Kinetic Simulations of Photoelectron Driven Instabilities Applied to 150 Km Echoes with Improved Collision Models

Equatorial VHF radars looking near-perpendicular to the Earth's magnetic field observe strong coherent echoes from around 150 km altitude. These echoes almost invariably start at ~170 km after sunrise, then descend to 130 km by noon, before rising again to 170 km at sunset and then disappearing overnight. This daily pattern motivated kinetic simulations showing that photoelectron driven instabilities can produce echo with characteristics matching 150 km echoes ( Oppenheim and Dimant , 2016). Those simulations, however, did not correctly account for the inelastic scattering processes from N₂ that create a reduced population of photoelectrons centered at 2.5eV. This research presents improved simulations with a new inelastic and velocity dependent collision algorithm. Using this new algorithm, large-scale particle-in-cell simulations explore the importance of the inelastic N₂ collisions. It also examines the effects of photoelectrons generated near 20 eV resulting from solar Helium EUV spectral lines. Current theory argues that the inclusion of multiple photoelectron distributions can either stabilize or destabilize the electron cyclotron instability perpendicular to B, depending on where the energy peaks exist. This should enable researchers to better understand the source and structure of 150 km echoes.