Statistical properties and geoefficiency of interplanetary coronal mass ejections and their sheaths during intense geomagnetic storms

Abstract

In this paper, we examine and compare the statistical properties of interplanetary coronal mass ejections (ICMEs) and their sheath regions in the near‐Earth space, mainly focusing on the distributions of various physical parameters and their geoefficiency. The 53 events studied are a subset of events responsible for intense (Dst ≤ −100 nT) geomagnetic storms during the time period from 1996 to 2005. These events all fall into the single‐type category in which each of the geomagnetic storms was caused by a well‐isolated single ICME, free of the complexity of the interaction of multiple ICMEs. For both sheaths and ICMEs, we find that the distributions of the magnetic field strength, the solar‐wind speed, the density, the proton temperature, the dynamic pressure, the plasma beta, and the Alfvén Mach number are approximately lognormal, while those of the Bz component and the Y component of the electric field are approximately Gaussian. On the average, the magnetic field strengths, the Bz components, the speeds, the densities, the proton temperatures, the dynamic pressures, the plasma betas, and the Mach numbers for the sheaths are 15, 80, 4, 60, 70, 62, 67, and 30% higher than the corresponding values for ICMEs, respectively, whereas the Y component of the electric field for the sheaths is almost 1 s of that for ICMEs. The two structures have almost equal energy transfer efficiency and comparable Newell functions, whereas they show statistically meaningful differences in the dayside reconnection rate, according to the Borovsky function.