Significance of Soft Electron Precipitation for the Neutral Upwelling in the Cusp: A High-resolution Numerical Modeling Approach

We investigate the effects of particle precipitation and thermospheric heating on the neutral mass density anomaly in the cusp. Recent satellite observations have shown that the neutral mass density around 400 km altitude in the cusp is significantly larger than that of ambient regions. The mass density enhancement is 20 % under geomagnetically quiet conditions on average, and can be over 100 % during geomagnetic storms. It is considered that soft electron precipitation and local heating in the F region play significant roles in generating the mass density anomaly. However, we still have not understood how such processes contribute to the mass density anomaly in detail. Previous modeling studies are partially successful in reproducing the mass density anomaly under geomagnetically disturbed conditions, whereas previous studies have difficulties reproducing sufficient mass density enhancements under quiet conditions. In the cusp ionosphere-thermosphere system, various processes, such as ion-neutral drag, Joule heating, heating and ionization by particle precipitation, chemical processes, and diffusion, are strongly coupled. Furthermore, the mass density anomaly has small scale features, such as kilometer-scale field-aligned currents and strong vertical dependence of heating. In order to study the mesoscale density anomaly in the cusp, we thus employ a new high-resolution numerical model including various physical and chemical processes. In contrast to previous studies, it is shown that not only Joule heating but also particle heating caused by soft electron precipitation is important for thermospheric heating. Soft electron precipitation causes ionization at F region altitudes, which increases Joule heating significantly. In addition, direct heating by soft electron precipitation can be comparable to Joule heating. We will report how thermospheric processes such as electron precipitation and Joule heating contribute to the mass density anomaly using the two-dimensional numerical model.