Analysis of a severe geomagnetic storm on August 26, 2018 and the related effects on the GRACE-FO mission

On August 20, 2018 a complex interplanetary coronal mass ejections (ICME) occurred on the Sun, which subsequently triggered an unexpected large geomagnetic storm on August 25. We present a detailed analysis of the ICME eruption and explore the occurred perturbation of the neutral mass density in the upper Earth's atmosphere. The analysis is based on accelerometer observations from the satellite mission GRACE Follow-On as well as interplanetary magnetic field measurements by the DSCOVR and ACE spacecraft. Through the evaluation of solar observations by the SECCHI instrument on-board of the STEREO-A satellite in form of white-light, the early evolution of the ICME can be aptly illustrated. Furthermore, due to the heating and the subsequent expansion of the thermosphere also the drag force acting on the spacecraft is enhanced. This leads to an additional storm induced orbit decay, which we calculate by means of variations in the semi-major axis. The findings are compared with predictions from our preliminary thermospheric forecasting tool, which is based on the study by Krauss et al. 2018.