Magnetic latitude and local time distributions of ionospheric currents during a geomagnetic storm

Abstract

In order to clarify the global distribution of ionospheric currents during a geomagnetic storm, we analyzed ground magnetic disturbances from high latitudes to the magnetic equator for the storm on September 7–8, 2002, with the minimum SYM‐H value of –168 nT. In this analysis, we investigated magnetic field deviations in the northward component from the SYM‐H, as functions of the dipole magnetic latitude (DMLAT) and the magnetic local time (MLT). During the main phase of the storm, the deviations at the low latitudes (10°−35° in DMLAT) were positive/negative in the dawn/dusk (0–9/11–24 h MLT) sector. On the other hand, the deviations at the dayside middle latitudes (35°–55° in DMLAT) were negative/positive in the morning/afternoon (6–12/13–15 h MLT) sector. The local time distribution at the low latitudes may represent the dawn‐dusk asymmetry of the storm time ring current, while that at the dayside middle latitudes coincides with the DP2 currents due to the convection electric field associated with the Region 1 field‐aligned currents (R1 FACs). All over the nightside middle latitude, the deviations were positive. This implies the direct effect of the R1 FACs through the Biot‐Savart's law. At the geomagnetic equator, the eastward and westward electrojets were intensified on the day and nightside, respectively, being caused by the penetrated dawn‐to‐dusk convection electric field. We found that the MLT distribution of the magnetic deviations during the recovery phase was in opposite sense to that during the main phase at the dayside middle latitudes. The reversed magnetic disturbances must be due to the overshielding electric field associated with the Region 2 field‐aligned currents (R2 FACs). Similarly, the deviations at the dayside and nightside equator were reversed, indicating penetration of the dusk‐to‐dawn overshielding electric field into the equatorial ionosphere. Based on the above results, we propose a current system including the ionospheric currents at middle latitudes caused by the R1/R2 FACs, equatorial EEJ/CEJ, and asymmetric ring current, during the main/recovery phase of the geomagnetic storm.