Ultraviolet Remote Sensing of Space Weather A tutorial

Report
Topics in Space Weather
Lecture 12
Ionosphere
Robert R. Meier
School of Computational Sciences
George Mason University
[email protected]
CSI 769
22 November 2005
1
Topics
• Photoionization & Photoelectrons
• Photoionization & Chapman Layer
• Ionospheric Layers
– F-Region
– E-Region
– D-Region
• Ionospheric Regions
– Equatorial
– Midlatitudes
– High Latitudes
2
3
Photoionization and
Photoelectrons
• Important source of
–Secondary ionization
–Dayglow emissions
• Heat source for plasmasphere
• Conjugate photoelectrons important
• Concepts analogous to auroral electron
precipitation
4
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 Dissociation
(Å)
(eV)
O
O2
N2
2423.7
1270.4
5.11
9.76
Ionization
(Å)
Ionization
(eV)
910.44
1027.8
796
13.62
12.06
15.57
5
Photoelectron Energy
• Example: Ionization of O by solar He+
emission at 30.4 nm
– Photon energy: Es (eV)= hs = 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?
S = Sun
PE = photoelectron
IP = ionization potential
 Kinetic energy of photoelectron
6
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
7
From Rees, Phys. Chem. Upper Atmos.
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
8
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
9
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
10
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]
11
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
12
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
• Loss
LN2 = (z,E) N2(E) nN2(z)
 = photoelectron flux (PE cm-2s-2 eV-1)
N2 = total energy loss cross section for e*+N2
collisions
13
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 :
(z,E) =
nN2 (z)IN2 (z,E)e  eff  nO (z)IO (z,E)e  eff
N2 (E)nN2 (z)  O (E)nO (z)
 IN2

 R

IO  IO
 e  eff

O  N2

 R

 O

R
nO
nN2
14
Photoelectron Flux, cont.
IN2
If
IO
then
≈
(z,E) =
σN2
σO
IO (E) eff (z)
e
O (E)
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
15
Photoelectron Flux, cont.
Simple and full
PE flux
calculations
Some
differences < 20
eV and > 50 eV
Richards and Torr [1983]
16
Photoelectron Flux, cont.
Simple and full
PE flux
calculations
Compare with
AE-E PE
measurements
Richards and Torr [1983]
17
Altitude Dependence of Photoelectron Flux
Full PE flux calculations
Note small change in
energy shape with
altitude
- Supports Richards and Torr
simple concepts
18
Photoelectron Energy Distribution
Function
Structure due
To He+ 30.4 nm
Thermal Electrons
Photoelectrons
19
Photoionization and
the Classic Chapman
Ionosphere
20
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
21
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-z )/H
o
q(z) = no(zo) e-(z-z )/H Fs(,) e-(z,) 
o
22
Photoionization Rate cont.

(z)   n(z ')dz '  n(zo )He

 z  zo 
H
z
q(z) = n(zo ) e
-
z-zo
H
Fs ( ) e n( zo )He
q(z) = n(zo ) Fs ( ) e
Peak in layer occurs when

z  zo
H
 z  zo  


 z  zo  
H


 n( zo )He



 H 


dq(z M )
0
dz
Working through, this occurs at (zM )  1  n(zo )He

zM zo
H
23
Photoionization Rate cont.
Substituting and rearranging terms leads to:

z  zM
1
e
H
 z  zM 
H
q(z) = q(zM ) e
For Sun at zenith, s

z  zM
1
 sec s e
H
q(z) = q(zM ) e
 z  zM 
H
24
Recombination
• Radiative Recombination
O+ + e  O + h
• Recombination Coefficient
 = 1.2 x 10-12 (1000/T)1/2 cm-3 s-1
• Electron loss rate
L(z) = nO+(z) ne(z) = ne2(z)
25
Chapman Layer
Production = Loss
(Steady-state: dne/dt =q-L = 0)
q(z) = L(z) = ne2(z)
Solving for the electron density
ne(z) = [q(z) / ]1/2
or
ne (z) = ne (zM ) e

z  zM
1
1
 sec s e
2
H

z  zM
H




s = 80
0
60
40
26
Ionospheric Layers
•
•
•
•
•
D-Region
E - region
F1 – Region
F2 – Region
Plasmasphere
27
28
Ionospheric Layers Similar to
Chapman Layers
F2
F1
Total ne
E
29
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 Dand lower E-region, J. Atm. Terr. Phys., 45, 369-385, 1983.
• Tens of species—some models have more
• Few measurements
– Requires rockets
30
D-Region Chemical Scheme
From Torkar, K. M., and M. Friedrich, 1983
31
Example Comparison of D-region
Observations and Model
32
From Torkar, K. M., and M. Friedrich, 1983
E-region
• Production--photoionization
O2 + h  O2+ + e
N2 + h  N2+ + e
O + h  O+ + e (smaller)
j = photoionzation rate
• 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
33
Electron Density in Lower Part of
E-Region
• O2+ is dominant ion
• Ignore dynamics, diffusion
dne (z)
 Pr oduction  Loss
dt
 j(z)nO2 (z)  k O nO (z)ne (z)  j(z)nO2 (z)  k O ne2 (z)
2
2
2
34
Electron Density in Lower Part of
E-Region, cont.
In steady state,
n2e (z)  j(z)nO2 (z)
or
ne (z) 
j(z)nO2 (z)

 ne (zM ) e

z  zM
1
1
 sec s e
2
H

z  zM
H




35
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
36
F1-Region
• Similar to E-region
• Must include O+, the dominant ion
• Diffusion begins to be important
37
F2 Peak Region
- Assume photochemical equilibrium
- Ignore transport and diffusion
F2-region ion chemistry
O +hν  O + e
+
O+ + N2 ,O2   NO+ ,O2+  + N,O
rates
jO
kN2 ,O2
O+ balance yields:
jO nO = kN2 nN2 + kO2 nO2
Ignoring O2 in the upper ionosphere yields:
jOnO nO
nO   ne  

kN2 nN2 nN2
38
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
39
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
40
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
41
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
42
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
43
Ambipolar Diffusion, cont.
or
and
with
dp i
= -ni  mi g - eE 
dz
dp e
= -n e  me g + eE 
dz
ni = ne
solving electron pressure equation for
dpe
nieE = - nimeg
dz
44
Ambipolar Diffusion, cont.
substituting into ion pressu re equation :
dp e
dp e
dp i
= -nimig - nime g  -nimig dz
dz
dz
d pi + p e 
= -nimi g
dz
Since
p i  nikTi and p e  nikTe
d ni
nimi g
1
=
dz
k  Ti  Te 
Hi
Gombosi, Equation 10.48
45
Ambipolar Diffusion, cont.
• If Te = Ti = Tn, then
(Assuming mi=mn)
2kTn
Hi =
= 2Hn
mi g
• Thus the ion scale height is twice the neutral
scale height
• The ion density profile is then:

ni (z) = ni (zo ) e
z-zo
2Hn
• More complete physics requires numerical
solutions of differential equations
46
F-region Diffusion Times
• Plasma Diffusion
time:
Atmosphere:
from Homework 1
– D = Hi2 / Din
– Din ~ 1x1019 / nn
(Banks and Kockarts,
Aeronomy)
• Chemical lifetime:
C
D
– C = (kN2 nN2)-1
(from slide 24)
Diffusion is faster above 280 km and chemistry is, below
47
See Section 7.5 of Tascione
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
48
Plasmasphere, cont.
• 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
n (H
n (O
) n (H )


) n (O )

Tn
Ti
• As altitude increases, H/O increases, and H+
becomes the dominant ion
Gombosi, 10.6
49
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)+ h30.4  He+ (2P)
 He+(2S)+ h30.4
From IMAGE Satellite
50
Ionospheric Regions
Low Latitudes
Mid Latitudes
High Latitudes
51
Ionosphere Cross Section
Fig 8.6 Tascione
52
Equatorial Ionosphere is Anomalous
• Key: Electric Fields
– E-Region Dynamo
– F-region Dynamo
• ~ Horizontal Magnetic Field
v
B
E x B drift upward
E
53
Map of Ionospheric Critical Frequency
TIME-GCM Model
• 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
Evening
Morning
54
Winds Can Affect Ionization Peaks
55
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
56
scintillation of radio signals
GUVI FUV Ionosphere Observations
e + O+  O* (135 nm)
I =   ne2 ds
Latitude
Equatorial Anomaly
Depleted Flux Tubes
9/22/2002
Longitude
Magnetic Equator
57
SOLAR CYCLE CHANGES INDUCE
IONOSPHERIC IRREGULARITIES THAT AFFECT
ELECTROMAGNETIC PROPAGATION
Noon
18
Midnight
Noon
18
Midnight
L-Band
20dB
Solar Maximum
15dB
10dB
5dB
2dB
1dB
Solar Minimum
58
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 midlatitudes 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
59
Mid-Latitude Ionosphere, cont.
• Various diurnal and seasonal
“anomalies”
– See Tascione, 8.4
• Strong solar cycle variation
associated with Solar EUV Radiation
60
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
61
Magnetosphere-IonosphereAtmosphere Coupling Processes
Particle
Popoulation
H+, He+,O+
Polar
Wind
Electric
Field
Convection,
Heating,
Composition
Changes
Particle
Precipitation
Particle
Popoulation
H+, He+,O+
Ionization,
Conductivity,
Heating
Energetic
Auroral
Ion Outflow
Neutral Motion,
Composition
Changes,
Dynamo E Field
Schunk & Nagy, Ionospheres
62
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
63
Electric Field
Schunk & Nagy, Ionospheres
64
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
65
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
66
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
67
Electric Field, cont.
Collisional “friction”
between ions moving
in response to E fields
and neutrals causes
joule heating and
momentum transfer
Schunk & Nagy, Ionospheres
68
Particle Precipitation
Akasofu, Scientific American, May 1, 1989
69
Auroral Energetic Electron Spectrum
Model
Calculations
Dynamics Explorer 2
Measurement
70
From Rees, Phys. Chem. Upper Atmos.
Ionization Rates by Energetic Auroral
Electrons
71
From Rees, Phys. Chem. Upper Atmos.
Polar Ionospheric Phenomena
Magnetospheric Electric Fields
Particle Precipitation
Field-aligned Currents
Polar Holes
Ionization Troughs
Tongues of Ionization
Plasma Patches
Auroral Ionization Enhancements
Electron and Ion Temperature Hot
Spots
Depend on:
Phase of Solar Cycle
Season
Time of Day
Type of Convection Pattern
Strength of Convection
Infinite Possibilities &
Infinite Opportunities to
Study the Physics
Schunk & Nagy, Ionospheres
72
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://uapwww.nrl.navy.mil/models_web/hwm/hwm_hom
e.htm
73

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