Importance of Shock Compression in Enhancing ICME’s Geoeffectiveness

Abstract

Abstract Shock embedded interplanetary coronal mass ejections (ICMEs) are of great interest in the solar and heliosphere physics community due to their high potential to cause intense geomagnetic storms. In this work, 18 moderate to intense geomagnetic storms caused by shock-ICME complex structures are analyzed in order to show the importance of shock compression in enhancing ICMEs’ geoeffectiveness. Based on the characteristics of the shocks inside ICMEs, including the shock velocity, shock normal direction, and the density compression ratio, we recover the shocked part in the ICME to the uncompressed state by using a recovery model developed by Wang et al. according to the Rankine–Hugoniot relationship. Comparing the observational data and the recovered parameters, we find that the maximum southward magnetic field in the ICME is doubled and the dawn–dusk electric field is increased 2.2 times due to the shock compression. Then, the parameters of the observed and recovered solar wind and magnetic field are, respectively, introduced into various Dst prediction models. The prediction results show that, on average, the shock compression can enhance the intensity of the geomagnetic storm by a factor of 1.4. Without shock compression, the geoeffectiveness of these ICMEs would be markedly reduced. Moreover, there is a significant correlation between the shock density compression ratio and the shock’s capacity of strengthening geomagnetic storms. The larger the shock density compression ratio is, the more obvious Dst index decrease is caused.