Geomagnetic Response to a Chain of Interplanetary Coronal Mass Ejections

The most geoeffective storms in the Space Age have been driven solely by the sheath preceding an interplanetary coronal mass ejection (ICME) or by a combination of the sheath and an ICME magnetic cloud. In the present study, the magnetospheric response to a chain of three independent and well-spaced ICMEs (P_dyn > 15 nPa and Sym-H < -50 nT) spanning one month (December 12, 2015 – January 12, 2016) is investigated using WIND, Cluster, and MMS fields and plasma measurements. The first of the three ICMEs consists of a sheath preceding an ICME (ICME-SH). The latter two ICMEs are preceded by a combination of the sheath and an ICME magnetic cloud (ICME-SH-MC).

Following the passage of the first ICMEs (ICME-SH) the interplanetary environment was made up of moderate Alfvenic Mach number (M_A ~ 10) and average magnetopause standoff distance (R_MP ~ 11 R_E). The arrival of the ICME-SH-MC then initiated a sudden storm commencement (SSC) phase. During the SSC, the storm index (Sym-H ~ +50 nT) remained positive through the ICME shock and sheath regions. The storm index and the Alfvenic Mach number sharply declined (Sym-H~ -200 nT and M_A ~ 1.0, respectively) with the arrival of the leading edge of the magnetic cloud (B_{IMF, core} ~ 20 nT) and the associated sharp IMF B_z reversal (B_z<0). The Alfvenic Mach number and IMF B_z are found to directly correlate with the Sym-H index. ICME-SH-MC compressed the magnetopause standoff distance (∂R_MP/∂t ~ -1 R_E/min), resulting in a sudden reduction in the total magnetospheric volume (∂V_MP/∂t ~ -3×10² R_E³/min), as determined by cross-scale observations. In particular, the sharp drops in the magnetospheric volume (relative change in volume >30%) with the arrival of each of the three independent ICMEs are shown to start with the SSC and remain low through the main phase, before slowly recovering (∂V_MP/∂t ~ +1 R_E³/min) to the pre-ICME conditions during the recovery phase.