Contribution of the partial ring current to the SYMH index during magnetic storms

Abstract

[1] To identify which magnetospheric current system contributes the most to the SYMH index during magnetic storms, we performed a statistical analysis of 299 magnetic storms from 1996 to 2006 and investigated the distribution of the H depressions with magnetic local times (MLT), by using data from 25 geomagnetic stations distributed almost uniformly in magnetic longitudes over magnetic latitudes ranging from 9° to 45°. As expected, a significant dawn-dusk asymmetry of H depression during the storm main phase and early recovery phase reveals the importance of the partial ring current during magnetic storm processes. The location, evolution, and quantitative contribution of the partial ring current during magnetic storms with different intensities are all further obtained. The partial ring current locates in the dusk sector, peaking in 18:00∼20:00 MLT. It forms in the early main phase, increases gradually until the SYMH index reaches its minimum, and then quickly decreases in the recovery phase. The contribution of the partial ring current weakens gradually as the storm intensity increases. For moderate (−100 < SYMH ≤ −50 nT) and intense (−300 < SYMH ≤ −100 nT) storms, the partial ring current is the predominant contributor during the main phase. However, the partial ring current is no longer the predominant contributor for the super storms with SYMH ≤ −300 nT, which may suggest the saturation of the partial ring current under extreme solar wind conditions.