High-Frequency Variations in Thermospheric Densities from GRACE C Data

Thermospheric density measurements are crucial for satellite operations and for understanding the dynamic behaviour of the upper atmosphere, where thousands of LEO satellites orbit. In this study, we use thermospheric densities derived from high-precision accelerometer data from GRACE-FO, provided by Delft University, sampled at 0.1 Hz (every 10 seconds). We focus on high-frequency density variations (f > 1 mHz) observed between 2018 and 2024, a period covering the solar minimum of cycle 24 and the ascending phase of cycle 25. As solar maximum approaches, the densities increase by two orders of magnitude.

After removing diurnal, semi-diurnal, and seasonal periodicities, higher-frequency disturbances become apparent, even during low geomagnetic activity (Kp = 3), with stronger signatures near the North Pole. We observe a consistent downward trend when GRACE-C is within the Earth’s shadow, while more intense disturbances occur near the solar terminator. During the ascending phase of cycle 25, strong equatorial signals also emerge. A correlation analysis with the Hp30 geomagnetic index highlights the importance of using high-cadence geomagnetic indices to capture short-term disturbances. Additionally, spectral analysis during major geomagnetic storms over the six-year period shows how thermospheric densities respond to increased geomagnetic activity at higher latitudes in both hemispheres.