Modeling the day-to-day variability of midnight Equatorial Plasma Bubbles with SAMI3/SD-WACCM-X

It is well-known that equatorial plasma bubbles (EPBs) are highly correlated to the post-sunset rise of the ionosphere on a climatological basis. However, when proceeding to the daily EPB development, what controls the day-to-day/longitudinal variability of EPBs remains a puzzle. In this study, we investigate the underlying physics responsible for the day-to-day/longitudinal variability of EPBs using the Sami3 is A Model of the Ionosphere (SAMI3) and the Specified Dynamics Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (SD-WACCM-X). Simulation results on October 20, 22, and 24, 2020 were presented. SAMI3/SD-WACCM-X self-consistently generated midnight EPBs on October 20 and 24, displaying irregular and regular spatial distributions, respectively. However, EPBs are absent on October 22. We investigate the role of gravity waves on upwelling growth and EPB development and discuss how gravity waves contribute to the distributions of EPBs. We found the westward wind associated with solar terminator waves and gravity waves induces polarization electric fields that map to the equatorial ionosphere from higher latitudes, resulting in midnight vertical drift enhancement and retrograde plasma flow. The upward vertical drift and retrograde flow further lead to shear flow instability and midnight plasma vortex, creating background conditions identical to the post-sunset ionosphere. This provides conditions favorable for the upwelling growth and EPB development. The converging and diverging winds associated with solar terminator waves and midnight temperature maximum also affect the longitudinal distribution of EPBs. The absence of EPBs on October 22 is related to the weak westward wind associated with solar terminator waves.