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Ionosphere

CSI 662 / ASTR 769 Lect. 11 Spring 2007 April 17, 2007. Ionosphere. References: Prolss: Chap. 4, P159-205 (main) Gombosi: Chap. 10, P176 – P205 (supplement) Tascione: Chap. 7, P. 89 – 97 (supplement). Fast and Slow Wind. Topics.

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Ionosphere

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  1. CSI 662 / ASTR 769 Lect. 11 Spring 2007 April 17, 2007 Ionosphere • References: • Prolss: Chap. 4, P159-205 (main) • Gombosi: Chap. 10, P176 – P205 (supplement) • Tascione: Chap. 7, P. 89 – 97 (supplement)

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

  3. Fast and Slow Wind Brief History • 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)

  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 • O + h ( 91.0 nm)  O+ + e • O2 + h ( 102.8 nm)  O2+ + e • N2 + h ( 79.6 nm)  N2+ + e Ionization Energies

  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) = 10-21 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 • 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 0 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 30000 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

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