M. Galand (1), I.C.F. Müller-Wodarg (1), L. Moore (2), M. Mendillo (2), S. Miller (3) , L.C. Ray (1)

(1) Department of Physics, Imperial College London, London, U.K. (2) Center for Space Physics, Boston University, Boston, MA, USA(3) Department of Physics and Astronomy, University College London, U.K.

Thermosphere - Ionosphere - Magnetosphere Coupling

1. Energy crisis at giant planets2. TIM coupling3. Modeling of IT system4. Comparison with observations5. Outstanding questions

(Gladstone et al., 2007) Cassini/UVIS

[UVIS team]

JUPITER

Credit: J. Clarke (BU), NASA

Credit: NASA/JPL/Space Science Institute

Cassini/ISS (false color)

SATURN

Canada

Cassini/VIMS (IR)[VIMS team/JPL, NASA, ESA]

Cassini/UVIS(Pryor et al., 2011)

1. SETTING THE SCENE: THE ENERGY CRISIS AT THE GIANT PLANETS

Exosphere

Thermosphere

Mesosphere

Stratosphere

85 km

50 km

~ 15 km

500 km

Troposphere

TexoTHERMAL PROFILE (EARTH)

Ionosphere

Key transition region between the space environment and the lower atmosphere

ion, e-

ion, e-

Solar photons

Neutral

Suprathermalelectrons

SOLAR ENERGY DEPOSITION IN THE UPPER ATMOSPHERE

Thermal e-

B

Ionospherice- heating

+

Ther

mos

pher

e

Ne, NionSP, SH

TeAirglow

Neutral atmospheric heating

*

Exothermic reactions

[after Mendillo et al., 2002]

Outer planetsEarth

CO2 atmospheres

W

Exos

pher

ic te

mpe

ratu

re (K

) Main energy source:UV solar radiation Main energy source?

IS THE SUN THE MAIN ENERGY SOURCE OF PLANERATY THERMOSPHERES?

[After Yelle and Miller, 2004; Melin et al., 2011 (+poster)]

Observed values at low to mid-latitudes

Modeled values (Sun only)

ENERGY CRISIS AT THE GIANT PLANETS

solstice

equinox

ENERGY BUDGET OF THE THERMOSPHERE

HEATING SOURCES

Solar heating through excitation/dissociation/ionization + exothermic chemical reactions

Auroral particle heating via collisions + chemistry [Grodent et al., 2001]

“Ionospheric Joule heating” via auroral electrical currents and ion-drag heating [Vasyliũnas and Song, 2005]

Dissipation of upward, propagating waves (such as gravity waves, …)

- Solar EUV/FUV heating*: 0.5 TW (Earth), 0.8 TW (Jupiter), 0.2 TW (Saturn)- Auroral part./Joule heating*: 0.08 TW (Earth), 100 TW (Jupiter), 5-10 TW (Saturn)

[*: Strobel, 2002]

2. THERMOSPHERE-IONOSPHERE-MAGNETOSPHERECOUPLING

MAGNETOSPHERE-IONOSPHERE-THERMOSPHERE COUPLING

Examples of ITM coupling:• Angular momentum transfer• Ion outflow, particle precipitation

MAGNETOSPHERE

Exchange ofparticles, momen-tum & energy

AURORALTHERMOSPHERE

IONOSPHERE

• Overall, at high latitudes:the magnetosphere extracts angular momentum from the upper atmosphere through the magnetic field-aligned currents [e.g., Hill, 1979]

The magnetosphere “swims” on the ionosphere.

MAGNETOSPHERE-IONOSPHERE-THERMOSPHERE COUPLING

MAGNETOSPHERE

IONOSPHERE

MAGNETOSPHERE-IONOSPHERE-THERMOSPHERE COUPLING

Examples of ITM coupling:• Angular momentum transfer• Ion outflow, particle precipitation

MAGNETOSPHERE

Exchange ofparticles, momen-tum & energy

AURORALTHERMOSPHERE

IONOSPHERE

S

Pedersen

current

Field-alignedcurrent

Fiel

d-al

igne

dcu

rren

t

AURORAL THERMOSPHERE

TO MAGNETOSPHERE

IONOSPHERE

Ionospheric Joule heating(resistive + frictional heating)[Vasyliũnas and Song, 2005]

TIM coupling through current system

FROM MAGNETOSPHERE

QJ = J . Ei

MAGNETOSPHERE-IONOSPHERE-THERMOSPHERE COUPLING

Energy redistribution towards lower

latitudes?

MAGNETOSPHERE

ATMOS

STIM

Strongercooling

Local time averaged

Ion drag fridge mechanism [Smith et al., 2007; Smith and Aylward, 2009]

wind direction

Polar sub-corotation due to auroral forcing (westward ion velocities due to ambient E fields) drives equator-to-pole circulation

[Mueller-Wodarg et al., 2011]

Strongerheating

EXOSPHERICTEMPERATURE

Does the ion drag fridge mechanism rule out auroral energy in solving the global energy crisis at Giant Planets?

3. MODELING THE THERMOSPHERE/IONOSPHERE SYSTEM

Electron and ion densities & temperaturesElectrical conductances

Beer-Lambert Lawapplied to the solar flux

Ionospheric densities (Ne, Ni) , drifts, &

temperatures (Te, Ti)[Moore et al., JGR, 2008]

Boltzmann Equationapplied to suprathermal

electrons

3D Thermosphere-Ionosphere Model

Thermospheric densities (Nn), winds, & temp.

[Müller-Wodarg et al., Icarus, 2006]

1D Energy Deposition Model[Galand et al., JGR, 2009, 2011]

Nn

Ne, TePe, Pi, Qe

Pe, Pi

Incident AuroralElectron

Distribution

Solar Flux

Flexible model which allows us to explore the parameter space and assess the effect of it on ionospheric/thermospheric quantities, such as Ne, S, Tn.

Neutral temperatures

COUPLED FLUID/KINETIC STIM MODEL

* Full ion-neutral dynamical coupling

*

Electric field

with Dt = 25 min

STIM RESULT 1: Ionospheric conductances in auroral regions

-Pedersen conductivities peak at the homopause conductances peak near 2.5 keV- At low energies, conductances are driven by the solar source

Q0 = 0.2 mW m-2

SOLAR VALUES

12 LT

[Galand et al., 2011]

€

Q0(t − Δt)

€

SP

Ped

erse

n co

nduc

tanc

e (m

ho)

with Dt = 25 min

STIM RESULT 1: Ionospheric conductances in auroral regions

-Pedersen conductivities peak at the homopause conductances peak near 2.5 keV- At low energies, conductances are driven by the solar source

Q0 = 0.2 mW m-2

12 LT

[Galand et al., 2011]

€

Q0(t − Δt)

€

SP

Ped

erse

n co

nduc

tanc

e (m

ho)

Sun only12 LT

78°S LAT

EquinoxSolar min

EquinoxSolar max

SummerSolar min

Pedersen conductances

0.7 mho 1.6 mho 2.6 mho

Auroral electron mean energy &

energy flux

Earth[Fuller-Rowell and

Evans, 1987]

Saturn[Galand et al.,

2011]

Jupiter[Millward et al.,

2002]

10 keV1 mW m-2

SP = 4-6 mho

SP = 11-12 mho

SP = 0.1-0.2 mho

Composition Altitude range

SP=max(SP)/10 over 70 km (E) and 500 km (S)

Strength of B fieldB field 20 times

stronger at J cp w/ S

Slippage parameter(1) for Jupiter(2) & Saturn(3):

€

k =Ω −ωn

Ω −ω i

= ~ 0.5

(1) Bunce et al. [2003]; (2) Cowley et al. [2004]; (3) Galand et al. [2011]

STIM RESULT 1: Ionospheric Conductances in auroral regions

• Charge exchange reaction H+ + H2(v≥4) H2+ + H(1)

controls the abundance of H3+ as it is quickly followed by:

H2+ + H2 H3

+ + H

• Reaction rate k1* = k1 [H2(v≥4)]/[H2]

– Low k1* means less charge exchange reaction and increase in ionospheric densities

k1 = 10-9 cm3 s-1 [Huestis, 2008]

At low- and mid-latitudes: Moore et al. (2010) found best match between model and Cassini RSS data for a reduction of ([H2(v≥4)]/[H2]) from Moses and Bass [2000]

In the auroral regions, expected to be larger: Galand et al. (2011) assumed 2 x ([H2(v≥4)]/[H2]) from Moses and Bass [2000]

• How does this affect thermospheric circulation?

STIM RESULT 2: sensitivity to vibrationally excited H2 rate

Local time averaged[Mueller-Wodarg et al., 2011]

3 cells

STIM RESULT 2: sensitivity to vibrationally excited H2 rate

wind direction

EXOSPHERICTEMPERATURE

4. OBSERVATIONS OF THE THERMOSPHERE/IONOSPHERE SYSTEM

(1) STIM, 3 cells(2) STIM, 1 cell(3) Voyager UVS(Vervack and Moses, 2011)(4) Voyager UVS (Smith et al., 1983)(5) Cassini UVIS (Nagy et al., 2009)(6) UKIRT (Melin et al., 2007)

3

4

5

1

26

Implication for exospheric temperatures

[Mueller-Wodarg et al., 2011]

+

Type of observat.

Radio occultations

H3+ IR

emissionsUV

emissionsRadio

emissions

Physical quantities

Electron density

Effective H3+

column density, temp, and velocity

vector

Auroral e- energy flux & energy

(generating aurora)

Auroral e- energy flux and energy

(within accele region)

Which observations can help constrained the problem?

Sample of relevant observations: Earth-based + space missions

UV occultations In situ measurements(particle, fields)

Atmosph. densities,Texospheric

Down/upgoing auroral particles (if conditions allows),

Magnetic field strength/direction, Electric

currents

Combine as many as possible to better constrain the problem [e.g., Melin, talk]

JUNO over the polar regions

Credit: Juno Team

Credit: Juno Team

JUNO observations through the magnetic field lines connected to the auroral ionosphere, close/within the acceleration region (expected to be 2-3 RJ from center [Ray et al., 2009]):- Electric currents along magnetic field lines- Plasma/radio waves revealing processing responsible for particle acceleration [see Hess, tutorial]- Energetic particles precipitating into atmosphere creating aurora- Ultraviolet/IR auroral emissions regarding the morphology of the aurora

• Can the energy crisis be solved via auroral forcing alone as proposed here?– Is the mechanism proposed efficient at Jupiter, Uranus (seasonal

asymmetry [Melin et al., 2011 + poster]), and Neptune?– At Saturn, beside the solar contribution which is dominant [Moore et al.,

2010], are they additional energy sources at low- and mid-latitudes?[e.g., break-down in co-rotation of the ions in the ionosphere [Stallard et al., 2010; Tao, poster; Ray, talk], molecular neutral torus of Saturn through charge-exchange (ENA) [e.g., Jurak & Johnson, 2001], wave heating (super-rotat≠IR)]

Further constraints on ionospheric densities at different LT [dawn/dusk RSS, Max Ne SEDs, ground-based IR in H3

+ (noon!)]

• What drive the hemispheric differences observed at Saturn in the magnetosphere and auroral, ionospheric regions?– Asymmetry in B field? Hemispheric (seasonal) differences in the

atmosphere? If the latter, should reverse now as going out of equinox?• Is the variable rotation rate observed in the magnetosphere linked

to atmospheric dynamics? [e.g., Jia/Kivelson talk] (two-way MI coupling)

Outstanding questions

EXTRA SLIDES

Ionospheres in the solar system

Mendillo et al. (2002)

PLANETARY IONOSPHERES in the SOLAR SYSTEM

[LASP, TIMED/SEE]

Ionization Heating of thermosphere

ORIGIN of the soft X-RAY and UV SOLAR SPECTRUM

SP P z dzionosphere

SH H z dzionosphere

Pedersen (SP) & Hall (SH)conductances:

Pedersen (sP) & Hall (sH)conductivities:

with

eBm

H z nieB

e2

en2 e

2 i

2

in2 i

2

i

n

P z nieB

ene

en2 e

2 in i

in2 i

2

i

n

IONOSPHERIC CONDUCTIVITIES IN THE AURORAL REGION

Sun only (0.1 mW m-2 including 4x10-3 mW m-2 for photoe-)Sun + Soft e- (500 eV, 0.2 mW m-2)Sun + Hard e- (10 keV, 0.2 mW m-2)

12 LT

IONOSPHERIC CONDUCTANCES IN THE AURORAL REGION

€

αP Q0(t − ΔtP )

Em = 10 keV<Q0> = 0.2 mW m-2

LT-dependence based on HST/UV brightness analysis [Lamy et al., 2009]

[Galand et al., 2011]

0800 UT

0400 – 1400 UT

DtP = 23 min

Pedersen

Electrical conductances proportional to but shifted in time

€

Q0

• Polar sub-corotation due to auroral forcing (westward ion velocities due to ambient E fields) drives equator-to-pole circulation

• NOTE: this is not simply the return-flow of thermally driven pole-to-equator winds higher up!

• Therefore, the poleward flow cools the equatorial regions• Does this rule out magnetospheric energy in solving the Gas Giant energy

crisis?

Pole-to-equator flow

Equator-to-pole flow

Smith et al., Nature (2007)Smith and Aylward (2009)

Subcorotating Region

ENERGY REDISTRIBUTION FROM HIGH TO LOWER LATITUDESThe “Ion Drag Fridge”