Simulations of photoelectron driven ionospheric waves predict many features of 150 km VHF radar echoes

150 km echoes are ionospheric daytime density perturbations observed between 130 and 160 km in altitude by VHF radars near the geomagnetic equator. Oppenheim and Dimant (2016) proposed photoelectron induced waves as a likely mechanism and Longley et al. (2020, 2021) provided a partial theoretical explanation. In the 2016 paper, the simulations showed that that photoionized electrons can drive a bump on tail instability, exciting high frequency upper hybrid modes that decay into lower frequency ion-acoustic modes that explain many aspects of the radar observations. However, this paper only presented a few small simulations and did not accurately model e- - N2 collisions above 1eV. Here, we use fully kinetic Particle-in-Cell (PIC) simulations to extend the earlier simulations to larger systems in order to investigate the theoretical work by Longley et al.. Specifically, we will study the upper hybrid resonance condition presented as the driving mechanism. We also investigate wave mode coupling as a mechanism for exciting 150 km echoes. These simulations reveal enhancement in the upper hybrid and ion acoustic modes, as well as significant differences based on a resonance condition that could account for the structure of the echoes.