Ionosphere References: Prolss: Chap. 4, P159-205 (main) 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)

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

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

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+

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

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

Ionization Production Photoionization Primary Secondary Charge Exchange Particle Precipitation

Photoionization Processes Ionization Energies 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) (eV) Ionization O O2 N2 2423.7 1270.4 5.11 9.76 910.44 1027.8 796 13.62 12.06 15.57

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

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

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

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

Particle Precipitation Play an important role in high latitude

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

Ionization Loss Radiative Recombination of Atomic Ions Charge Exchange

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

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

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

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

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

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

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

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

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

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+)

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

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

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

Natural Oscillation in a Plasma: Plasma Frequency

Forced Oscillation in a Plasma: Plasma Frequency

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

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

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

Radio Wave

Radio Wave Elapsed time  height Frequency  electron density ionosonde

The End