1. Ionosphere References: Prolss: Chap. 4, P159-205 (main)
CSI 662 / ASTR Lect Spring 2007
April 17, 2007
Ionosphere
References:
Prolss: Chap. 4, P (main)
Gombosi: Chap. 10, P176 – P205 (supplement)
Tascione: Chap. 7, P. 89 – 97 (supplement)

2. Topics Fast and Slow Wind Height profile and layers
Ionization production
Ionization loss
Density profiles
Systematic Variation of density
Radio waves
Fast and Slow Wind

3. Brief History Fast and Slow Wind
Fluctuation of geomagnetic field by atmospheric current (Kelvin, 1860)
First transmitting radio waves across Atlantic (Marconi, 1901)
Solar UV radiation responsible for the charge carriers (Kennelly, Heaviside and Lodge 1902)
Radio wave experiment on ionosphere (Appleton 1924)
Fast and Slow Wind

4. Structure Classified by Composition D region h < 90 km
Negative ions, e.g., NO3-
E region
90 km < h < 170 km
O2+, NO+
F region
170 km < h < 1000 km O+
F1 region, F2 region
Plasmasphere
h > 1000 km H+

5. Structure Height of maximum density: 200 – 400 km
Maximum Ionization Density:
1 – 30 X 1011 m-3
Column Density:
1 – 10 X 1017 m-3 F2 F1
Total ne E

6. Structure Ionosphere: Weak ionization
Electrons and ions represent trace gases
Ion/neutral ratio (n/nn)
10-8 at 100 km
10-3 at 300 km
10-2 at 1000 km

7. Ionization Production
Photoionization
Primary
Secondary
Charge Exchange
Particle Precipitation

8. Photoionization Processes Ionization Energies
O + h ( 91.0 nm)  O+ + e
O2 + h ( nm)  O2+ + e
N2 + h ( 79.6 nm)  N2+ + e
Ionization Energies
Species
Dissociation
(Å)
(eV)
Ionization O O2 N2
2423.7
1270.4
5.11
9.76
910.44
1027.8
796
13.62
12.06
15.57

9. Photoionization
Consider monochromatic photon in a single gas atmosphere of species X
X + photon ( 100 nm)  X+ + e
Ionization rate:
Ionization efficiency:
1: wavelength larger than ionization limit, atomic gas
0: wavelength greater than ionization limit
Photon absorption cross section
e.g., O (EUV) = m2
Photon flux

10. Photoionization Photon extinction function
Introduce Ionization Frequency, which is independent of height
Then
Chapman production function: single peak
Different elements have different peaks

11. Photoionization Primary Photoionization Secondary Ionization Process
O + h (34.0 nm)  O+ + e + 27 ev
Produce a hot photoelectron
Secondary Ionization Process
Photoelectron with energy large than the ionization energy can do further ionization through collision with neutral
Contribute 20% of the ion production

12. Charge Exchange Charge Exchange Process Charge Exchange Rate
Does not change the total ionization density
Important source for NO+ and O2+ in the lower ionosphere
Important source for H+ for the plasmasphere

13. Particle Precipitation
Play an important role in high latitude

14. Ionization Loss Dissociative Recombination of Molecular Ions
Ion loss Rate
Dissociation Recombination Reaction constants for O2+,N2+, and NO+
Largest reaction constant

15. Ionization Loss
Radiative Recombination of Atomic Ions
Charge Exchange

16. Ionization Loss E region (O2+)
Dissociative recombination is the quickest way of removing ions and elections

17. Ionization Loss F region (O+)
Charge exchange is the quickest way of removing O+ ions

18. Density Balance Equation
Density is determined by the ion production term, ion loss term and ion diffusion term, for species s
Day time: production-loss equilibrium
Night time: production is negligible

19. Chapman Layer Density profile in E-region
Production = Loss
q(Z) = L(Z) = ne2(Z)
Solving for the electron density
ne(Z) = [q(Z) / ]1/2 or
s = 80 60 40

20. Density profile in Lower F Region (h < hM)
Electron exponentially increases with height, where hql is the effective scale height

21. Density profile in upper F Region (h > hM)
Assuming photochemistry equilibrium, n would increase to infinity
Transport or diffusion sets-in in the upper F region
Diffusion shall be ambipolar to ensure the charge neutrality.
Electrons diffuse more rapidly than ions (initially)
Slight charge separation produces polarization electric field
Ions “feel” electric field (E) and are pulled along by electrons to ensure charge neutrality

22. F-region Diffusion Times
Plasma Diffusion time:
D = HP2 / DP
Din ~ 1x1019 / nn
Chemical lifetime:
C = (kO2 nO2)-1 D C
Diffusion is faster above 280 km

23. Plasma Scale Height In static state
Since mO is larger than me, electron scale height is much larger
This effectively causes charge separation
Polarization electric field would pull electrons down, and drag ions up

24. Plasma Scale Height
For Ti=Te, plasma scale height is twice the ion scale height.
Electron scale height is reduced more than a factor of 104
Plasma scale height

25. Polar Wind
Ions in the polar ionosphere can escape along the “open” magnetic field line.
Ambipolar electric field results in a net upward acceleration
Causes supersonic outflow of light ions (H+, He+)

26. Variation of Ion Density
The ionization production depends on the solar radiation intensity and the zenith angle
The ion density shows daily, seasonal variation as well solar rotation and solar cycle effects
After sunrise
TEC (Total Electron Content) diurnal variation

27. Variation of Ion Density
D and F1-layers may disappear at night

28. Radio Waves in the Ionosphere
Radio wave is altered during its passage through the ionosphere
Propagation direction changes: refracted, reflected
Intensity changes: attenuated, absorbed
“radio echo”, ionosonde, is used to probe the ionosphere: electron density versus height

29. Natural Oscillation in a Plasma: Plasma Frequency

30. Forced Oscillation in a Plasma: Plasma Frequency

31. Ionosphere as a Dielectic
Interaction depends on frequency
Nref < 1, radio wave will be refracted according to the familiar Snell’s law. Θ2 > Θ1

32. Ionosphere as a Dielectic
Wave damping due to electron interaction with neutral particles
Radio wave (e.g., 5 Mhz) refraction and damping usually occur in the upper D region and lower E region

33. Ionosphere as a Conducting Reflection
Wave interacts strongly with plasma, inducing a large current. Ionosphere acts like a conductor
Radio wave is reflected
This often occurs in the F-region
Radio wave passes through the ionosphere

34. Radio Wave

35. Radio Wave Elapsed time  height Frequency  electron density
ionosonde

36. The End