Spatial profile of magnetic field in the near‐Earth plasma sheet prior to dipolarization by THEMIS: Feature of minimum B

Abstract

We present THEMIS observations of the near‐Earth plasma sheet that permit us to assess the geometrical structure of the magnetotail prior to dipolarization. Latitudinal profiles of the magnetic field were obtained by five spacecraft simultaneously around the magnetic equator at a distance of X GSM ∼ −10 R E in the premidnight for two events. It is found that the strength of normal magnetic field B z was increased with a distance from the magnetic equator, which differs considerably from the standard magnetic field model. Instead, the observation can be explained by a magnetic field model that has a minimum in the equatorial field strength (minimum B), as required from the force‐balanced magnetotail model with steady earthward convection. Moreover, the analyses showed that the feature of minimum B sustained continuously for ∼20 min before dipolarization onsets.