Abstract/Summary

We study Universal Time (UT) variations in the magnetospheric response to Coronal Mass Ejection (CME) impacts, using the example of the two CMEs that led to the destruction of 38 out of 49 Starlink satellites in early February 2022. We employ the Expanding-Contracting Polar Cap model to analyse the variation in the size of the ionospheric polar caps and an eccentric dipole model of the geomagnetic field and thereby quantify the UT variations caused by the inductive effect of the diurnal motions of the geomagnetic poles in a \geocentric-solar" frame of reference (i.e., a frame with an X axis that points from the centre of the Earth to the centre of the Sun). The results show that use of a quasi-steady convection model predicts a similar global power deposition into the thermosphere as that inferred here, but does not give the same division of that power between the northern and southern hemispheres. We demonstrate that, through the combined effects of the Russell-McPherron dipole-tilt mechanism on solar-wind magnetosphere coupling and of the diurnal polar cap motions in a geocentric-solar frame, the power deposited varies signiffcantly with the arrival UT of the CMEs at Earth. We also show that in the events of early February 2022, both CMEs arrived at almost the optimum UT to cause maximum thermospheric heating.