Impact of Solar Radiation Pressure, Coriolis Force, and Exobase Conditions on Terrestrial Exospheric Density: MATE Simulation Results

Abstract

We improved the Model for Analyzing the Terrestrial Exosphere (MATE), a kinetic exosphere model, by incorporating solar radiation pressure and Coriolis force arising from the Earth's revolution around the Sun, in addition to Earth's gravity, to investigate their impacts on the terrestrial exospheric hydrogen density. Under constant and uniform exobase conditions, we examined the governing force effects. Solar radiation pressure enhances the exospheric density and induces a day‐night asymmetry that increases with radial distance, whereas the Coriolis force has a minor effect (<10%). In separate simulations, we applied globally averaged exobase conditions derived from NRLMSIS‐2.0, covering a realistic solar activity range (F10.7 = 70–340). The density response to solar activity exhibits a distinct radial dependence. At 2 R _E , the density peaks during solar minimum and reaches its minimum during solar maximum, closely following the exobase hydrogen density. In contrast, beyond 6 R _E , the density is lowest during solar minimum and peaks at moderate solar activity (F10.7 ∼ 160), suggesting that the exobase temperature plays a dominant role in controlling the outer exosphere. Lastly, we simulated one synodic solar rotation during the solar minimum using time‐varying and non‐uniform MSIS exobase conditions. The resulting day‐night asymmetry is weaker than in the uniform exobase case because the dayside solar EUV heating increases the hydrogen population reaching higher altitudes, partially compensating for the asymmetry produced by solar radiation pressure. Although geomagnetic activity remained quiet, a weak disturbance (Ap∼11) enhanced the exospheric density by up to 21%, highlighting geomagnetic activity as an important driver of short‐term exospheric variability.