Electron collision effects on radar temperature measurements of the ionosphere
Longley, William James
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Most plasmas in astrophysics are hot, tenuous gases where macroscopic electric and magnetic fields dominate the plasma dynamics. In the Earth’s ionosphere the plasma is cold and dense, and the small-scale Coulomb force between charged particles becomes important. In this dissertation I implement a grid-based Coulomb collision algorithm in the Electrostatic Parallel Particle-in-Cell (EPPIC) simulator to model spectra observed by incoherent scatter radars (ISR). The modeled spectra are then compared to observations from Millstone Hill to show that current radar techniques can systematically underestimate plasma temperatures in the ionosphere. ISRs transmit radio waves that are Thomson scattered by electrons in the ionosphere, and then measure the Doppler shift spectra off the ion-acoustic wave. The measured spectra are then fit to a forward model in order to estimate altitude profiles of plasma density, electron temperature, ion temperature, and ion drift speed. For radars looking at aspect angles within 5° of perpendicular to the Earth’s magnetic field, the magnetic field constrains electron movement and Coulomb collisions add an additional source of damping that narrows the spectral width. Fitting the collisionally narrowed spectra to collisionless theories leads to underestimates of plasma temperatures by as much as 25%. Using EPPIC, I present the first fully kinetic, self-consistent, particle-in-cell simulations of ISR spectra with electron-ion and electron-electron Coulomb collisions. For intermediate aspect angles between 0.5° and 2° off perpendicular, the damping effects of electron-ion and electron-electron collisions are the same and the resulting spectra are narrower than what current theories predict. For aspect angles larger than 3° away from perpendicular, the simulations with electron-ion collisions match collisionless ISR theory well, but spectra with electron-electron collisions are narrower than theory predicts at aspect angles as large as 5° away from perpendicular. I use the Millstone Hill radar to measure spectra at small aspect angles and show that current theories produce incorrect temperature measurements at aspect angles of 4.6° or less. The EPPIC simulations show that a nonlinear interaction between electron-electron collisions and Landau damping causes the errors in temperature measurement, which is not accounted for in current theories.
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