Topics in Space Weather Lecture 12

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Topics in Space WeatherTopics in Space Weather

Lecture 12

Topics in Space WeatherTopics in Space Weather

Lecture 12

Ionosphere

Robert R. Meier

School of Computational SciencesGeorge Mason University

[email protected]

CSI 76922 November 2005

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Topics

• Photoionization & Photoelectrons• Photoionization & Chapman Layer• Ionospheric Layers

– F-Region– E-Region– D-Region

• Ionospheric Regions– Equatorial– Midlatitudes– High Latitudes

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Photoionization and Photoelectrons

• Important source of–Secondary ionization–Dayglow emissions

• Heat source for plasmasphere• Conjugate photoelectrons important• Concepts analogous to auroral electron precipitation

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Photoionization

Processes– O + h ( 91.0 nm) O+ + e– O2 + h ( 102.8 nm) O2

+ + e– N2 + h ( 79.6 nm) N2

+ + e

Ionization Energies

Species Dissociation

(Å)

Dissociation

(eV)

Ionization

(Å)

Ionization

(eV)

O

O2

N2

2423.7

1270.4

5.11

9.76

910.44

1027.8

796

13.62

12.06

15.57

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Photoelectron Energy

• Example: Ionization of O by solar He+ emission at 30.4 nm– Photon energy: Es (eV)= hs = hc/s

= 12397/304 = 40.78 eV

• Ionization into ground state of O+

– Ionization potential is 13.62 eV– Excess energy:

E = Es - EIP = 40.78 – 13.62 = 27.16 eV

– What happens to excess energy?

Kinetic energy of photoelectron

S = SunPE = photoelectronIP = ionization potential

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Photoelectrons, cont.

What about ionization into excited states of ions?

• O(4S): 13.62 eV– Ground state of ion

• O(4P): 28.49 eV– First allowed state

He+ Photon can ionize into 4P state:28.49 eV < E30.4 = 40.78 eV

• Kinetic energy of photoelectron:40.78 – 28.49 = 12.29 eV

Ground state of atom From Rees, Phys. Chem. Upper Atmos.

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• Photoelectrons produced when O+ is in the ground state have sufficient energy to ionize O– EPE = 28.49 eV > 13.62 eV (ionization potential

of O)– Note that if O+ is in the 4P state, the excess

energy is 12.29 eV• Not sufficient to ionize N2 or O, but is for O2

• Therefore photoelectrons are an important source of secondary ionization– ~25% at higher altitudes– More at lower altitudes where X-rays can

produce very energetic photoelectrons

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Full computation of photoelectron flux requires solution of Boltzmann transport equation– Production

• Photoionization into ground and excited states of N2+,

O2+,O+,N+

• Secondary ionization by energetic photoelectrons

• Doubly ionized species not significant

– Loss• Elastic scattering

– Scattering by neutrals

• Coulomb collisions

• Inelastic scattering– Ionization – Excitation of electronic, vibrational, rotational states– Dissociation

– Transport

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Dominant Energy Losses:

EPE > 50 eV: ionization and excitation of atoms and molecules

EPE ~ 20 eV: excitation of atoms and molecules

EPE < 5 eV: excitation of vibrational states of N2

EPE < 2 eV: coulomb collisions with ambient electrons

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• Full solutions of Boltzmann equation– Mantas [Plan. Space Sci, 23, 337, 1975]– Oran and Strickland [Plan. Space Sci.,

26, 1161, 1978]– Link [J. Geophys. Res., 97, 159, 1992]

• Simpler approach– Richards and Torr [J. Geophys. Res., 90,

2877, 1985]

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Photoelectron Flux

• Following Richards and Torr, ignore– Transport– Coulomb collisions– Cascade of high energy photoelectrons to

lower energy photoelectrons

– EPE < 20 eV

– O2

• Simple Production = Loss gives insight into photoelectron flux spectrum

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Photoelectron Flux

• Production

qN2(z,E) = nN2(z) Fs(z,E()) (E) dE

nN2(z) I(E) exp(- eff(z,E))

Similar expression for O

• LossLN2 = (z,E) N2(E) nN2(z)

= photoelectron flux (PE cm-2s-2 eV-1)

N2 = total energy loss cross section for e*+N2 collisions

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Photoelectron Flux

Production = Loss or qtotal = Ltotal

nN2(z) IN2(E) exp(- eff(z,E)) + nO(z) IO(E) exp(- eff(z,E))

= (z,E) N2(E) nN2(z) + (z,E) O(E) nO(z)

Solving for :

eff eff

2 2

2 2

2

eff

2 2

N N O O

N N O O

N

OO O

NO N

O

n (z)I (z,E)e n (z)I (z,E)e(z,E) =

(E)n (z) (E)n (z)

IR

II ne R

nR

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Photoelectron Flux, cont.

If

then

and the photoelectron flux – altitude dependence is from the effective

attenuation of the solar flux

– energy dependence is from the production frequency and energy loss cross section ratio

– is independent of composition

2 2N N

O O

I σ≈

I σ

eff (z)O

O

I (E)(z,E) = e

(E)

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Simple and full PE flux calculations

Some differences < 20 eV and > 50 eV

Richards and Torr [1983]

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Simple and full PE flux calculations

Compare with AE-E PE measurements

Richards and Torr [1983]

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Altitude Dependence of Photoelectron Flux

Full PE flux calculations

Note small change in energy shape with altitude

- Supports Richards and Torr simple concepts

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Photoelectron Energy Distribution Function

Thermal Electrons

Photoelectrons

Structure dueTo He+ 30.4 nm

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Photoionization and Photoionization and the Classic Chapman the Classic Chapman

IonosphereIonosphere

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Photoionization

• Example:O + h O+ + e*, …

• Photoionization Frequency

j(z) = Fs(z,) () d (# s-1)

Fs(z,) = Fs(,) e-(z,)(photon cm-2 s-1 nm-1)

() = photoionization cross section

no(z) = O number density

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Photoionization Rate

• q(z) = no(z) j(z) (# cm-3 s-1)– no(z) = O number density

• Assume single constituent, isothermal atmosphere, photoionized by a single wavelength emission:

n(z) = no(z) = no(zo) e-(z-zo)/H

q(z) = no(zo) e-(z-zo)/H Fs(,) e-(z,)

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Photoionization Rate cont.

o

z zooH

o

z zoo H

o

z z

Ho

z

z-z- n(z )HeH

o s

z zn(z )He

H

o s

(z) n(z ')dz ' n(z )He

q(z) = n(z ) e F ( ) e

q(z) = n(z ) F ( ) e

Peak in layer occurs when

Working through, this occurs at

)Mdq(z

0dz

M oz z

HM o(z ) 1 n(z )He

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Photoionization Rate cont.

Substituting and rearranging terms leads to:

For Sun at zenith, s

z zM

M Hz z1 e

HMq(z) = q(z ) e

z zM

M Hs

z z1 sec e

HMq(z) = q(z ) e

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Recombination

• Radiative Recombination O+ + e O + h

• Recombination Coefficient= 1.2 x 10-12 (1000/T)1/2 cm-3 s-1

• Electron loss rateL(z) = nO+(z) ne(z) = ne

2(z)

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Chapman Layer

Production = Loss (Steady-state: dne/dt =q-L = 0)

q(z) = L(z) = ne2(z)

Solving for the electron densityne(z) = [q(z) / ]1/2

or

z zMM H

sz z1

1 sec e2 H

e e Mn (z) = n (z ) e s = 8060

40

0

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Ionospheric LayersIonospheric Layers• D-Region• E - region• F1 – Region• F2 – Region• Plasmasphere

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Ionospheric Layers Similar to Chapman Layers

F2

F1

E

Total ne

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D-Region

• Ugly ion chemistry• See:

– Turunen, E., H. Matveinen, J. Tolvanen, and H. Ranta, D-region ion chemistry model, in STEP Handbook of Ionospheric Models, R. W. Schunk (ed.), pp. 1-25, 1996.

– Torkar, K. M., and M. Friedrich, Tests of an ion-chemical model of the D- and lower E-region, J. Atm. Terr. Phys., 45, 369-385, 1983.

• Tens of species—some models have more• Few measurements

– Requires rockets

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D-Region Chemical Scheme

From Torkar, K. M., and M. Friedrich, 1983

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Example Comparison of D-region Observations and Model

From Torkar, K. M., and M. Friedrich, 1983

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E-region

• Production--photoionizationO2 + h O2

+ + e j = photoionzation rate

N2 + h N2+ + e

O + h O+ + e (smaller)

• Chemistry– N2

+ + O NO+ + N or O+ + N2

– O+ + N2 NO+ + N

• Loss—Dissociative Recombination– NO+ + e N + O

– O2+ + e O + O kO2+ = recombination rate

• Net Result:– Major ions in E region are O2

+ and NO+

– To first order, diffusion & dynamics slow compared with photochemistry

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Electron Density in Lower Part of E-Region

• O2+ is dominant ion

• Ignore dynamics, diffusion

2 22 2 2

e

2O e O eO O O

dn (z)Pr oduction Loss

dt

j(z)n (z) k n (z)n (z) j(z)n (z) k n (z)

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Electron Density in Lower Part of E-Region, cont.

In steady state,

2

z zMM H

s

2

2e O

z z11 sec e

2 HOe e M

n (z) j(z)n (z)

or

j(z)n (z)n (z) n (z ) e

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Recombination Rates and Electron Lifetimes

Lower E-Region• O2

+ + e O + O kO2+ = 1.9 x 10-7 (Te/300)-0.5 cm3s-1

• nO2+ ~ 105 cm-3 & Te ~ Tn = 300K at ~ 110 km• Rate = kO2+ nO2+ = 0.019 s-1

• Lifetime = 1/Rate = 53 s

Upper E-Region• NO+ + e N + O kNO+ ~ 4.2 x 10-7 (Te/300)-0.85 cm3s-1

• nNO+ ~ 105 cm-3 & Te ~ Tn = 587K at ~ 140 km• Rate = kNO+ nNO+ = 0.024 s-1

• Lifetime = 1/Rate = 42 s

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F1-Region

• Similar to E-region

• Must include O+, the dominant ion

• Diffusion begins to be important

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F2 Peak Region - Assume photochemical equilibrium - Ignore transport and diffusion

2 2

+O

+ + +2 2 2 N ,O

O +hν O +e j

O + N ,O NO ,O + N,O k

O+ balance yields:

jO nO = kN2 nN2 + kO2 nO2

2 2 2

O O OeO

N N N

j n nn n

k n n

F2-region ion chemistry rates

Ignoring O2 in the upper ionosphere yields:

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Important Result when Chemistry Dominates: ne nO/nN2

• Problem: As z increases, nN2 decreases much more rapidly than nO

– Therefore ne exponentially as z increases

• Solution– Transport becomes faster at high altitudes– Also at other times when electrodynamics

become important

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Recombination Rates and Electron Lifetimes in F-Region

• Production: O + h O+ + e– Rate = j = 2 – 6 x 10-7 s-1

– Lifetime = 5 – 1.6 x 106 s ( 58 - 19 days)

• Intermediate step: O+ + N2 NO+ + N– (or O2)– Rate for N2: kN2 nN2 = 10-12cm3s-1 5.5x108cm-3 = 5.5 x 10-4 s-1

– Lifetime = 1800 s

• Loss: NO+ + e N + O– ne ~ 106 cm-3 & Te ~ 1800K at ~ 250 km– Rate = kNO+ ne = 0.129 s-1

– Lifetime = 1/Rate = 7.8 s

• Loss: O+ + e O– ne ~ 106 cm-3 & Te ~ 1400K at ~ 250 km– Rate = kO+ ne = 1.2x10-12 (1000/T)0.5 x ne s-1 = 1 x 10-6s– Lifetime = 1/Rate = 106 s = 11 days

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Simplified Ambipolar Diffusion

• Electrons diffuse more rapidly than ions (initially)

• Slight charge separation produced strong electric field

• Ions “feel” electric field (E) and are pulled along by electrons to ensure charge neutrality

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Ambipolar Diffusion, cont.

• Again, diffusive equilibrium– net diffusion velocity is zero– Ignore ion chemistry

• Assume plasma flow parallel to magnetic field lines – taken to be vertical (upper mid to high

latitudes)

• Assume single ion species– Same as neutral species– Note: can be generalized to multiple ions

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• Force on ions and electrons:Fi = eE (upward pull)

Fe = - eE (downward pull)

• Force balance for ions and electrons in slab of area, A:Ions: dpi A = - ni mi g Adz + ni eE Adz

Electrons: dpe A = - ne me g Adz - ne eE Adz

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or

and

ii i

ee e

dp=-n mg-eE

dzdp

=-n mg+eEdz

i e

ei i e

with n = n

solving electron pressure equation for

dpn eE = - - nm g

dz

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i i i e

e i

nkT and nkT

1

T H

e eii i i e i i

i ei i

i e

i i i

i

dp dpdp=-nmg-nmg- -nmg-

substitutingintoionpressu

dz dz dzd p +p

=-nmgdz

p p

d

reequatio

n nmg=-

d

n

z k T

:

Since

Gombosi, Equation 10.48

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• If Te = Ti = Tn, then(Assuming mi=mn)

• Thus the ion scale height is twice the neutral scale height

• The ion density profile is then:

• More complete physics requires numerical solutions of differential equations

ni n

i

2kTH = =2H

mg

o

n

z-z

2Hi i on (z) = n (z ) e

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F-region Diffusion Times

• Plasma Diffusion time: D = Hi

2 / Din

– Din ~ 1x1019 / nn (Banks and Kockarts, Aeronomy)

• Chemical lifetime: C = (kN2 nN2)-1

(from slide 24)

CD

Atmosphere: from Homework 1

Diffusion is faster above 280 km and chemistry is, belowSee Section 7.5 of Tascione

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Plasmasphere

• Top of ionosphere

• Strong interactions with magnetosphere, esp. during geomagnetic storms

• Consider simple processes only– More complicated interactions

with magnetosphere

Add fig 10.7 from Gombosi

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• Photochemical equilibrium– Near resonant charge exchange

O+(4S) + H(2S) O(3P) + H+ E

E = EIP(O) - EIP(H) = 13.618 - 13.598 = 0.02 eV

• Source and sink of H+

• Assuming photochemical equilibrium

• As altitude increases, H/O increases, and H+ becomes the dominant ion

Plasmasphere, cont.

i

n

TT

n(On(H

n(On(H

)

)

)

)

Gombosi, 10.6

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Plasmasphere, cont.

• He+ is second most populous ion– Tracks He+

• Can image plasmasphere by observing resonant scattering of solar He+ 30.4 nm emission line

He+(2S)+ h30.4 He+ (2P)

He+(2S)+ h30.4

From IMAGE SatelliteSun

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Ionospheric RegionsIonospheric Regions

Low LatitudesLow LatitudesMid LatitudesMid LatitudesHigh LatitudesHigh Latitudes

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Ionosphere Cross Section

Fig 8.6 Tascione

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Equatorial Ionosphere is Anomalous

• Key: Electric Fields– E-Region Dynamo– F-region Dynamo

• ~ Horizontal Magnetic Field

E x B drift upward

vB

E

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Map of Ionospheric Critical Frequency

• F-region peak density

nemax (cm-3) = 1.24 x 104 f (Mhz)

• Maximum separation in arcs near twilight

• Electric field reversal weakens anomaly through night

TIME-GCM Model

Evening Morning

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Winds Can Affect Ionization Peaks

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Instabilities in Equatorial Ionosphere

• Recombination at night removes E-Region

– F-Region recombination slower

• Vertical density gradient produces Rayleigh-Taylor instability

• Low density plasma drifts up field lines

- Produces “bubbles”- Empty field lines- Horizontal gradients cause

scintillation of radio signals

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GUVI FUV Ionosphere Observations

Longitude

Latit

ude

Depleted Flux Tubes

e + O+ O* (135 nm) I = ne2 ds

9/22/2002

e + O+ O* (135 nm)

Equ

ator

ial A

nom

aly

Magnetic Equator

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18MidnightNoon

Solar Maximum

Noon Midnight18

Solar Minimum

20dB

15dB10dB

5dB2dB

1dB

L-Band

SOLAR CYCLE CHANGES INDUCE SOLAR CYCLE CHANGES INDUCE IONOSPHERIC IRREGULARITIES THAT AFFECT IONOSPHERIC IRREGULARITIES THAT AFFECT

ELECTROMAGNETIC PROPAGATIONELECTROMAGNETIC PROPAGATION

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Mid-Latitude Ionosphere• Simple concepts apply more readily

– Magnetic field closer to vertical– Usually not much particle precipitation

• Electrodynamics less important (except during geomagnetic storms)– But, plasma irregularities more prevalent at mid-

latitudes than previously thought• Closer to photochemical equilibrium

– neutral composition is crucial:ne nO / nN2

• Neutral winds can blow plasma up or down field lines– Up: Lower recombination rate (fewer molecules)– Down: Higher recombination rate (more molecules)

• Plasma flow from plasmasphere can be important– Helps maintain nighttime ionosphere

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Mid-Latitude Ionosphere, cont.

• Various diurnal and seasonal “anomalies”– See Tascione, 8.4

• Strong solar cycle variation associated with Solar EUV Radiation

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High Latitude Ionosphere

• Magnetic field lines – “Open” over polar region– Closed in auroral oval, but extend deep

into magnetotail

• Main coupling region to magnetosphere– But, during geomagnetic storms, E

fields can penetrate to lower latitudes

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Magnetosphere-Ionosphere-Atmosphere Coupling Processes

ParticlePopoulation

H+, He+,O+

ElectricField

ParticlePrecipitation

PolarWind

ParticlePopoulation

H+, He+,O+

Convection,Heating,

CompositionChanges

Ionization,Conductivity,

Heating

EnergeticAuroral

Ion Outflow

Neutral Motion,Composition

Changes,Dynamo E Field

Schunk & Nagy, Ionospheres

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Polar Wind

• O+ is major ion in F-region

• Upward acceleration of H+, He+

– Ambipolar electric field – fewer collisions with O+

• Causes supersonic outflow of light ions

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Electric Field


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Electric Field, cont.

• Solar wind motion (Vsw) contains electric field: E = - Vsw x B

• Near-Earth sees electric field that points in dawn to dusk direction

• E-field maps down highly conducting field lines into ionosphere

• This “convection” E causes E x B drift of ionospheric plasma in anti-sun direction

• Farther from Earth, E x B drift is toward the equatorial plane

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• Charges on polar cap boundary induce E fields on nearby closed field lines, opposite to convection electric field

• Ionospheric plasma on closed field lines drifts sunward in response

• On boundary of open and closed field lines, field-aligned currents flow between the magnetosphere and ionosphere

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Auroral Plasma Drifts

• High altitudes– No net current– Collisions impart

momentum and cause neutral winds

• Low altitudes– Ion-neutral

collisions cause heating

– Ions lose mobility– Current carried by

electrons

• Plasma drifts in two-cell pattern

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Collisional “friction” between ions moving in response to E fields and neutrals causes joule heating and momentum transfer


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Particle Precipitation

Akasofu, Scientific American, May 1, 1989

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Auroral Energetic Electron Spectrum

Dynamics Explorer 2 Measurement

ModelCalculations

From Rees, Phys. Chem. Upper Atmos.

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Ionization Rates by Energetic Auroral Electrons

From Rees, Phys. Chem. Upper Atmos.

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Polar Ionospheric Phenomena

Magnetospheric Electric FieldsParticle Precipitation

Field-aligned CurrentsPolar Holes

Ionization TroughsTongues of Ionization

Plasma PatchesAuroral Ionization EnhancementsElectron and Ion Temperature Hot

Spots

Depend on:Phase of Solar Cycle

SeasonTime of Day

Type of Convection PatternStrength of Convection

Infinite Possibilities &Infinite Opportunities to

Study the Physics


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Other Empirical Models

• International Reference Ionosphere (IRI)– MSIS-like model of ionosphere– http://modelweb.gsfc.nasa.gov/models/iri.html

• Horizontal Wind Model (HWM)– Model of horizontal components of the neutral

wind– http://uap-www.nrl.navy.mil/models_web/

hwm/hwm_home.htm

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Topics in Space Weather Lecture 12