QUASI-STATIC EVOLUTION OF THE MAGNETOSPHERE - THE SUBSTORM GROWTH-PHASE

The growth phase of a substorm is marked by ''energy loading'' in the magnetosphere prior to energy dissipation in the substorm expansion ph ase. This loading takes place primarily in the geomagnetic tail and is identified by the growth of total magnetic flux in the tail lobes, th e increase of magnetic field intensity in the near-Earth portion of th e tail, and the field line stretching leading to thinning of the plasm a sheet in that region where a ''neck'' is thus formed. The last two p rocesses are shown here to be natural consequences of the magnetic flu x accumulation by considering theoretically a quasi-static evolution o f tail equilibrium. One of the model inferences in the growth phase is a strong diminishment of the dimension of the transition region in th e nightside where field lines change from dipolar to tail-like. This a ccounts for the growth phase development often seen in the near-synchr onous region. This study also reveals a qualitative difference between the growth phase and the ''quiet'' condition or the ''ground state'': the tail lobe flux should exceed a certain threshold for the neck for mation to occur and the size of the transition region to diminish.