ISR spectra simulations with electron-ion Coulomb collisions

Incoherent scatter radars (ISR) rely on Thomson scattering of very high frequency or ultrahigh frequency radio waves off electrons in the ionosphere and measure the backscattered power spectra in order to estimate altitude profiles of plasma density, electron temperature, ion temperature, and ion drift speed. These spectra result from the collective behavior of coupled ion and electron dynamics, and, for most cases, existing theories predict these well. However, when the radar points nearly perpendicular to the Earth's magnetic field, the motion of the plasma across the field lines becomes complex and Coulomb collisions between electrons and ions become important in interpreting ISR measurements. This paper presents the first fully kinetic, self‐consistent, particle‐in‐cell simulations of ISR spectra with electron‐ion Coulomb collisions. We implement a grid‐based Coulomb collision algorithm in the Electrostatic Parallel Particle‐in‐Cell simulator and obtain ISR spectra from simulations both with and without collisions. For radar directions greater than 5° away from perpendicular to the magnetic field, both sets of simulations match collisionless ISR theory well. For angles between 3° and 5°, the collisional simulation is well described by a simplified Brownian motion collision process. At angles less than 3° away from perpendicular the Brownian motion model fails, and the collisional simulation qualitatively agrees with previous single particle simulations. For radar directions exactly perpendicular to the magnetic field the simulated collisional spectra match those from the Brownian motion collision theory, in agreement with previous single particle simulations.This work was supported by NASA grants NNX14AI13G and NNX16AB80G and NSF grant PHY-1500439. This work used the XSEDE and TACC computational facilities, supported by NSF grant ACI-1053575. The work by Alex Fletcher was supported by NSF-AGS Postdoctoral Research Fellowship award 1433536 while at the Center for Space Physics, Boston University. Simulation produced data are archived at TACC and are available upon request. We thank John Swoboda of MIT Haystack Observatory for his suggestions on processing the simulated ISR spectra. (NNX14AI13G - NASA; NNX16AB80G - NASA; PHY-1500439 - NSF; ACI-1053575 - NSF; 1433536 - NSF-AGS Postdoctoral Research Fellowship at the Center for Space Physics, Boston University)First author draf