Variations of upper thermospheric scale height based on neutral mass density measurements from coplanar low-Earth-orbit satellites

The mass density scale height in the upper atmosphere gives the vertical distance over which the neutral mass density decreases by a factor of e (the base of natural logarithms). The change in scale height may depend on changes in neutral temperature and/or composition. This study uses simultaneous measurements of neutral mass density from coplanar low-Earth-orbit satellites to derive the neutral mass density scale height and analyses the variations of the scale height during quiet time and geomagnetic storms. The coplanar events are found in the satellite missions from 2014 to 2023, including Swarm, GRACE, and GRACE-FO.

Our study shows several interesting findings. During geomagnetic storms, the scale heights are increased significantly (by up to 15 km), probably mainly due to increased upper thermospheric temperature. The increase in scale height depends on latitude, local time, and season. In the summer hemisphere and on the dayside, the upper thermospheric temperature (or exospheric temperature) can be estimated by assuming the dominant composition of the neutrals is the atomic oxygen at the LEO satellite altitudes. Additionally, during quiet time, the semi-diurnal tides are revealed in neutral mass density scale height. The results provide strong evidence of the propagation of the atmospheric tides from below to the topside ionosphere-thermosphere, which affects the upper thermospheric temperature and/or composition. This is also a new way for atmospheric tidal diagnostics based on LEO satellite measurements.