limit

results from a limit either on how rapidly the species in

question is created through ion-neutral chemistry or a limit

on the rate at which a species can diffuse through the

frictional medium of other species or neutral gas atoms

[Barakat et al., 1987]. For current purposes, it is sufficient

to note that H+ in the polar wind is limited by typical

ionospheric conditions to a flux on the order of 2 �108 cm�2 s�1 (at 1000 km altitude). On the other hand, the

flux of O+ has a substantially higher limit on the order of

3� 109 cm�2 s�1, depending somewhat on local conditions.

[43] In summary, depending on the amount of energy

available in either imposed magnetospheric convection

(Poynting or electromagnetic energy flux), or imposed

electron precipitation (heat flux), auroral wind fluxes of

heavy ions will range from negligible to more than an order

of magnitude greater than polar wind fluxes. In this section,

we consider the various types of energy and their contribu-

tions to localized heavy ion escape from the ionosphere.

4.2. Polar Rain Precipitation

[44] One of the most basic sources of energy to the

ionosphere, erroneously cited by most encyclopedia

accounts as the cause of the aurora, is the direct ‘‘polar

rain’’ of solar particles entering along reconnected magnetic

field lines linking the Earth and the solar wind [Gosling et

al., 2004]. The ion polar rain is confined largely to the

cusp proper, where freshly injected magnetosheath plasma

penetrates quite low and precipitates into the conjugate

ionosphere, where it can be imaged as proton aurora, and

has been by the IMAGE mission [Frey et al., 2002]. Most

such solar wind ions are mirrored and reflected back into the

boundary layers, forming the plasma mantle flow along the

flanks of the magnetosphere, following the convective flow

of plasma along the magnetopause.

[45] Solar wind thermal electrons, though much faster

than the thermal ions, are required to stay with those ions

unless replaced by addition of electrons from the Earth. A

subset of them is able to move along reconnected field lines

all the way to the ionosphere, barring adverse electrostatic

potentials. The region of polar rain can include much of the

polar cap when linked to the upstream solar wind, because

the solar wind electron gas often includes a highly field

aligned, superthermal component known as the ‘‘strahl’’

[Fitzenreiter et al., 1998], the presence of which will

enhance the energy flux of electrons precipitating into the

atmosphere in the polar regions.

[46] Polar rain precipitation has been noted and studied

[Fairfield and Scudder, 1985], but it does not appear to be

associated with geophysically significant ionospheric out-

flows, at least in the observations obtained to date. It is

possible that polar rain precipitation is responsible for

certain O+ outflows that have been suggested to originate


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from the polar cap region rather than from the auroral zones

[Tam et al., 1995, 1998], but this needs more study.

4.3. Auroral Precipitation

[47] A major driver for F region/topside upward flows in

the topside ionosphere appears to be the effect of soft

auroral electron precipitation. Seo et al. [1997] examined

field-aligned flows at topside ionospheric altitudes

(850�950 km) from Dynamics Explorer 2 (DE-2) and found

strong correlations between the upward flows observed in the

auroral zones and elevated electron temperatures, as indicated

in Figure 5. They also found significant correlations of the

electron temperatures themselves with the soft electron

precipitation energy fluxes, and anticorrelation with the

characteristic or average energy of the precipitating electrons

as indicated in Figure 5. Moreover, they found that unusually

large upward ion fluxes exceeding 1010 ions/cm2-s were only

observed during periods when the average soft electron

precipitation energy was less than 80 eV, or ‘‘ultrasoft’’

precipitation cases (Figure 5).

[48] These results were interpreted by Seo et al. [1997] as

indicative that soft electron precipitation played a signifi-

cant role in driving upward F-region/topside ionospheric

plasma flows. The principal ionospheric effects of the

soft electrons in this regard were to directly enhance the

F-region ionization and to heat the thermal electrons, either

via Coulomb collisions or more indirectly through wave-

particle processes. Elevating the ionospheric electron

temperatures increases the local ambipolar electric field to

lift the ionospheric ions. The fact that unusually high

ionospheric ion upward fluxes were observed only during

periods when the average electron precipitation energy was

very low is consistent with the concept that ultrasoft

precipitation is most efficient at both ionization and heating

at upper F-region and topside altitudes. Higher-energy

precipitation penetrates to lower ionospheric altitudes, in

which ionization tends to remain in local equilibrium and

not diffuse upward.

[49] Modeling of specific auroral ionospheric upflow/

outflow observations in which effects of soft electron

precipitation were incorporated was performed by Liu et

al. [1995] and Caton et al. [1996], while Su et al. [1998a]

performed systematic fluid-based ionospheric modeling of

the effects of electron precipitation on upflows by examining

the effects of a range of electron precipitation parameters on

ionospheric parameters, such as the upflows versus time

and altitude, the electron and ion temperatures. Wu et al.

[1999, 2002] incorporated the effects of soft electron

precipitation into a fluid-kinetic transport model that exam-

ined the synergistic effects of soft electron precipitation and

transverse ion heating on the transport of ionospheric plasma

through the auroral ionosphere-magnetosphere region.

4.4. Auroral Heating

[50] Transverse ion heating in the auroral zone was

observed well before high-altitude observations of the polar

wind were made [Sharp et al., 1977; Whalen et al., 1978;

Klumpar, 1979]. This is owed at least in part to the fact that

it is relatively easier to observe, as directional hot tails of the

ionospheric energy distribution, extending readily up to tens

of eV and often up to hundreds of eV or even several keV,

depending on event intensity and instrument sensitivity

[Hultqvist et al., 1991; Yau and Andre, 1997; Andre et al.,

1998; Norqvist et al., 1998].

[51] The basic observation of transversely accelerated

ions (TAI) clearly implies energy gain principally transverse

to the local magnetic field. This was often envisioned as

taking place in a narrow altitude range, and it was assumed

that the subsequent behavior of the ions would be essen-

tially that of a free particle gyrating and mirroring up out

of the divergent magnetic field of the auroral zone, leading

to a ‘‘conical’’ distribution whose cone angle would vary

with altitude according to the conservation of the first

adiabatic invariant and total energy. However, it soon

became apparent that transverse heating is in general

Figure 5. Electron precipitation as a driver of outflows via electron heating. (left) Ion outflow velocitiesin the auroral topside ionosphere (850�950 km altitude) from DE-2 versus electron temperature. Straightline fits and correlation coefficients are based on binned data. (middle) Ion upflow velocities versusaverage energy of soft electron precipitation. (right) Ion upflow fluxes versus average energy of softelectron precipitation. From Seo et al. [1997].


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widely distributed along auroral flux tubes in altitude [e.g.,

Moore et al., 1999a].

[52] Transverse heating also occurs through multiple

different physical processes. At the lowest altitudes, in the

F region, the dominant mechanism is ion-neutral friction as

magnetospheric motions drag the ionized plasma through

the embedded neutral gas. The collisions become infrequent

relative to the gyro frequency at some altitude, with charge

exchange interactions persisting to the greatest heights.

Subsequent to a collision, the scattered or newly formed

ion is picked up by the convection electric field, imparting

a perpendicular energy that exceeds the ion thermal

energy for strong convection. Transverse ion heating is

the relatively straightforward result, but it is limited to

energies corresponding to a few km/s or a few eV for O+

ions, and much less for H+ [e.g., Moore et al., 1996].

[53] Plasma waves with substantial power in the electro-

magnetic ion cyclotron range (EMIC waves) are frequently

observed in the auroral ionosphere, with frequencies well

above those associated with MHD instabilities. These are

thought to result from turbulent cascades of MHD wave

energy, or to be associated with free energy of auroral

current systems, or possibly with the occurrence of certain

types of ion anisotropy, for example ion shell distributions.

EMIC waves or broadband extreme low frequency (BBELF)

waves have sufficient power in the cyclotron range to raise

transverse tails on the ion velocity distributions and are

thought to do so frequently. Some of these are Alfven waves

propagating into the ionosphere along flux tubes, which are

thought to be effective in heating ionospheric ions, and have

been associated with the most productive region of nightside

outflows at the poleward boundary of the midnight auroral

oval [Tung et al., 2001; Keiling et al., 2001]. These low-

frequency waves are thought to be generated by boundary

layer instabilities or other sources of turbulent motions, such

as reconnection.

[54] At higher plasma wave frequencies, ion transverse

motions are though to be coupled with parallel electron

motions by means of lower hybrid waves [Retterer et al.,

1994; Kintner et al., 1992]. These waves are not thought

capable of influencing the core of the ion velocity distribu-

tions, but are thought to create or extend transverse tails of

the distribution to energies well above thermal. The waves

are thought to be current-driven, deriving energy from the

parallel motion of current-carrying electrons.

[55] Recent observations from the FAST Mission have

added many details to our knowledge of auroral transverse

ion heating. These include events in which He+ ions are

resonantly accelerated perpendicular to the magnetic field to

energies of several keV [Lund et al., 1998]. These He+

energizations occur during periods of EMIC waves and

conic angular distributions of up to a few hundred eV in H+

and a few keV in O+. This was identified by Lund et al.

[1998] as a process similar to that which may occur in solar

flares. Lund et al. [1999] also examined FAST observations

indicating that for ion conics produced by interactions with

BBELF waves, the energies of different ion species are

often nearly equal. They suggested that this discrepancy

with theoretical predictions, which predict preferential

heavy ion acceleration, can be understood through a version

of the ‘‘pressure cooker’’ mechanism. With simple single

particle computations, Lund et al. [1998] showed that for a

significant downward electric field within the transverse

heating region, ions of different masses are energized to

approximately the same energy as required to escape the

downward electric field ion trapping region.

[56] It seems likely that all these processes contribute to

the inferred wide altitude range of auroral ion transverse

heating, beginning in the F region with frictional heating,

extending through the topside with EMIC and LHR wave

heating of ions, with the latter extending to altitudes of

several RE. All of the processes derive energy from the

dissipation of MHD energy in the form of medium frequency

(approximately cyclotron) waves or turbulence, including

the auroral current systems and wave driven by them.

4.5. E// Acceleration

[57] Parallel electric fields occur in the auroral region in

both upward and downward current regions. Their presence

and significance for auroral acceleration had been discussed

for many years, certainly in Scandinavian perspectives, and

evidence for their existence had been found in rocket [e.g.,

Evans, 1974] and satellite [e.g., Frank and Gurnett, 1971]

electron measurements. However, the first systematic and

relatively conclusive measurements of such parallel electric

fields probably came from the measurements in the mid-

1970s by the S3-3 spacecraft near 1 RE altitude, an altitude

region hitherto largely unsampled. Direct measurements of

parallel electric fields [Mozer et al., 1977] and upgoing ion

beams [Shelley et al., 1976] confirmed the existence of

strong parallel electric fields somewhere below �6000 km

altitude in the auroral zones. Moreover, in addition to the

upward parallel electric field observations, associated with

upward current regions and generally linked with discrete

auroral arcs, S3-3 observations also indicated evidence of

downward electric fields having important effects ion out-

flows by trapping ions in transverse heating regions for

extended periods of time and thus leading to higher trans-

verse energization [e.g., Gorney et al., 1985].

[58] An example of a recent simulation investigation

which sought to elicit the effects of parallel electric fields

on ionospheric ion outflows is the work of Wu et al. [2002],

mentioned earlier. As noted above, Wu et al. [1999] used a

Dynamic Fluid Kinetic (DyFK) code to simulate the effects

of soft electron precipitation and transverse ion heating on

ionospheric outflows. Wu et al. [2002] added to these

influences a distributed field-aligned potential drop pro-

duced self-consistently by a high-altitude (magnetospheric)

population of hot ion and electron populations. When

the presumed upper boundary magnetospheric population

had significant differential anisotropy between the ion

distribution and the electron distribution, significant parallel

potential drops resulted. When the upper boundary ion

population was more perpendicularly peaked than the

electron distribution, such as a trapped type of ion distri-

bution with a more isotropic electron distribution, this led to


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a positive potential peak and downward directed parallel

electric fields. When the upper distribution of magneto-

spheric electrons was more perpendicularly peaked than that

of the magnetospheric ions, an upward electric field distri-

bution resulted. In both cases, the upward flow of iono-

spheric plasma into these hot plasma regions gradually

neutralized and reduced the potentials over timescales of

the order 30 min.

[59] In the case of the hot plasma differential anisotropy

circumstances associated with upward electric fields, these

fields of course accelerated the ionospheric ions, which

were first transversely heated above the topside by trans-

verse waves, to parallel beam energies of the order of

100�200 eV of energy. In the case of the hot plasma

differential anisotropies leading to downward electric fields,

various types of ionospheric distributions were produced,

including classical conics as well as ‘‘butterfly’’ distribu-

tions, in which occurred simultaneous upward and down-

ward conical distributions, at times with ‘‘holes’’ or

absences of particles at the low-energy part of phase space.

These holes presumably were the result of exclusion of

ionospheric ion access by the positive potential barrier.

Some observations of toroidal or quasibutterfly distributions

which were observed by POLAR [e.g., Huddleston et al.,

2000] could possibly be interpreted along such lines,

although other alternative explanations might be involved

as well. While anisotropies clearly require parallel potential

drops, it is field-aligned currents that are thought to

ultimately determine their necessity [e.g., Lyons, 1980].

[60] More recently, instruments aboard the FAST space-

craft have been used to observe effects of parallel electric

fields on ion beams and other ion outflows. For example,

McFadden et al. [1998] exploited very high time resolution

measurements of ion distributions to show kilometer-scale

spatial structures, which involve ‘‘fingers’’ of parallel

electric potential structures which extend hundreds of kilo-

meters along the magnetic field line but are only a few to

tens of kilometers wide. McFadden et al. [1998] were able

to show that integrations of the electric field along the

spacecraft track to calculate the parallel potential below the

spacecraft were typically in reasonable agreement with

the observed ion energy. In another paper based on FAST

measurements in the cusp region, Pfaff et al. [1998] found a

complex mixture of DC electric fields and plasma waves in

association with observed particle accelerations as mani-

fested by upgoing ion beams, ion conics, and related electron

distributions. In a particular case, injections of magneto-

sheath ions were associated with upgoing ion beams and

electrons going earthward, indicative of potential structures

set up by the magnetosheath injections.

[61] In another result from FAST, Ergun et al. [2000]

analyzed data from the upward current region of the auroral

zone, which inspired Vlasov simulations of the static

particle distributions and potential structures. From

the observed electron distributions, they inferred density

cavities, and regions in which the densities were dominated

by either plasma sheet electrons or cold ionospheric elec-

trons. Ergun et al. [2000] conducted Vlasov calculations

with one-spatial and two-velocity dimensions to obtain

Figure 6. Observed relationship between ionospheric outflow flux in the dayside auroral zone and thehourly standard deviation of the solar wind dynamic pressure [Fok et al., 2005].


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large-scale self-consistent solutions of the parallel electric

field. Depending on the ionospheric H+ density level and

other parameters, the electric potential along the magnetic

field lines shows three different potential and plasma

population regions, and includes distinct electron and ion

‘‘transition’’ layers. Such potential distributions could then

be responsible for the upward ion beams emerging from

these types of regions.

4.6. Specifying Global Outflows

[62] To compute and appreciate their consequences for

the solar wind interaction with Earth, as well as their

ultimate loss into the downstream solar wind, it is essential

to specify how these outflows are driven by that interaction.

Prior to the advent of upstream solar wind observations,

efforts focused on the dependence of outflows upon

internal magnetic indices such as Kp (planetary activity),

AE (auroral electrojet), or Dst (storm time disturbance)

[Yau and Lockwood, 1988]. The results were usually stated

in statistical terms of global aggregate outflow (ions/s),

sometimes treating the Earth as a point source, but usually

sorted by longitude and latitude [e.g., Giles et al., 1994].

More recently, outflows have been regarded as directly

driven by solar wind parameters [Pollock et al., 1990;

Moore et al., 1999b; Cully et al., 2003a; Lennartsson et

al., 2004]. It has become clear from these studies, as shown

in Figure 6, that the dynamic pressure of the solar wind is a

very strong driver of outflows, particularly from the dayside

auroral zone where the solar wind interaction is quite direct.

Somewhat surprisingly, the IMF Bz appears to be more

weakly correlated with auroral wind outflows [Pollock et

al., 1990]. While illuminating, these observed relationships

clearly suffer from a large amount of scatter indicating that

multiple processes mediate the interaction with the solar

wind. Any attempt to relate global outflow with typical

upstream solar wind conditions begs the question of how

auroral wind outflows respond to the detailed local distri-

bution of energy inputs into the topside ionosphere gas, as

determined by magnetospheric global dynamics [Andersson

et al., 2004]. Only when such detailed outflows can be

specified as functions of global dynamics can a realistic

global simulation be constructed. Even then, the solar wind

is far from uniform across the magnetopause and the system

responds to prior history as well as instantaneous condi-

tions, and no solar wind sequence is ever repeated exactly.

[63] A significant advance in our ability to observe global

patterns and time variability of ionospheric outflows and

atmospheric escape came with the IMAGE mission and its

neutral atom imagers, including especially the Low Energy

Neutral Atom (LENA) Imager [Moore et al., 2000]. LENA

provides a new capability to both remotely sense ion plasma

heating and acceleration, and to observe the escape of fast

neutral atoms from the atmosphere [Moore et al., 2003].

Using LENA it was confirmed that heavy ion outflows are a

prompt response to solar wind dynamic pressure enhance-

ments [Fuselier et al., 2001, 2002; Khan et al., 2003]. It was

also confirmed that the creation of outflows is strongly

related to internal geomagnetic activity of the magneto-

sphere [Wilson and Moore, 2005]. A hot neutral oxygen

exosphere was shown to be created by auroral activity

[Wilson et al., 2003]. The related outflows of fast neutral

atoms were initially surprisingly intense [Moore et al.,

2003], but recent theoretical work suggests that this is

Figure 7. A flowchart depicting the relationships between energy inputs to the ionosphere andionospheric ion outflows detected by FAST near 4000 km altitude. From Strangeway et al. [2005].


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expected when ion acceleration begins sufficiently low in

the ionosphere [Gardner and Schunk, 2004].

[64] Recently, Strangeway et al. [2000, 2005] examined

ionospheric outflows near 4000 km altitude using the FAST

spacecraft in order to understand the factors that drive the

outflows. Zheng et al. [2005] performed a similar study for

the Polar data set. Both found high correlations between the

logarithms of the observed outflow ion fluxes and both

the density of the soft electron precipitation as well as

the electromagnetic energy flux. Both concluded that the

principal effects were Poynting flux and electron precipita-

tion inputs in the low ionosphere, which would be broadly

consistent with incoherent scatter radar observations by

Wahlund et al. [1992] and others, and the DyFK simulations

of Wu et al. [1999]. A summary ‘‘flowchart’’ of their

findings is indicated in Figure 7.

[65] There is a strong internal correlation between the

precipitation and Poynting energy fluxes themselves, and

therefore it is somewhat difficult to separate their effects on

the outflows purely from such statistical observational

methods. Recently, Moore et al. [2007] and Zeng and

Horwitz [2007] concluded that both are necessary at some

level for outflow to occur. Owing to the transit times of ions

from F-region/topside ionosphere to the observation alti-

tude, as the observation altitude increases, spatiotemporal

discrepancies between responsible energy inputs and out-

flow plumes are likely to be more significant. Such transit

time–produced uncertainties may well have reduced possi-

ble correlations, especially at Polar altitudes of 5000 km.

However, the soft electron precipitation and the Poynting

flux appear to have somewhat separable effects on the

outflows. If so, this relationship is consistent with the

concept of ‘‘two-stage’’ processes driving auroral iono-

spheric outflows, as modeled for instance by Wu et al.

[1999, 2002]. In this paradigm, auroral outflows consist

typically first of ‘‘upwelling’’ in the F-region and topside

ionosphere (driving the mass density of outflows), followed

by stronger energization at higher altitudes (driving the

velocity and therefore escape of outflows). The processes

that are thought to drive ‘‘upwelling’’ at low altitudes, such

as soft electron precipitation-driven ionospheric electron

heating and Joule/frictional ion heating, may propel signif-

icant fluxes to higher altitudes but only to relatively small

energies (typically below 1 eV), which are too small alone

to cause O+ escape. Thus a second energization stage, for

instance involving wave-driven transverse ion heating pro-

cesses, at altitudes down extending toward 1000 km or even

lower, is then required to produce significant fluxes of

escaping O+ ions that could overcome the gravitational

barrier, approximately 10 eV from the Earth’s surface, and

attain magnetospheric altitudes.

[66] Another important factor in the acceleration of ions

to magnetospheric energies and altitudes is the parallel

electric field, generally considered to be significant in the

altitude range of 1 RE, as discussed above. From a global

magnetospheric perspective, E// is a relatively low altitude

influence on the properties of outflowing auroral ions. E// is

thought to be determined largely by the sense and magni-

Figure 8. (top) Hemispheric distribution of NeHot, Poynting energy flux, and parallel current density,mapped to ionospheric heights in the Northern Hemisphere, for a snapshot of the LFM MHD simulationof the magnetosphere during SBZ. (bottom) The hemispheric distribution of ion Tperp, V//, and escapeflux corresponding to the conditions in the top row (simulation results courtesy of S. Slinker (personalcommunication, 2005); ionospheric outflow conditions computed per Strangeway et al. [2005]; R. J.Strangeway (private communication, 2005) for ETH).


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tude of the local parallel or Birkeland current density [e.g.,

Lyons, 1980; Carlson et al., 1998]. High-altitude flow shear

(or vorticity) producing J// away from the ionosphere, and

exceeding a threshold current density that is easily supplied

by low-density magnetospheric electrons, implies parallel

potential drop that increases with the current density. In

contrast, when the current J// flows toward the ionosphere, it

is relatively easily carried by ionospheric electrons, and

implies quite small potential drops.

[67] In Figure 8, we exhibit the inner boundary condi-

tions relevant to auroral wind outflows, mapped to iono-

spheric heights, from the LFM simulation as a snapshot

during an interval of southward IMF. In Figure 8 (bottom),

we have translated these boundary conditions into ion

outflow conditions using relations based on those of

Strangeway et al. [2005] and Zheng et al. [2005]. Here

we have assumed that the total ion flux has superposed

contributions from the Poynting energy flux and from the

hot electron precipitation density, but a geometric mean has

proven more realistic [Moore et al., 2007; Zeng and Horwitz

2007]. We have taken the Poynting flux to drive the ion

temperature, and we have taken the parallel current density

to drive the parallel ion velocity according to the modified

Knight relationship of Lyons [1981]. It can be seen that the

result is highly structured outflow distribution, in both flux

and velocity, and it can readily be appreciated that this

pattern is highly variable in response to system dynamics as

simulated by a global MHD simulation.

5. GLOBAL CIRCULATION

5.1. Centrifugal Acceleration

[68] The terminology ‘‘centrifugal acceleration’’ for

convection-driven acceleration of high-latitude ionospheric

outflows was introduced by Horwitz [1987a], while the

basic effect itself was identified by Cladis [1986], Mauk

[1986], and Delcourt et al. [1988] as a curvature drift. The

effect may be described [e.g., Horwitz, 1987b] as similar to

the acceleration of a bead moving easily on a rotating stiff

wire, in which the bead is flung radially outward along the

wire as a result of the rotation. In the case of diverging

magnetic field lines of the Earth, convection can be viewed

as a quasirotation of charged particles along these field

lines, again effectively accelerating them outward toward

higher altitudes in the magnetosphere.

[69] Several authors have included and investigated the

significance of this effect on high-latitude and magneto-

spheric particle motion. For example, Horwitz et al. [1994]

examined the possible effect of centrifugal acceleration on

the polar wind in a kinetic ionospheric transport simulation,

and found that such acceleration can in principle explain

observed polar electron density profiles and elevated O+

densities for moderate convection speeds. However,

Demars et al. [1996] calculated that the effect of centrifugal

acceleration on the polar wind H+ may be slight for normal

convection values. Centrifugal acceleration has been incor-

porated in numerous single particle trajectory studies of

ionospheric and other particle flow into and through the

magnetosphere [e.g., Delcourt et al., 1990, 1994]. Also, the

one current global MHDmodel of magnetospheric dynamics

[Winglee, 1998; Winglee et al., 2002] that incorporates the

effects of ionospheric plasma outflow into the magneto-

spheric dynamics at present relies chiefly on centrifugal

acceleration as the agent for energization of ionospheric

plasma to be ejected into the magnetosphere.

[70] Arguably the most compelling discrete observational

evidence for centrifugal acceleration of ionospheric

ions comes from the observations of energized H+, He+,

and O+ ions in the middle magnetosphere during substorm

‘‘dipolarization’’ events [Liu et al., 1994], as illustrated in

Figure 9. Liu et al. [1994] observed energized ionospheric

ion species having similar parallel (and convection) veloci-

ties, and thus characteristic energies roughly in the proportion

H+:He+:O+ = 1:4:16. This is a telltale characteristic of

centrifugal acceleration and should be observed when it

dominates other processes.

5.2. Fountain Flows

[71] The concept of a localized fountain of outflowing

ionospheric ions ejected into a horizontal wind of convec-

tion was introduced in the form of the ‘‘cleft ion fountain’’

(CIF) during the mid-1980s [Lockwood et al., 1985;

Horwitz and Lockwood, 1985]. These formulations were

based on observations of low-energy ionospheric ions from

DE-1 in the region of the cleft and on into the polar cap

lower-intermediate altitude magnetosphere. Observations

and corresponding simulations depicted a cleft-associated

source of upward moving ions that spread into the polar

cap magnetosphere under the influence of antisunward

(or sunward) convection.

[72] Among the interesting phenomena observed and

simulated were the effects of separation of ion species,

Figure 9. Velocities of H+, He+, and O+ during a substormdipolarization event observed by DE-1 in the middlemagnetosphere [Liu et al., 1994].


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termed the ‘‘geophysical mass spectrometer’’ [Moore et al.,

1985], or ‘‘geomagnetic mass spectrometer’’ [Lockwood et

al., 1985] and ‘‘parabolic’’ heavy ion flows [e.g., Horwitz,

1984, 1986; Horwitz and Lockwood, 1985]. The picture of a

‘‘fountain’’ followed from the discovery of the geophysical

mass spectrometer effect, in which midaltitude spacecraft

observe, in sequence ‘‘downstream’’ from the cleft source,

first H+, then He+, and then O+ as the spacecraft moves

antisunward from near cleft threading magnetic field lines

into the polar cap region. This results because, for similar

thermal energies, light H+ ions have larger field-aligned

velocities than do heavier He+ and O+, while for all energies

and all species the horizontal convection velocity is the

same. Thus the faster H+ ions tend to remain closer to the

source field lines, while the slower O+ ions are transported

further into the polar cap by convection during the transit

time required to attain similar spacecraft altitudes. ‘‘Para-

bolic’’ flows, involving upward and then downward motion

of O+ ions with energies below the gravitational escape

barrier, were indicated in the trajectory modeling of Horwitz

[1984], and the downward O+ flows in the polar cap were

observed near 1 RE altitude by Lockwood et al. [1985].

[73] Soon thereafter, Chappell et al. [1987] incorporated

the cleft ion fountain concept, together with polar wind and

‘‘auroral ion fountain’’ sources, to consider whether circu-

lation of ionospheric origin plasma into the magnetosphere

could account for the observed magnetospheric plasmas.

They concluded that indeed this was the case and in

particular that the ionosphere was a ‘‘fully adequate source’’

for the plasma sheet. Several other efforts have employed

large-scale trajectory models to track ionospheric particles

through the magnetosphere, including Delcourt et al. [1989,

1994] Ashour-Abdalla et al. [1992], Peroomian [2003], and

Fok et al. [1999], seeking to account for observed beams

and other specific ionospheric ion distributions in the larger

magnetosphere and also under the influence of centrifugal

acceleration and related effects. An example from a recent

effort [Tu et al., 2005] is used to visualize the auroral

plasma fountain in Figure 10.

[74] One of the ongoing issues concerning ‘‘fountain

flows’’ is the question of whether or not the cleft ion

fountain, or more generally the auroral fountain, is the

dominant source of observed O+ in the polar cap magneto-

sphere. Additional observations of frequent downward

flows of O+ in the main polar cap under southward IMF

conditions [e.g., Chandler, 1995] have led others [e.g.,

Horwitz and Moore, 1997] to at least tentatively conclude

that indeed the cleft ion fountain, and not the polar cap

ionosphere, must be the source of the O+ in the polar cap

magnetosphere proper, at least during southward IMF when

the polar cap is not normally threaded by transpolar arcs that

might be sources of energized upflowing O+. More recently,

papers by Tu et al. [2004, 2005] have demonstrated that

sophisticated ionospheric plasma transport modeling based

on the CIF paradigm can produce close agreement

with observed polar cap density profiles and from observed

O+ field-aligned velocity and density profiles seen near

5000 altitude from cross-polar spacecraft passes. However,

other authors [e.g., Abe et al., 2004] maintain that the solar-

illuminated polar cap ionosphere can directly supply the O+

observed in the midaltitude polar cap magnetosphere,

through effects such as photo-electron acceleration of the

polar wind and/or elevated effective electron temperatures

Figure 10. Simulated variations of the O+ densities and velocities along a noon-midnight orientedconvection trajectory with the horizontal distance and altitude. Vertical bars on the bottom x axis mark thelocations of the equatorward and poleward boundaries of the cusp/cleft region. (Reprinted/modified fromTu et al. [2005], with permission from Elsevier.)


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(such as through effects of the polar rain [e.g., Barakat and

Schunk, 1983]).

[75] As indicated previously, the idea that photo-electron

effects can accelerate relatively low altitude heavy iono-

spheric ions to escape energies comes in large part from

papers by Tam et al. [1995, 1998], whose modeling calcu-

lations suggested that O+ could attain supersonic speeds

at altitudes as low as 500 km. However, as also noted

earlier, these predicted low-altitude photo-electron acceler-

ation effects have never been identified in observations, and

other efforts on modeling the photo-electric polar wind by

Khazanov et al. [1997], Wilson et al. [1997], and Su et al.

[1998a] have instead indicated that photo-electron accelera-

tion occurs at relatively high altitudes of 2–3 RE, in con-

junction with a thin double layer of tens of volts. Hence we

maintain that the cleft, or more broadly the auroral, ion

fountain, is the principal source of O+ in the polar cap

magnetosphere ‘‘proper,’’ that is, on magnetic field lines

not threaded by polar cap/transpolar arcs such as occur during

northward IMF conditions. However, the most powerful

evidence that photothermal outflow cannot in itself supply

significant heavy ion plasmas at high altitudes is the well-

known light ion dominance of the plasmasphere, a region

that is supplied almost exclusively by the sunlit low latitude

ionosphere.

5.3. Lobal Wind

[76] The polar and auroral winds are easily distinguished

in terms of their causative agents, but they are not so easily

distinguished when observed from a given point in geo-

space. Owing to the fountain flow effects discussed in

section 5.2, ions are dispersed according to their differing

parallel velocities, and are thus selected for velocity by

the geophysical mass spectrometer effect. The result is a

mixture of polar wind and auroral wind that varies substan-

tially with location within the high-latitude regions of

magnetospheric circulation, but tends to have a restricted

range of parallel velocities in any particular spot [Horwitz et

al., 1985]. This effect is accentuated for auroral wind ions

that are emitted from a region of relatively narrow extent

embedded within magnetospheric convective flow, as

described above, creating the geophysical mass spectrometer

effect.

[77] The resultant mixture flows outward through the

magnetospheric lobes, in part supplying the plasma sheet

with relatively cold source plasmas, and in part escaping

downstream beyond the region convecting earthward. To

preserve the distinction between polar and auroral winds,

we have adopted the term ‘‘lobal wind’’ for the resultant

mixed plasmas that flow out of the polar cap into the lobes

of the magnetosphere, and then convect into the plasma

sheet or escape beyond.

[78] Recently, Liemohn et al. [2005] have surveyed the

properties of the lobal wind observed from the Polar

spacecraft and determined that it is a pervasive feature of

the lobes, providing a continuous supply of cool plasmas to

the central plasma sheet inside the location of any persistent

reconnection X line. The unidirectional outward flows are

transformed into bidirectional interpenetrating streams from

the two lobes as they convect onto closed field lines. These

bidirectional streams are isotropized and strongly heated

when the neutral sheet thins during periods of higher

activity. This behavior is associated with the onset of

spatially nonadiabatic trajectories as the plasma sheet thins.

Temporal nonadiabatic behavior is also triggered when

reconnection or other effects change the neutral sheet on

gyroperiod timescales for specific ion species.

5.4. Geospace Storms

[79] Downstream of the midtail plasma sheet, during

normal slow magnetospheric convection, lies the magneto-

pause along the dawn and dusk flanks of the magneto-

sphere. When dayside reconnection complements nightside

reconnection, the plasma circulation is drawn deeply

through the inner magnetosphere toward the subsolar

magnetopause region. Then heated ionospheric and solar

plasmas supply the inner magnetosphere and ring current

region, and a lesser or greater geospace storm results,

depending upon the strength and persistence of these

effects, combined with the number and frequency of sub-

storm dipolarizations that heat and compress plasma sheet

plasmas into the inner magnetosphere.

[80] Mitchell et al. [2003] have shown that substorms, in

particular, modulate primarily the heavy ion component of

ring current hot plasmas, while the proton component is

relatively unresponsive to them. Moore et al. [2005a] have

argued that this is a natural consequence of the different

circulation paths of solar wind and auroral wind contribu-

tions to the ring current. In the global test particle simu-

lations they performed in MHD fields, solar wind proton

entry occurred principally through the low-latitude flank

magnetopause boundary layers, with direct transport into

the ring current region. In contrast, auroral wind O+ was

transported via the traditional route through the polar caps

and lobes, into the plasma sheet, and then back to the inner

magnetosphere via the midnight plasma sheet. They argued

that this implies a greater substorm dipolarization effect on

plasmas of ionospheric origin. Recent work with simulated

substorms has confirmed this conclusion [Fok et al., 2006].

[81] The amount of auroral wind O+ in the energetic

plasmas of the ring current is a direct function of the

magnitude of that current [Sharp et al., 1985, 1999, 1994;

Daglis and Axford, 1996; Daglis, 1997; Moore et al., 2001,

2005b]. Recently, Nose et al. [2005] have studied the

October 2003 superstorm and reviewed its place in the

existing observations of storm plasma composition, as

shown in Figure 11. They show that a trend line can be

identified in which the ratio of O+ to H+ pressure increases

exponentially with the magnitude of storms as measured by

Dst. The pressure ratio rises from near zero for very small

Dst and passes through unity at Dst � 200, making the

origin of the largest geospace storm plasmas primarily

ionospheric.

[82] Energetic neutral atoms (ENA) imaging from the

IMAGE mission has confirmed that the principal mecha-

nism of ring current decay is charge exchange production


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of fast neutral atoms that immediately escape from the

magnetosphere [Burch et al., 2001]. Thus even that part

of the auroral wind outflow that is trapped within the

magnetosphere and fails to escape to the downstream solar

wind eventually manages to escape through charge exchange

(on cold geocoronal hydrogen) back to a neutral state. Since

the Earth is such a small object within the realm of the ring

current, a small fraction of such fast atoms will precipitate

back into the atmosphere, and most of them will escape into

the solar wind.

5.5. Plume Flows

[83] The polar wind fills the plasmasphere to pressure

equilibrium with the ionosphere and then shuts off in the

stagnant, corotating, inner magnetosphere. Beyond the

plasmasphere, magnetospheric circulation carries flux tubes

away to the dayside magnetopause and boundary layers.

Some of these flux tubes are opened up to the solar wind by

dayside reconnection and then dragged downstream through

the polar caps or low-latitude boundary layers. Viscosity in

the ionosphere spreads this motion to neighboring flux

tubes even if they are not actually opened up by reconnec-

tion, and these flux tubes are also stretched dramatically as

they flow down the low-latitude boundary layers.

[84] Magnetospheric circulation varies greatly in its depth

of penetration into the inner magnetosphere, depending on

the synchronization, or lack thereof, between dayside and

nightside reconnection. When reconnection is weak, and

circulation is restricted to the outer magnetosphere, the

plasmasphere grows larger as flux tubes fill with plasma,

with longer timescales for the outer, longer flux tubes at

greater radius. When reconnection increases simultaneously

on the dayside and on the nightside, the night-day plasma

pressure gradient increases and deep convection occurs

through the innermost magnetosphere. Then plasma built

up on outer flux tubes of the plasmasphere is entrained in

the flow and transported to the dayside magnetopause where

it enters the reconnection regions there, or becomes part of

the low-latitude boundary layer flows.

[85] This process was hypothesized in the late 1960s and

early 1970s [Grebowsky, 1970], and shown then to create

a plume of plasma leaving the outer plasmasphere and

moving toward the postnoon sector of the dayside magne-

topause. Borovsky et al. [1997] and Elphic et al. [1997]

further hypothesized that such transient release of plasma

from the plasmasphere could create superdense polar wind,

and possibly a superdense plasma sheet. Evidence of this

was observed during events when the magnetopause was

compressed to geosynchronous orbit [Su et al., 2000, 2001],

but our understanding of this process was greatly expanded

when the IMAGE spacecraft was able to image the forma-

tion of such plumes [Sandel et al., 2001], permitting

detailed studies of their formation and variability [Goldstein

et al., 2002].

[86] More recently, direct observations of cold convecting

plasmas have been obtained at the normal equatorial mag-

netopause [Chandler and Moore, 2003]. Chen and Moore

[2004] found further that sunward flow bursts approaching

the local Alfven speed were observed in these flows,

correlated with southward IMF in the upstream solar wind.

More recently, statistical studies [Chen and Moore, 2006]

have shown that the occurrence and sense of convection

of these flows is fully consistent with global convection

as driven by dayside reconnection, and its variation

with interplanetary magnetic field, Bz. Figure 12 gives an

overview of these results in terms of the distribution of cold

convecting plasmas observed in the dayside magnetopause

region.

Figure 11. Relationship between ring current composition and magnitude of the ring current asmeasured by Dst or Sym-H (limited Dst). The ‘‘Halloween 2003’’ event appears at the top right of theplot [Nose et al., 2005].


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[87] The significance of these plasmas is that they have

not been observed systematically before, yet they substan-

tially reduce the Alfven speed of the inflow to the low-

latitude reconnection region at the dayside magnetosphere.

Freeman et al. [1977] have previously noted that this

could create a negative feedback on deep magnetospheric

convection, whereby reconnection would be slowed if a

sufficient amount of ionospheric or plasmaspheric plasma

was convected to the dayside subsolar region. This may be

related to recent interest in the saturation of the transpolar

potential [Siscoe et al., 2002a]. More recently, it has

been shown that enhanced densities of plasma, especially

cold protons, will produce essentially a ‘‘cessation of recon-

nection’’ [Hesse and Birn, 2004]. O+ ions are observed to be

part of the hot magnetospheric plasma when arriving at the

dayside magnetosphere [McFadden et al., 2003]. This is

consistent with most O+ outflow being driven by auroral

processes, as part of what we have called the auroral wind.

Such hot plasmas may also load the dayside reconnection

processes, especially for heavy ions.

5.6. Global Simulations

[88] We have learned from observations that ionospheric

plasma permeate the magnetosphere and come to dominate

even the energetic particle populations during great geo-

space storms. Many of the groups performing global sim-

ulations have begun to modify their codes to take account of

ionospheric plasmas as circulating dynamical elements of

the system, literally above and beyond their role as a

resistive medium in the thin F layer. The most needed

improvements are, first, the specification of ionospheric

outflows in terms of energy inputs and, second, the simple

inclusion of multiple fluids in global simulations [Winglee,

1998, 2000]. This will slow the calculations of such

simulation codes proportionately to the number of fluids.

However, we have made the case earlier that it is a serious

oversimplification to assume that the ionospheric load is

well described by friction in the topside ionosphere, that is,

that no ionospheric energy appears in the magnetosphere

proper, when the observed fact is that ionospheric pressure

dominates the largest storms.

[89] Nevertheless, the full task is more difficult than it

appears because heavy ions are nonadiabatic in many

magnetospheric regions. Even when they are reasonably

adiabatic, their transport departs substantially from the

convective paths for cold particles. We must move in the

direction of accounting for ion drifts and finite gyroradius

effects, which will ultimately require hybrid simulations,

with fluid electrons and particle ions. In the meantime,

progress is being made in understanding and assessing the

character of ionospheric circulation in the magnetosphere

using global simulation fields to compute the motions of test

particles [Winglee, 2003; Peroomian, 2003; Cully et al.,

2003b; Moore et al., 2005a, 2005b].

[90] In Figure 13, we illustrate this work with an example

result from Cully et al. [2003b]. Here the supply of iono-

spheric plasmas to the plasma sheet has been assessed in

aggregate as a function of time during two different step

changes in the IMF, one a northward turning and the second

a southward turning. It can readily be seen that an abrupt

northward turning results in a sharp and relatively prompt

cutoff in the supply of O+ ions to the plasma sheet. This is

because convection in the lobes stops and reverses direction

in response to this change. In contrast, when the IMF turns

southward, convection through the lobes is enhanced, but

enhanced ion outflow delivery to the plasma sheet increases

gradually and with a delay owing to the transport time of

relatively slow O+ ions from their source regions.

[91] An important point made by Cully et al. [2003b] is

that auroral wind outflow through the lobes is constant and

ongoing, though the magnitude of the flux can vary greatly

in response to ionospheric energy inputs. Even though the

transit time from the ionosphere to the plasma sheet is long,

the outflow stream responds instantaneously to changes in

Figure 12. Distribution of cold convecting ions at the dayside magnetosphere for (a) northward and(b) southward IMF. After Chen and Moore [2006].


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magnetospheric convection or configuration. Cully et al.

suggest further that a cutoff in the supply of O+ to the near-

Earth magnetotail may deprive the region of current carriers

and cause a reconfiguration of the magnetic field in that

region as a result, with possible implications for substorm

dynamics [Lyons et al., 1998]. Cully et al. did not consider

an abrupt compression of the magnetosphere, as would be

produced by a sudden increase in solar wind dynamic

pressure. Such compression would likely have an energizing

and intensifying effect on the plasma sheet and the auroral

winds feeding it, with no significant transport delay. Simu-

lation of this is best done with MHD fields for which the

global response can be calculated self-consistently, as

reported by Moore et al. [2007].

[92] In addition to supplying current carriers, the presence

of polar and auroral winds in the magnetosphere also

represents a moderating load on the system, and this appears

when they are treated as true dynamical elements of the

global simulation, as noted by Winglee et al. [2002]. While

illustrating the processes of transport and energization that

operate on polar and auroral wind plasmas, current single

particle simulations must in the future be adapted to run

within self-consistently calculated global fields so that their

responses to dynamical solar wind events can be properly

assessed.

6. COMPARATIVE PLANETARY ABLATION

[93] It is interesting to compare the situation at Earth with

our best knowledge of other planets. Within the solar

system, the gas giant planets with large magnetospheres

are so massive and cold that the loss rates of their atmos-

pheres are negligible, even though hydrogen dominates

them, accounting for the tremendous thickness of their

atmospheres compared with those of the inner terrestrial

planets. Replete with moons of varying sizes and composi-

tions, the outer planet magnetospheres tend to be dominated

by gas and heavy ion plasmas escaping from those satellites

rather than hydrogen escaping from the planets themselves

[Belcher, 1983; Belcher et al., 1990]. The focus of interest

in ablation is then upon the atmospheres of the moons.

These are generally well shielded from direct solar wind

impact by the large magnetospheres of these planets. Still,

substantial amounts of energy derived either from the solar

wind or the rotation of these large magnetospheric systems

is clearly present, and it generates robust populations of

energetic particles comparable to or far exceeding the

intensity of the Van Allen radiation belts at Earth [Bolton

et al., 2004; Jun and Garrett, 2005]. The moons are

themselves highly diverse, with or without their own

magnetospheres. As this is written, a handful of flyby

missions have briefly explored the gas giant magnetospheric

systems in the past, and the Saturnian system is now being

explored by the orbiting Cassini Mission. New results are

appearing rapidly, so the detailed exploration of gas giants

and their satellite systems is therefore beyond the scope of

this review.

[94] Among the terrestrial planets, only Earth has a

planetary magnetic field strong enough to hold the solar

wind off at about 10 times the planetary radius for typical

solar wind conditions. Earth is also unique in having

acquired and retained substantial amounts water in all

forms. Mercury has a relatively weak magnetic field that

holds the solar wind off the planet for average conditions,

but stronger solar winds are directly incident on the planet,

beginning in the magnetic cusps and spreading with

increasing solar wind dynamic pressure [Delcourt et al.,

2002]. Mars has scattered regions of remnant crustal mag-

netization [Connerney et al., 1999], which will produce

features in the solar wind interaction, but do not hold the

solar wind off Mars’ ionosphere. Venus has no measurable

magnetodynamo so the solar wind is continuously incident

upon the ionosphere [Luhmann, 1991].

Figure 13. Simulated supply of O+ to the CPS in the region 30 RE < x < 5 RE, jyj < 10 RE during a stepchange in the convection field. The electric field changes at time t = 0. Supply is cut off by a northwardturning, and enhanced with a delay for southward turning [Cully et al., 2003b].


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[95] Mars and Venus are both are believed to have

become deficient in water relative to their earlier states.

Their solar wind interactions have been observed extensively

[Lundin and Dubinen, 1992; Brace and Kliore, 1991] and

are known to involve considerable rates of loss of water

products into space. The Hermean solar wind interaction

has not been observed since Mariner 10 [Glassmeier, 1997],

but is about to be revisited by the NASA Messenger

Mission. Mercury is believed to lack any significant water

or water product escape and has an exosphere in which

sodium is prominent [Killen and Ip, 1999].

[96] It is tempting to hypothesize causality in the mag-

netized Earth’s retention of water, but we should first

consider what we have learned about the expansion of

Earth’s ionosphere into space. Water loss begins with

photodissociation of water in the upper atmosphere. Photo-

thermal expansion of water products into space is oxidizing

for larger planets that trap heavier species. Smaller planets

like Mars may be an exception. Solar wind ablation tends to

balance the loss of light species, or to be reducing (when

oxygen loss exceeds light species loss). Jeans escape and

photothermal escape via the creation of an ionosphere are

common to the inner planets. Mars and Mercury are smaller

and can lose heavy species such as O+ photothermally.

Venus (and Earth) loses only lighter species to any signif-

icant degree owing to photothermal effects, but solar wind

ablation may be active at Venus, though different in

character from Earth [Fok et al., 2004].

[97] The solar wind is directly incident upon the iono-

spheres of the unmagnetized planets, Mars and Venus,

exposing dayside ionospheres to an energy flux on the

order of 0.3 mW m�2. A number of processes are active

in these cases, including charge exchange between atmo-

spheric atoms and solar wind ions, impact ionization of

atmospheric atoms by solar wind electrons, and some high-

altitude photoionization [Breus et al., 1991; Brecht et al.,

1993; Kallio et al., 1997, 2006]. Some atoms and ions are

sputtered to escape energy by direct collisions with solar

wind ions, and ions are picked up by the interplanetary

electric field. All of these processes contribute to the

contamination of the planetary magnetosheath by planetary

material that is lost from the planet.

[98] Similar fluxes (scaled for distance from the Sun) are

incident upon the regions of the ionosphere conjugate with

the magnetospheric cusps for the magnetized Mercury and

Earth. On the other hand, an appreciable fraction of the solar

wind energy incident upon the entire magnetospheric cross

section (15–20 RE radius) is dissipated in the terrestrial

auroral zones, with energy fluxes at the ionosphere ranging

from 3 to 100 mW m�2. The lower end represents a typical

value for average conditions, while the upper end represents

very active solar wind and aurora. Thus the energy flux

available for heating and ablation of ionospheric plasmas is

10 to 300 times larger in the localized auroral zones than it

would be if the solar wind were simply incident upon an

unmagnetized Earth, as it might possibly be during a

geomagnetic field reversal.

[99] Given that heavy ion expansion out of the gravita-

tional well of the planet is dependent upon auroral levels of

energy flux, it might seem that an unmagnetized Earth

would lose only lighter species such as H and He, while

the magnetized Earth loses substantial quantities of N, O,

and the diatomic molecules of those species. However,

studies of Mars and Venus [Cravens, 1991; Luhmann and

Kozyra, 1991; Fok et al., 2004] (the latter based on the

MHD simulation of Tanaka and Murawski [1997] with

embedded aeronomy models) indicate that they suffer

nearly as much total heavy ion and heavy atom loss as

the Earth does, and more steadily over time. The interplan-

etary magnetic field diffuses into the dayside ionosphere of

Venus and couples energy from the solar wind, even though

there is no planetary magnetic field. Thus it appears that the

solar wind finds ways to carry off as much of a planetary

atmosphere as is possible (given its momentum flux),

independent of planetary magnetization, though this needs

much more investigation.

7. EXTRASOLAR PLANETS

[100] So far, more than 160 extrasolar planets have been

identified (J. Schneider, Interactive Extra-Solar Planets

Catalog, http://www.obspm.fr/planets, retrieved on 4November

2005), and almost all of them have masses comparable to

(within a factor of 5) or larger than Jupiter. About 20% of

the planets are classified as close-in extrasolar giant planets

(CEGP) (or ‘‘Hot Jupiters’’), as they orbit their center stars

within a distance of 0.10 AU or less, noting that the smallest

star-planet separation distance identified to date is 0.021 AU

[Rivera et al., 2005], equivalent to 4.5 solar radii. Magnetic

interaction between CEGPs and their host stars was first

postulated by Cuntz et al. [2000] and later confirmed by

Shkolnik et al. [2003] for HD 179949 through the detection

of temporal variations of Ca II H, K line emission in the

stellar plasma, which were found to be synchronized with

the planetary orbit rather than the stellar rotation.

[101] Direct observations of planetary atmospheric plasma

flows, revealed through hydrogen Lyman-a [Vidal-Madjar

et al., 2003], oxygen and carbon [Vidal-Madjar et al.,

2004], and sodium detections [Charbonneau et al., 2002]

have been obtained for the planet of HD 209458, which

constitutes the ‘‘best studied to date’’ extrasolar star-planet

system. Vidal-Madjar et al. [2003] concluded that the

Lyman-a absorption feature is formed outside the planetary

Roche limit, thus constituting evidence of escaping planetary

hydrogen plasma. As a way of visualizing the planet in

question, these authors developed an artist’s conception,

which is shown in Figure 14, accompanied by their principle

result plot.

[102] A further possible consequence of planetary plasma

flows and evaporation is the existence of ‘‘close-in hot-

Neptunes,’’ which have been found around GJ 436, r Cancri,and m Ara and are believed to be caused by extreme stellar-

induced mass loss [Baraffe et al., 2004, 2005] of formerly

intact Jupiter-mass planets. This interpretation is strongly

supported by planetary evolution studies, which in particu-


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lar take into account effects of stellar irradiation. An even

more extreme outcome of this type of process is suggested

by observational evidence that CEGPs can even be engulfed

by their host stars. Israelian et al. [2001] reported the

presence of the rare 6Li isotope in the stellar atmosphere

of HD 82943. This isotope is normally destroyed in the

early evolution of solar-type stars, but is preserved intact in

the atmospheres of giant planets. In fact, the 6Li isotope has

hitherto not reliably been observed in any metal-rich star

besides HD 82943. The authors interpret this finding as

evidence for a planet (or planets) previously engulfed by the

parent star.

[103] While most of the extrasolar planets discovered to

date are in circumstances that are far more extreme than

those of the Earth in our own solar system, they serve as

reminders of the range of possible conditions over which we

would like our theories to be applicable. As observing

capabilities are refined, future discoveries are likely to

reveal planets that are much more similar to the terrestrial

planets.

8. FUTURE MEASUREMENT REQUIREMENTS

[104] This section deals with the requirements for future

progress in the observation of ionospheric outflows as

driven by stellar wind energy dissipation. Some of the areas

of theory and simulation of ionospheric outflows that appear

to be ready for near-term rapid advances include the

following.

8.1. Removing Low-Energy Plasma Obstacles

[105] Recent observations have largely overcome the

problems of observing cold plasmas from spacecraft in

sunlight, owing to photoelectric charging. Another problem

has been the prevalent assumption that cold plasma densi-

ties were so low that they were negligible, which made it

difficult to justify the expenditure of resources to measure

them. These problems delayed a beginning survey of

the polar wind circulation throughout the magnetosphere

from the first theoretical predictions in the 1960s until

the Dynamics Explorer-1 (DE-1) mission in the 1980s

[Chappell, 1988]. Akebono, Polar, and Cluster have estab-

lished the technology [Moore and Delcourt, 1995; Comfort

et al., 1998; Singh et al., 2001; Torkar et al., 2001] to

actively control spacecraft floating potential, leading to

successful tracing of polar and auroral winds from the

ionosphere to the magnetotail [Moore et al., 1997; Su et

al., 1998b; Liemohn et al., 2005; Huddleston et al., 2005].

On the Cluster mission, Active Spacecraft Potential Control

(ASPOC) is an ion emitter that effectively neutralizes posi-

tive spacecraft charging in sunlight. By avoiding emission

of electrons, it creates a benign amount of plasma wave

Figure 14. (left) Total intensity of lines as function of the orbital phase for extrasolar planet HD209458. Circles, squares, triangles, and diamonds are for first to fourth transits, respectively. Verticaldotted lines correspond to the position of the first to fourth contacts of the planetary disk [Vidal-Madjar etal., 2004] (reproduced by permission of the AAS). (right) Artist’s conception of the extrasolar planet ofHD 209458.


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noise, and appears to improve the accuracy of double probe

electric field measurements [Eriksson et al., 2006] in low-

density polar wind regions. This technology is included in

plans for the Magnetospheric MultiScale Mission currently

entering development.

8.2. Observing the Heating Mechanisms

[106] Sounding rockets offer direct low velocity probing

of topside ionospheric heating processes, albeit with limited

scope and duration. They have contributed a great deal to

our knowledge and should certainly continue with new

strategies and observing technologies. The Polar Outflow

Probe (POP) [Yau et al., 2002] spacecraft is being devel-

oped for the Canadian Space Agency by the University of

Calgary Institute for Space Research, and will explore

ionospheric outflow origins in the altitude range from

300 to 1500 km, where energy goes into creating mass flux

(and where photoelectric charging is tolerable). The science

objectives of POP are to quantify the microscale character-

istics of plasma outflow in the polar ionosphere and probe-

related microscale and mesoscale plasma processes at

unprecedented resolution, and explore the occurrence

morphology of neutral escape in the upper atmosphere.

This type of mission is much needed to better understand

the fundamental causes of ionospheric outflows so they can

be predicted on the basis of their causal physics rather than

through empirical scaling.

8.3. Determining Exospheric Structure and Variability

[107] Recently, owing in large part to the IMAGE mission

[Burch, 2005], it has been possible to better observe and

understand the ionospheric topside, geocoronal, and plas-

maspheric media that supply gas and plasma to magneto-

spheric reservoirs encompassing the plasmaspheric plumes,

polar lobes, plasma sheet, and ring current regions. Both the

gas and plasma of the ionosphere expand into the magne-

tosphere [Gardner and Schunk, 2004, 2005], impeded

mainly by gravity. The gas creates a relatively steady, nearly

spherical geocorona of light hydrogen gas with a scale of a

few Earth radii. The denser gas species form a smaller

geocorona or exosphere with a height scale that about

100 km at normal F-layer heights but increasing with

altitude above that in a highly variable way that depends

on the amount of energy deposition into the auroral iono-

sphere. The actual configuration and response of the oxygen

exosphere has been very poorly known, but considerable

information has recently been obtained by the IMAGE

mission and especially the LENA imager [Moore et al.,

2003; Wilson et al., 2003; Shematovich et al., 2005].

8.4. Resolving Dynamic Escape Processes

[108] The heavy ion plasmas are normally confined to the

F layer proper, similar to the heavy gas atoms. Exceptions

are important in the auroral ionosphere, where oxygen flows

locally in an auroral wind with fluxes greatly exceeding

those of the steadier and more widespread polar wind [Cully

et al., 2003a, 2003b; Strangeway et al., 2005]. The time

variations of auroral wind have been poorly known until

recently, when considerable information has been obtained

on the drivers of such outflows by the IMAGE/LENA

imager [Wilson and Moore, 2005; Fuselier et al., 2006].

Plasmas in the magnetosphere are heated strongly by

geospace storm events such that they substantially inflate

the inner magnetosphere, producing a ring current that adds

to the Earth’s dipole moment [Moore et al., 2001; Seki et al.,

2001; Cully et al., 2003a, 2003b]. Recently, it has been

found that this ring current is overwhelmingly composed of

O+ from the Earth during the largest geospace storm events.

This surprising result grew out of a collaboration between

the Geotail investigators and the IMAGE LENA Imager

[Nose et al., 2005; Moore et al., 2007].

8.5. Imaging Missions

[109] The IMAGE mission, at nearly 6 years of age and

long past its design lifetime of 2.5 years, unfortunately

failed a few days before this paper was submitted to

Reviews of Geophysics. After pioneering many aspects of

geospace imaging, including those based on energetic

neutral atoms from the ring current, the Polar Mission is

scheduled to terminate its full solar cycle of operations in

the next month or two. Owing in large part to the transfor-

mation of NASA to revive human space flight, no geospace

imaging missions are currently in development or have firm

plans for the future, though several are ‘‘in the wings’’ and

waiting for a suitable opportunity. There is a pressing

need for future magnetospheric and auroral imaging to

better understand the terrestrial atmospheric contribution

to geospace storms. Imaging has proven so fruitful in

understanding how our planetary atmosphere is disturbed

and ablated by solar wind energy inputs that it has become

an essential feature of the toolbin of both space meteorol-

ogists and space weather forecasters.

8.6. Extending Observations Beyond the Geosphere

[110] An important priority for the space exploration

initiative will be the exploration of planetary atmosphere

ablation by the solar wind, which is thought to have been an

important factor in the evolution of the Martian atmosphere

[Lundin et al., 2004; Vaisberg et al., 1995, 2005]. As noted

above, Venus is thought to lose atmosphere at a rate

comparable to that of the Earth, but this is based largely

on the Pioneer Venus Orbiter, which did not have instru-

mentation designed to observe such loss. Moreover, this

comparison needs an update for recent observations of the

Earth’s ablation rate. A number of strategic studies have

placed high priority on aeronomy probes for Mars and

Venus, but the current fascination appears to be with

Mercury, which has a magnetosphere but very little in the

way of an atmosphere. In the past, planetary exploration

missions have understandably focused on the condensed

parts of the planets. Two current missions, the ESA Mars

Express and Venus Express, are now making strides toward

a better understanding of how those planets compare with

the Earth in terms of solar wind coupling. In the future,

missions may devote more resources to the study of effects

that could have a long-term evolutionary impact on those


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planets, now that those effects are better appreciated as a

result of closer study at Earth.

8.7. Extending Observations Beyond the Heliosphere

[111] There is no foreseeable opportunity for in situ

measurements of an extrasolar planet, but the planets being

remotely sensed are slowly becoming more Earth-like as

technology advances and smaller planets become more

detectable. It is hoped that the synergy between extrasolar

planetary studies and in situ measurements of our own

planet and its nearby neighbors will be fully exploited by

researchers. Though the techniques are vastly different at

present, they clearly have the capability to inform each

other.

9. FUTURE THEORY REQUIREMENTS

[112] Theory and modeling progress in ionospheric out-

flows, as with other phenomena, is in large part driven and

inspired by new observational findings, such as those

projected in the previous section. For example, advances

in spacecraft potential control may enable new comprehen-

sive findings on the characteristics and behavior of <2 eV

ions in regions such as the high-altitude polar cap, tail lobes,

and plasma sheet. This will inspire and suggest new

directions of theory and simulation to understand and

replicate those observational discoveries. Some of the areas

of theory and simulation of ionospheric outflows that appear

to be ready for near-term rapid advances include the

following.

[113] 1. Auroral field-aligned plasma transport can be

better simulated by incorporating ionospheric plasma and

energy production and loss and auroral and kinetic processes

[e.g., Wu et al., 1999, 2002; Tu et al., 2004, 2005]. This

would be done by implementing more sophisticated, self-

consistent treatments of auroral processes involving fine-

structured parallel electric fields, for example, kinetic

Alfven waves and solitary waves [e.g., Ergun et al., 1998;

Singh, 2002] and waves which cause transverse heating,

including ion cyclotron and lower hybrid waves [e.g., Andre

and Yau, 1997].

[114] 2. Molecular ion outflows in the auroral regions

[e.g., Peterson et al., 1994; Wilson and Craven, 1999] have

thus far not been systematically treated with transport

models, but can readily be added to multifluid codes.

[115] 3. Plasmaspheric drainage plumes, involving sub-

stantial ionospheric F-region plasma densities, are observed

convecting toward the cleft region and across the polar caps

following large magnetic storms [e.g., Foster et al., 2002].

These should be incorporated into ionospheric plasma

transport models of the cleft ion fountain and global

magnetosphere. Foster [2005] has suggested that such large

F-region densities in these drainage plumes could be

sources of plasma content, in particular heavy ions, for

the cleft ion fountain at these times.

[116] 4. Improved ionospheric plasma transport models

and processes can be incorporated as ionospheric outflow

plasma sources in global magnetospheric circulation and

dynamics models. At the present time, the global magneto-

spheric dynamics model of Winglee et al. [2002] has

pioneered the incorporation of ionospheric plasma sources,

but more and more researchers are reporting efforts to move

in a similar direction. There is room for improvement in the

manner in which ionospheric plasma outflows occur in such

models. As a first step, it would be possible to use empirical

specifications of ionospheric response to magnetospheric

inputs [Strangeway et al., 2005; Zheng et al., 2005], as

reported by Moore et al. [2007] and Fok et al. [2006].

Ultimately, we will need an embedded model of the

ionosphere and thermosphere, with all appropriate physics

included, within a global dynamical simulation.

[117] 5. The inner magnetosphere is centrally important to

terrestrial space weather. It is common for global simulation

codes to omit the inner magnetosphere from the simulation

space, setting an inner boundary at a radius of 3–4 RE. This

is mainly for reasons of resolution, though curvilinear and

adaptive calculation grids have largely rendered that reason

obsolete, and indeed, one of the codes now extends down

all the way through the ionosphere proper [Maynard et al.,

2001], albeit with a single fluid that can originate in either

the ionosphere or the solar wind. While this cannot readily

distinguish the separate roles of plasma from the iono-

sphere and solar wind, it allows for realistic momentum

and energy flows, which may ultimately be the most

important consideration.

10. CONCLUSIONS AND OUTSTANDINGPROBLEMS

[118] In this review we have focused upon observations

of the expansion of our ionized upper atmosphere into

geospace, and the theories that have been developed to

explain those observations as well as their consequences.

We assert that observations and theory have reached suffi-

cient convergence to form a strong basis for predicting the

behavior of similar systems under different conditions and

in different contexts. In particular, the knowledge we have

reviewed can be used to anticipate and predict the behavior

of the diverse other planets throughout our own solar

system, and for conditions around other stars, to the degree

that their stellar winds and intrinsic magnetic fields and

atmospheres have observed or inferred properties. This

includes, for example, the practical exercise of anticipating

terrestrial space weather changes during the next geomag-

netic field reversal, which may well be underway in view of

recent trends in the internal field of Earth.

[119] Thus this review should be understood in the

general context of coupling between a stellar atmosphere

and the volatile matter embedded within it as part of

a planetary system. Prior to stellar ignition, the overall

situation is dominated by gravitational accretion with rota-

tional dynamics reflecting the initial angular momentum of

the system. Once stellar ignition occurs, however, the

emanated UV energy flux in general ionizes these atmos-

pheres into fully or partially ionized plasmas, with higher

charge states and complete stripping of electrons from


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atoms emitted from the star itself. Energetic photon fluxes

then extend throughout the planetary system and ionize

much of the diffuse gas surrounding the more condensed

objects. Once the system contains a substantial amount

of plasma, electromagnetic effects and magneto-plasma

dynamic forces become important in coupling the entire

system together, and we expect to see telltale signs of

nonequilibrium activity, including reconnection, plasma

heating, and superthermal particle acceleration, together

with neutral atom and photon emission.

[120] As an aid to visualizing the global circulation of the

various plasmas in the geosphere, we offer Figure 15 as a

schematic diagram or plasma flowchart. The flows begin

with the upstream solar wind and the creation of an

ionosphere and light ion plasmasphere by solar photons.

The solar wind boundary layers carry the bulk of solar wind

around the geosphere, but entry occurs in the cusp/cleft and

low-latitude boundary layer regions. The high- and low-

latitude features are divided top to bottom in this schematic

to fit the three-dimensional structure on the page without

adding unnecessary complexity. The overall circulation is

controlled by IMF orientation via the distribution and effect

of dayside magnetic reconnection. Northward IMF Bz

interrupts the normal flow of boundary layer flow away

from the subsolar region, interrupting and diverting the

high-latitude flow cell to the low-latitude boundary layers,

closing open flux from the polar lobes and shrinking the

polar caps, while also thickening and drawing plasma from

the plasma sheet, as indicated by arrow labels. This also

interrupts the supply of the plasma sheet from the high-

latitude or auroral wind circulation, switching the source to

the low-latitude circulation cells, which are fed by solar

wind plasmas. Southward IMF has the reverse effect of

strengthening the high-latitude flow cells in comparison

with the low-latitude cells.

[121] Over the past 2 decades, considerable progress has

been made in the study of atmospheric ablation by helio-

spheric influences, leading to the following conclusions.

[122] 1. Both the solar wind and polar wind contribute

light ion plasmas to the magnetospheric coupling region.

During typical conditions (average solar wind properties;

spiral IMF with small Bz) a substantial contribution of

ionospheric plasma with significant heavy ion content

(mainly O+ but also N+ and molecular species during high

activity) is supplied via the auroral wind, to a degree that

varies in response to energy dissipation as a strong function

of solar wind dynamic pressure.

[123] 2. For events with strong northward IMF, the

auroral wind is reduced and circulated into the low-latitude

boundary layers and downstream rather than over the poles

into the magnetotail.

[124] 3. For events with strong southward IMF, the

eroded plasmasphere (supplied by polar wind) supplies

significant amounts of plasma to the dayside afternoon

magnetopause region, resulting in an interaction with

magnetospheric global circulation as driven by reconnection.

[125] 4. Substorm effects in the near-Earth midnight

sector significantly energize and enhance the auroral wind

plasmas that return to the inner magnetosphere via the

midnight plasma sheet. The solar wind and polar wind

proton plasmas are less influenced by substorm effects.

[126] 5. The storm time ring current becomes increasingly

auroral wind dominated with increasing ring current energy

density, implying that these enhancements of the quiet time

ring current are supplied with charge carriers largely

through atmospheric ablation by solar wind energy. Essen-

Figure 15. A schematic plasma flowchart of the heliosphere-geosphere interaction. Flow color codingruns from dotted line (energy) to solid line (mass), with dashed line for both (mass and energy). Flowdependences on IMF are indicated by labels above the arrows.


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tially, the storm time ring current is produced by the

pressure of heated ionospheric plasmas seeking to escape

from magnetospheric confinement.

[127] 6. The expanding ionosphere is lost into the down-

stream solar wind in both the dayside and nightside recon-

nection regions, where closed magnetospheric flux tubes

are first opened up to the solar wind, stretched into the

tail with acceleration of the plasmas residing in them, and

then reconnected after substantial plasma expansion has

occurred, beyond the nightside reconnection region.

[128] 7. Ionospheric plasmas that are retained in the inner

magnetosphere are ultimately lost, mostly by escaping

outward after being freed by charge exchange.

[129] Among the outstanding problems stemming

from the expansion of ionospheric plasma into space,

the following pose the highest priority requirements for

improved observations and theory: (1) continuing aware-

ness of the importance of relatively cold plasma populations

in the magnetosphere, as reflected in attention to spacecraft

floating potential measurement and control; (2) a global

theoretical and/or empirical model of ionospheric topside

response to solar wind electromagnetic energy and particle

precipitation inputs, including auroral potential structures

associated with circulation shear and field aligned currents;

(3) theory and observations of the dynamic role of

the extended ionosphere in mass loading, wave propagation,

and particle acceleration within the magnetosphere;

(4) theory and observations of the extended ionospheric

pressure that inflates the storm time magnetosphere and

escapes during the largest storms; and (5) theory and

observations of the role of extended ionospheric density in

moderating plasma and particle acceleration, and enhancing

particle losses from the magnetosphere, through both clas-

sical processes and plasma effects such as wave-particle

interactions.

[130] Beginning with the surprising discovery of O+ ions

in the magnetosphere [Shelley et al., 1972], the Solar-

Terrestrial Physics community has undergone a paradigm

shift that transformed our understanding of the storm time

plasmas of the magnetosphere. Once thought of as the result

of entry of the intense solar wind, storm-enhanced plasmas

are now seen to result from the dissipation of solar wind

energy in the terrestrial atmosphere and ionosphere, with

resultant expansion. The key to this understanding began

with ideas about ionospheric expansion in the polar wind

[Banks and Holzer, 1969], but early suggestions that the

magnetosphere was filled with hot plasmas accelerated from

the polar wind were found wanting [Hill, 1974]. With the

understanding that a substantial auroral wind blows as a

result of solar wind energy inputs, a spirited debate began

between those who advocated [Chappell et al., 1987]

and those who questioned [Ashour-Abdalla et al., 1992;

Borovsky et al., 1998] a substantial contribution of iono-

spheric material to the hot magnetospheric plasmas. With

more and more observations both of large auroral wind

fluxes and substantial pressures of auroral wind O+ in the

ring current, it gradually became clear that global circulation

models would need further development, since the iono-

spheric load is not confined to a thin boundary layer.

Accurate description of a magnetosphere in which the

ionosphere extends throughout the system will require

models with ionospheric fluids or particles as active dynamic

components.

[131] We have emerged from this transformation with

ample evidence and community acceptance that the iono-

sphere expands to the magnetospheric boundaries and

escapes continually into the downstream solar wind, its

composition and partial pressure varying with solar wind

drivers. Updated ionospheric models now produce the

observed heavy ion outflows from solar wind energy inputs.

We also have promising new or revised global circulation

models that incorporate the ionosphere as an extended load

within the system, and we are learning that this load can be

felt all the way out to the boundary layer reconnection

regions. There is much additional work to be done to work

out the consequences of this shift in our understanding, but

we are now well equipped for this work and also well

equipped to use our new simulation tools to predict and

understand the behavior of diverse other planetary systems

within and beyond our solar system.

[132] ACKNOWLEDGMENTS. We are indebted to Manfred

Cuntz of the University of Texas at Arlington for valuable inputs to

the section on extrasolar atmospheric ablation. This work was

supported by Polar/TIDE under UPN 370-08-43, IMAGE/LENA

under UPN 370-28-20, and by NASAROSES under UPN 432-01-34

at Goddard Space Flight Center, by NASA grant NNG05GF67G,

and NSF grant ATM-0505918 to the University of Texas at

Arlington.

[133] The Editor responsible for this paper was Peter Riley. He

wants to thanks two anonymous technical reviewer and one

anonymous cross-disciplinary reviewer.

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��J. L. Horwitz, Department of Physics, University of Texas at

Arlington, 502 Yates Street, Box 19059, Arlington, TX 76019, USA.(horwitz@uta. edu)T. E. Moore, Heliospheric Physics Branch, Code 612.2, NASA

Goddard Space Flight Center, Greenbelt, MD 20771, USA. ([email protected])


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