The relationship of storms and substorms determined from mid-latitude ground-based magnetic maps

Abstract

Using data from a worldwide chain of mid- and low-latitude magnetic observatories, we construct maps of the magnetic disturbance field that display the temporal and spatial development of the storm time ring current, partial ring current and the substorm current wedge. While there is significant variability between storms, in general, the ring current produces an asymmetric depression in the northward magnetic field that becomes symmetric in the late recovery phase. The initial depression, attributed to a partial ring current, is largest on the afternoon and night side. Magnetic substorms generally produce an enhancement in the northward magnetic field within a limited longitudinal region corresponding to the location of the substorm current wedge. From the mid-latitude maps of the magnetic disturbance we can measure the magnitude, location, and extent of the substorm expansion phase disturbance. We find that large substorm expansions are generally associated with an enhancement of the dusk-side partial ring current, but they appear superposed upon an independently developing ring current. This is consistent with recent model simulations using the Michigan Ring current Atmosphere interaction Model (RAM). Growth of the ring current appears more related to the enhancement of the cross-magnetospheric electric field than to individual substorm expansions.