Substorm currents in the equatorial magnetotail

Abstract

We have determined characteristics of magnetospheric equatorial currents during substorms from the vector magnetic field data acquired with the GOES 5 and GOES 6 satellites, separated about 1.9 hours in MLT in geosynchronous orbit. These data have been used to determine the local time (azimuthal) and radial variation of the equatorial current. The divergence of the equatorial current was computed from these variations, and systems of field‐aligned currents were deduced. During the growth phase to the maximum phase of the taillike reconfiguration of the near‐Earth magnetic field, a positive divergence (away from the equatorial plane) of the westward equatorial current occurs in the late evening to premidnight MLT sector, and a negative divergence (away from the equatorial plane) occurs in the premidnight to early morning MLT sector. The field‐aligned current associated with these divergences flows into the ionosphere in the late evening to premidnight MLT sector and flows away from the ionosphere in the premidnight to early morning MLT sector. This flow direction pattern is the same as that of the region 2 field‐aligned current system. During the expansion phase a field‐aligned current that is distinctive to the growth phase field‐aligned current is generated in the same near‐Earth plasma sheet region. The field‐aligned current flows away from the ionosphere in the late evening to premidnight MLT sector and flows into the ionosphere in the premidnight to morning MLT sector. These field‐aligned currents are due to a change in a sign of the divergence of the westward equatorial current. This flow direction pattern is same as that of the region 1 field‐aligned current system and also of the current‐wedge model. This region 1 sense field‐aligned current develops first near midnight at about 5 min after the expansion phase onset (as determined from the ground‐based magnetometer data), is delayed by 10‐25 min farther away from midnight in the evening and morning MLTs, and continues until the end of geomagnetic dipolarization at the site of either GOES 5 or GOES 6, whichever is located closer to midnight. We have also determined the presence of a radial current that flows toward the earth in the late evening to premidnight sector and flows away from the Earth in the midnight to morning sector. The intensity of the radial currents increases before the expansion phase. Consequently, the patterns of field‐aligned currents associated with various substorm phases are the superposition of currents driven by multiple sources with different temporal variations. We have identified at least three different, but related sources of field‐aligned currents during the growth and expansion phases. These sources are related to the divergence of the westward flowing equatorial current and to distributions of pressure and magnetic field gradients that evolve in the magnetotail. These patterns include the current‐wedge model during the expansion phase. When combined, these complicated systems support the basic region 1 to region 2 field‐aligned current flow pattern.