Revisiting the solar wind structures contributing crucially to the main phase of the major geomagnetic storms in May 1997

Abstract We re-examined the contributions of the sheath region and the magnetic cloud to the main phase of the major geomagnetic storm on May 15, 1997. Our analysis revealed that the maximum values of the southward component of the interplanetary magnetic field (IMF) and the solar wind electric field were both located within the magnetic cloud. However, the sheath region’s contribution to the main phase of the storm was significantly greater than that of the magnetic cloud, indicating that this storm was predominantly driven by the sheath region. Interestingly, we found that the time integral of the solar wind electric field in the magnetic cloud was greater than that in the sheath region, which suggests that storm intensity is not solely determined by the time integral of the solar wind electric field. The only advantage of the sheath region over the magnetic cloud was its density, which was substantially higher in the sheath region than in the magnetic cloud. This finding underscores that solar wind density is a crucial parameter influencing the evolution of the ring current, in addition to the southward IMF and the solar wind speed.