Earth’s Ionosphere Lecture 13 ESS200C Earth’s Ionosphere Lecture 13

Hydrostatic Equilibrium The force of gravity on a parcel of air is balanced by the pressure gradient Assume Tn is independent of height and integrate we obtain The density of an atmosphere falls off (generally) exponentially.

Photoionization As radiation passes through the atmosphere, it is absorbed and its intensity decreases If this absorption is due to ion production, then One ion pair is produced (generally) per 35eV in air Production is a maximum when Here Since So peak production occurs when or where , where Nnm is the integrated density. This is also where the optical depth is unity.

Chapman Production Function Peak production is Production as a function of height is Let y = (h - hm) / Hn then Below the peak y is negative and exp (-y) dominates Above the peak y is positive and –y dominates If we reference local production rate to maximum at subsolar point, we obtain where

Particle Impact Ionization In many situations, particle impacts can be the principal source of ionization Solar proton events in polar cap Auroral zone during substorms Satellites with atmospheres in planetary magnetospheres A primary particle can produce energetic secondary electrons that can ionize. These electrons can also produce x rays when they decelerate Charge exchange can occur for ions producing a fast neutral Process is very non-linear; often is numerically stimulated Range energy relation is a good approximation. Allow calculation of stopping altitude. Note: In this context nn(s) is a mass density

Particle Energy Deposition Need to calculate altitude distribution of energy loss Range-energy relation can also be written Assume that the depth of matter traversed at x is approximated by Where is energy particle has at point x Solving for Then Curves here are for mono-energetic beams. In practice, sum over a distribution of energies.

Bremsstrahlung/ Ion Loss Electrons scatter much more easily than ions. A decelerating or accelerating electric charge produces electromagnetic energy. This braking radiation tends to be in the x-ray range and this produces further ionization. Once produced electrons are lost by three processes: Radiative recombination e+x+→x+hυ Dissociative recombination e+xy+→x+y Attachment e+z→z-

Ionospheric Density Profile Photochemical equilibrium assumes transport is not important so local loss matches local production. If loss is due to electron-ion collisions, we get a Chapman layer If there is vertical transport Treating the pressure forces of electrons and ions and assuming neutrals are stationary, we obtain Where is the ambipolar diffusion coefficient and Hp the plasma scale height Vertical transport velocity becomes

The Earth’s Ionosphere The electron density in the ionosphere is less than the neutral density. For historical reasons, the ionospheric layers are called D, E, F D layer, produced by x-ray photons, cosmic rays E layer, near 110 km, produced by UV and solar x-rays F1 layer, near 170 km, produced by EUV F2 layer, transport important Enhanced ionization in the D-region leads to absorption of radio waves passing through because it is collisional with neutrals. At night, ionosphere can recombine, but transport, especially from high altitudes can be important In polar regions where field is vertical, a polar wind of light ions can form similar to the solar wind.

Collision Frequencies Ion and electrons collide with neutrals as they gyrate. How they move in response to electric fields depends very much on the collision frequency relative to the gyro-frequency. If the gyro-frequency is much lower than the collision frequency, ions and electrons move in the direction of the electric field or opposite to it. This will produce a current. If the collision frequency is much lower than the gyro-frequency, ions and electrons drift together perpendicular to the magnetic field. Since the ions and electrons have different gyro-frequencies and collision frequencies, a complex set of currents may be produced. This is treated with a tensor electrical conductivity.

Conductivity Parallel equation of motion Perpendicular equation of motion Conductivity tensor Petersen conductivity (along E┴) Hall conductivity (along E x B) Parallel conductivity

Force Balance - MI Coupling

Maxwell Stress and Poynting Flux

Currents and Ionospheric Drag

Weimer FAC morphology

FAST Observations IMF By ~ -9 nT. IMF Bz weakly negative, going positive. Questions: Where is the dawnside open/closed boundary? Where do the field-aligned currents go?

MHD FAST Comparisons

MHD FACs

dB’s Scaled to Ionosphere dt = -411s dt = -51s Time Time Scaled dB’s largely agree. Mapped by √B. Even small scale structures can show persistence. UT and ephemeris data for FAST only

38700 – MHD Comparison Bx ≈ 5 nT By ≈ 3 nT Bz ≈ -5 nT Density jump: 12 – 18 cm–3 at 11:15 dB’s generally consistent with MHD flow pattern. Weaker dB’s in polar cap because of lower conductivity.