Effects of Geometries and Substructures of ICMEs on Geomagnetic Storms

Abstract

To better understand geomagnetic storm generations by ICMEs, we consider the effect of substructures (magnetic cloud, MC, and sheath) and geometries (impact location of flux-rope at the Earth) of the ICMEs. We apply the toroidal magnetic flux-rope model to 59 CDAW CME–ICME pairs to identify their substructures and geometries, and select 20 MC-associated and five sheath-associated storm events. We investigate the relationship between the storm strength indicated by minimum Dst index $(\mathrm{Dst}_{\mathrm{min}})$ and solar wind conditions related to a southward magnetic field. We find that all slopes of linear regression lines for sheath-storm events are steeper (${\geq}\,1.4$) than those of the MC-storm events in the relationship between $\mathrm{Dst}_{\mathrm{min}}$ and solar wind conditions, implying that the efficiency of sheath for the process of geomagnetic storm generations is higher than that of MC. These results suggest that different general solar wind conditions (sheaths have a higher density, dynamic and thermal pressures with a higher fluctuation of the parameters and higher magnetic fields than MCs) have different impact on storm generation. Regarding the geometric encounter of ICMEs, 100% (2/2) of major storms ($\mathrm{Dst}_{\mathrm{min}} \leq -100~\mbox{nT}$) occur in the regions at negative $P_{Y}$ (relative position of the Earth trajectory from the ICME axis in the $Y$ component of the GSE coordinate) when the eastern flanks of ICMEs encounter the Earth. We find similar statistical trends in solar wind conditions, suggesting that the dependence of geomagnetic storms on 3D ICME–Earth impact geometries is caused by asymmetric distributions of the geoeffective solar wind conditions. For western flank events, 80% (4/5) of the major storms occur in positive $P_{Y}$ regions, while intense geoeffective solar wind conditions are not located in the positive $P_{Y}$. These results suggest that the strength of geomagnetic storms depends on ICME–Earth impact geometries as they determine the solar wind conditions at Earth.