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Abstract:
High-resolution thermospheric mass density low Earth orbit (LEO)-based measurements are valuable for accurately estimating short-term atmospheric abrupt disturbances triggered by solar flux forcing. To investigate the enhancing status of solar cycle 25 between August 29 and December 31, 2020, we processed Gravity Recovery and Climate Experiment Follow-on (GRACE-FO) 10-s accelerometer-derived thermospheric mass density (TMD) measurements normalized at 500 km altitude by the NRLMSIS-2.0 empirical model. These 4-month enhancing disturbance observations suggest a shift from relative quiescence to a much more active solar phase, revealing unexpected dependencies on temporal and spatial characteristics. The results indicated that the dominant driver is solar extreme ultraviolet radiation (EUV) during this ascending phase. Density enhancement was symmetric in both hemispheres around the autumn equinox. After the equinox, the neutral density enhancement intensity in the Southern Hemisphere surpasses that in the Northern Hemisphere. Density maxima occurred from high to low latitudes, accompanied by a 2–3 h time lag. The Wygant function was applied to model the response to solar wind geomagnetic field changes and quantify the impact of geomagnetic activities on upper atmosphere density, verifying the time lag of density disturbances. All these findings could potentially improve our understanding of the solar cycle and LEO orbital drag.
