Penetration of Magnetospheric Electric Fields to the Low Latitude Ionosphere During Storm/Substorms

Penetration of the magnetospheric electric fields to low latitude ionosphere is examined using magnetometer data from high latitude to the dip equator during substorms and geomagnetic storms. To detect the penetration electric fields, we analyzed magnetic disturbances at the dip equator, subtracted by those at low latitude (DP2). During substorm growth phase, the DP2 currents are enhanced by the dawn-to-dusk convection electric field, which are supplied by the Region-1 field-aligned currents (R1 FACs) via the mid and low latitude ionosphere. On the other hand, the DP2 currents decrease significantly during the substorm expansion, superposed by reversed currents flowing from the R2 FACs. In particular, when the IMF turns northward during the substorm, the DP2 currents change to the counterelectrojet (CEJ), i.e., overshielding currents at the dip equator. During the storm main phase, the DP2 currents are very much intensified by the enhanced convection electric field. However, the DP2 currents change to the CEJ at the beginning of the storm recovery phase. The overshielding electric field as well as the convection electric field causes dramatic changes in the low latitude ionosphere.