Geomagnetic storms caused by shocks and ICMEs

Abstract

We performed an event‐by‐event study of 47 geomagnetic storms (GSs) that occurred during the ascending phase of solar cycle 23. All the GSs are associated with the passage of a shock and an interplanetary coronal mass ejection (ICME). For each event, we identified the section in the interplanetary (IP) medium causing the GS (the sheath behind the shock, the main body of the ICME or the combination of both). On average, the most intense GSs are caused by sheaths, followed by sheath‐ICME combinations and by ICMEs. We obtained the correlation coefficients between the intensity of each GS (minimum Dst) and several solar wind parameters. We found that the well‐known correlation between the GS intensity and the solar wind convected electric field, Ey, stands for the GSs caused by ICMEs (CC = −0.88) and sheath‐ICME combinations (CC = −0.95), but it is very low for the GSs caused by sheaths (CC = −0.44). In contrast, we found a very good correlation between the GSs caused by sheaths and the total convected electric field (ΣEy) (CC = −0.89). On the other hand, we estimated the total perpendicular pressure (Pt) for each IP event associated with the GSs and identified the three different types of Pt profiles. The most intense GSs are related with IP events with Pt = 1, but moderate and less intense storms are associated with the three Pt profiles. The correlations between the Dst and the solar wind parameters results that the CCs decrease significantly for IP events having a Pt profile of 3.