Electron Dropout Events Caused by Low-Density Coronal Mass Ejections (CMEs) and Shocks Within CMEs

Abstract:

Interplanetary shocks are a well-known cause of intense geomagnetic effects, including storms and electron dropout. Here, we discuss how interplanetary shocks which are overtaking a previous coronal mass ejection (CME) may cause especially severe geomagnetic effects at Earth, even for weak shocks. We show two examples, one from solar cycle 23 (6 August 1998) with good coverage in the magnetosheath and one from the recent solar cycle with RBSP measurements (30 September 2012). For these events, the simultaneous dynamic pressure enhancement and increase in the southward component of the magnetic field resulted in a large earthward retreat of Earth's magnetopause, causing large electron dropout in the outer radiation belt due to magnetopause shadowing.

We also present for comparison a more typical event for which an interplanetary shock is followed by a low-density magnetic ejecta in January 2013. Combining interplanetary, magnetosheath, outer magnetosphere and radiation belt measurements by more than ten satellites, including RBSP, THEMIS and Cluster, we show how a period of extremely low Mach number during the passage of the magnetic cloud resulted in dramatic losses in the radiation belt and a large-scale reorganization of the entire day-side magnetosphere.