We use a bounce averaged drift model with realistic electromagnetic fields together with magnetic field and electron data obtained by CRRES to study energetic electron distributions during the growth phase of an isolated substorm on December 12, 1990. The magnetic field model includes the actual time evolution of the geomagnetic field as measured by CRRES. The inductive electric field caused by the time evolution of the magnetic field configuration is included in the drift model to consider fully electromagnetic fields. The drift motion is computed for all pitch angles and for the entire energy range covered by the medium‐energy spectrometer on CRRES. By using the Liouville theorem we are able to map electron distributions from orbit to orbit to model their time evolution in the model fields. To test the model predictions, we examine the substorm growth phase on December 12, 1990: A quiet period of about 20 hours preceded the growth phase that led to the expansion phase of a 500‐nT substorm. The outer belt energetic electron distributions showed a clear development of magnetic field‐aligned pitch angle anisotropy. This period was covered by two CRRES orbits, 339 and 340. During orbit 339, CRRES measured a quiet time distribution of energetic electrons. During 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. We conclude that pitch angle dependent energization of the drifting electrons caused by the inductive electric field plays an essential role in development of the outer belt electron distributions during the substorm growth phase.
