Direct Observational Evidence of Solar Wind Density Controlling the Evolution of the Ring Current During the 2024 October Major Storm

Abstract

We investigated the interplanetary source of the October 2024 major storm and determined that it was triggered by a sheath region and a magnetic cloud (MC), with the sheath region playing a decisive role. The MC has a larger and longer duration of southward interplanetary magnetic field (IMF) and solar wind electric field compared to the sheath, and the largest southward IMF and solar wind electric field were observed within the MC. As expect, the solar wind density in the sheath region is much larger than that in the MC. The results of this study not only provide direct evidence that solar wind density controls the evolution of the ring current, but also demonstrate that the correlation coefficients between the largest southward IMF and geomagnetic storm intensity, as well as between the largest solar wind electric field and geomagnetic storm intensity, lack physical meaning. The contribution of the sheath region to the intensity of the major storm, as estimated by the empirical formula developed by Burton, McPherron, and Russell (1975) (hereafter referred to as the BMR equation), was found to be smaller than that of the MC. However, actual observations indicate the opposite. This discrepancy suggests that the BMR equation is not capable of accurately estimating the ring current variation. The injection term in the BMR equation is merely a linear function of the solar wind electric field, without considering the solar wind density. This indicates that if we overlook the influence of solar wind density on the evolution of the ring current, estimating the intensity of a geomagnetic storm based solely on the integral of the solar wind electric field during the main phase of the storm would yield incorrect results. The October 2024 major storm also provides direct evidence that solar wind velocity, density, and the southward component of the IMF are all important parameters in the evolution of the ring current.