MAGNETIC-FIELD CHANGES AT THE NEUTRAL SHEET ASSOCIATED WITH SUBSTORM EXPANSION ONSET - A MODEL PREDICTION AND OBSERVATIONS

Using a simple numerical model of the magnetotail current system, subs torm-associated changes in the large-scale distribution of the neutral sheet current and the corresponding magnetic field at the neutral she et are examined. In the current model it is assumed that the radial gr adient of the neutral sheet current at the late expansion phase of sub storms is steeper than that just prior to the expansion onset. It is s hown that our current model can successfully reproduce the observed ch aracteristics of the so-called magnetic field dipolarization in the ne ar-Earth magnetotail. Comparing various satellite observations with wh at 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 magneto spheric conditions: quiet times, just prior to substorm expansion onse t, and near the end of the expansion phase (or the beginning of the re covery phase) of substorms.