The magnetic field of the equatorial magnetotail: AMPTE/CCE observations at R < 8.8 RE

Abstract

The Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) magnetic field experiment has provided extensive measurements of the vector field near the equatorial plane in the inner magnetotail. Taillike distortions from the dipole field direction may reach 80° near the AMPTE/CCE apogee of 8.8 RE. The Bz (north‐south) field component in dipole coordinates was always positive within 0.5 RE of the equatorial current sheet, indicating that neutral lines were never seen inside of 8.8 RE. South of −0.5 RE, Bz values are sometimes southward due to extremely taillike fields and the southward tilt of the tail in dipole coordinates during northern hemisphere winter. Fields are most taillike near midnight and during times of high Kp. At 8.5 RE the equatorial field magnitude depressions are roughly half the dipole field strength of 51 nT. These depressions are larger at lesser distances reaching −40 nT at 3.4 RE for Kp ≤ 2‐ and −80 nT for Kp ≥ 3+. The equatorial crossing points of midnight field lines are computed using the observed field depressions. Mapping auroral boundaries along these distorted field lines indicates that most auroral arcs map to magnetospheric locations beyond 12 RE, but that low latitude arcs may well occur on closed field lines earthward of this distance. Evidence is found confirming that a cross‐tail field component in the inner magnetotail is related to the interplanetary sector structure but, contrary to earlier results, the effect is found to be larger during geomagnetically disturbed times.