Impact of Interplanetary Coronal Mass Ejections (ICME) on the geomagnetic tail from THEMIS observations

We focus on a well-defined interplanetary coronal mass ejection (ICME) with a dynamic pressure Pdyn > 20 nPa, in December 2015. ACE observations and OMNI data allowed to identify ahead of Earth the expected features with shock and sheath regions preceding a magnetic cloud. This ICME triggered a storm in the magnetosphere with a storm sudden commencement (SSC) phase (SYM‐H ~ +50 nT) followed by a growth phase (SYM‐H < −150 nT at the minimum) and a long recovery phase lasting several days.

We investigate the global impact of this ICME on the Earth's magnetotail from observations by the NASA mission THEMIS. Indeed we estimate the total pressure exerted on the magnetotail current sheet. We find that the current sheet is compressed to ~ >2nP in the main phase, i.e. 4 times more than in the quiet phase before the event. In contrast, the pressure gradually decreases in the recovery phase and approximately comes back towards quiet phase values. According to the tracking of magnetic field lines using the Tsyganenko T96 magnetic field model, the current sheet appears stretched right from the SSC phase, and even more than during the main phase, before returning progressively to a shape comparable to the quiet phase. We quantify and discuss these effects to provide a more precise description of the magnetospheric geomagnetic activity during solar events.