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Topics in Space Weather Lecture 12. Ionosphere. Robert R. Meier School of Computational Sciences George Mason University rmeier@gmu.edu CSI 769 22 November 2005. Topics. Photoionization & Photoelectrons Photoionization & Chapman Layer Ionospheric Layers F-Region E-Region D-Region - PowerPoint PPT Presentation
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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
rmeier@gmu.edu
CSI 76922 November 2005
2
Topics
• Photoionization & Photoelectrons• Photoionization & Chapman Layer• Ionospheric Layers
– F-Region– E-Region– D-Region
• Ionospheric Regions– Equatorial– Midlatitudes– High Latitudes
4
Photoionization and Photoelectrons
• Important source of–Secondary ionization–Dayglow emissions
• Heat source for plasmasphere• Conjugate photoelectrons important• Concepts analogous to auroral electron precipitation
5
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
6
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
7
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.
8
Photoelectrons, cont.
• 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
9
Photoelectrons, cont.
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
10
Photoelectrons, cont.
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
11
Photoelectrons, cont.
• 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]
12
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
13
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
14
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
15
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)
16
Photoelectron Flux, cont.
Simple and full PE flux calculations
Some differences < 20 eV and > 50 eV
Richards and Torr [1983]
17
Photoelectron Flux, cont.
Simple and full PE flux calculations
Compare with AE-E PE measurements
Richards and Torr [1983]
18
Altitude Dependence of Photoelectron Flux
Full PE flux calculations
Note small change in energy shape with altitude
- Supports Richards and Torr simple concepts
19
Photoelectron Energy Distribution Function
Thermal Electrons
Photoelectrons
Structure dueTo He+ 30.4 nm
20
Photoionization and Photoionization and the Classic Chapman the Classic Chapman
IonosphereIonosphere
21
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
22
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,)
23
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
24
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
25
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)
26
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
27
Ionospheric LayersIonospheric Layers• D-Region• E - region• F1 – Region• F2 – Region• Plasmasphere
30
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
33
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
34
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)
35
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
36
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
37
F1-Region
• Similar to E-region
• Must include O+, the dominant ion
• Diffusion begins to be important
38
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:
39
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
40
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
41
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
42
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
43
Ambipolar Diffusion, cont.
• 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
44
Ambipolar Diffusion, cont.
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
45
Ambipolar Diffusion, cont.
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
46
Ambipolar Diffusion, cont.
• 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
47
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
48
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
49
• 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
50
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
51
Ionospheric RegionsIonospheric Regions
Low LatitudesLow LatitudesMid LatitudesMid LatitudesHigh LatitudesHigh Latitudes
53
Equatorial Ionosphere is Anomalous
• Key: Electric Fields– E-Region Dynamo– F-region Dynamo
• ~ Horizontal Magnetic Field
E x B drift upward
vB
E
54
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
56
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
57
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
58
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
59
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
60
Mid-Latitude Ionosphere, cont.
• Various diurnal and seasonal “anomalies”– See Tascione, 8.4
• Strong solar cycle variation associated with Solar EUV Radiation
61
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
62
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
63
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
65
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
66
Electric Field, cont.
• 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
67
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
68
Electric Field, cont.
Collisional “friction” between ions moving in response to E fields and neutrals causes joule heating and momentum transfer
Schunk & Nagy, Ionospheres
70
Auroral Energetic Electron Spectrum
Dynamics Explorer 2 Measurement
ModelCalculations
From Rees, Phys. Chem. Upper Atmos.
72
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
Schunk & Nagy, Ionospheres
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