Large Scale Kinetic PIC Simulations of 150 km Echoes

150 km echoes are 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, they showed that photoionized electrons can drive a bump on tail instability, exciting high frequency upper hybrid modes that decay into the lower frequency ion-acoustic modes that may explain radar observations. However, this paper only presented a few small simulations. Here, we use fully kinetic Particle in Cell (PIC) simulations to extend to larger systems and address the upper hybrid resonance 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 the resonance condition.