Magnetic field changes at the neutral sheet associated with substorm expansion onset: A model prediction and observations

Abstract

Using a simple numerical model of the magnetotail current system, substorm‐associated changes in the large‐scale distribution of the neutral sheet current and the corresponding magnetic field at the neutral sheet are examined. In the current model it is assumed that the radial gradient of the neutral sheet current at the late expansion phase of substorms is steeper than that just prior to the expansion onset. It is shown that our current model can successfully reproduce the observed characteristics of the so‐called magnetic field dipolarization in the near‐Earth magnetotail. Comparing various satellite observations with what our current model predicts, the location of the substorm initiation and the propagation of the magnetic field dipolarization is discussed. It is discussed that there are at least three distinct types of the radial distribution of the neutral sheet current, depending on magnetospheric conditions: quiet times, just prior to substorm expansion onset, and near the end of the expansion phase (or the beginning of the recovery phase) of substorms.