Time-dependent modeling of particles and electromagnetic fields during thesubstorm growth phase: Anisotropy of energetic electrons

We use a bounce-averaged drift model with realistic electromagnetic fields together with magnetic field and electron data obtained by CRRES to study e nergetic electron distributions during the growth phase of an isolated subs torm on December 12, 1990. The magnetic field model includes the actual tim e evolution of the geomagnetic field as measured by CRRES. The inductive el ectric field caused by the time evolution of the magnetic field configurati on is included in the drift model to consider fully electromagnetic fields. The drift motion is computed for all pitch angles and for the entire energ y range covered by the medium-energy spectrometer on CRRES. By using the Li ouville theorem we are able to map electron distributions from orbit to orb it to model their time evolution in the model fields. To test the model pre dictions, we examine the substorm growth phase on December 12, 1990: A quie t period of about 20 hours preceded the growth phase that led to the expans ion phase of a 500-nT substorm. The outer belt energetic electron distribut ions showed a clear development of magnetic field-aligned pitch angle aniso tropy. This period was covered by two CRRES orbits, 339 and 340. During orb it 339, CRRES measured a quiet time distribution of energetic electrons. Du ring orbit 340 the substorm onset was seen as a rapid dipolarization of the magnetic field and by a dispersionless electron injection. The quiet time fluxes were used as initial conditions for the model for fluxes during the growth phase. conclude that pitch angle dependent energization of the drift ing electrons caused by the inductive electric field plays an essential rol e in development of the outer belt electron distributions during the substo rm growth phase.