Thermospheric mass density variations based on GRACE-FO during the ascending phase of solar cycle 25

High-resolution thermospheric mass density (TMD) measurements from Low Earth Orbit (LEO) Satellites are valuable to accurately estimate the short-term atmosphere abrupt disturbances, triggered by magnetospheric forcing. A good characterization of TMD variation ahead of the arrival geomagnetic storms can benefit LEO operations and crucial for both orbit propagation and collision avoidance. In this contribution, we will reveal the most probable feature of TMD variation during the initial stage of solar cycle 25, at the same time, we proved Wygant function as a better geomagnetic events indicator.

In this study, GRACE-FO 10s accelerometer-derived TMD measurements were employed and normalized at altitude of 505km by the NRLMSISE-00 (Naval Research Laboratory Mass Spectrometer and Incoherent Scatter Radar Exosphere 2000) empirical atmosphere model to investigate the status of solar cycle 25 between September 1 and December 31, 2020. With the high-inclination orbit global coverage, three magnetic latitude regions were separated and divided into day and nighttime using magnetic local times (MLT). 4-month enhancing disturbances observations suggest solar activities will shift from its relatively quiet condition to a much more active behavior, which reveal unexpected dependencies on the temporal and spatial characteries. Our detailed analysis shows that (1) TMD spreads from high latitudes to low latitudes and as same as time lag, (2) TMD enhancement in the Southern hemisphere is more intense than in the Northern one, reaching peak value around 15:00 MLT; geomagnetic activities cause TMD to increase up to 0.86×10^-13 kg/m³ at night side, 3.4×10^-13 kg/m³ at day side, and (3) the TMD enhancement was symmetric in both N- and S- hemispheres before the equinox. In general, thermospheric mass density analysis reveals the significant impact of solar and geomagnetic activities, providing the most relevant and probable characteristic of the TMD disturbances driven by solar wind.

Additionally, we try to use different geomagnetic indices for a complete description of geomagnetic storms and their phases. The S_10.7 index is used as a proxy for solar irradiation. These indicators show high correlation with the TMD variation during recurrent geomagnetic activities. What’s more, the cross-correlation analysis reflects a high correlation of to the Wygant function E_WAV found both at three latitude bins.

Even thought our study is considered a minor to moderate geomagnetic storm of the upcoming solar cycle 25 maximum, the high-speed stream injection into the thermosphere still caused thermosphere expansion that significantly enhanced the neutral density in the LEO environment. Therefore, all these findings provide a possibility to improve our understanding of LEO orbital drag.