Electron pitch angle distributions following the dipolarization phase of a substorm: Interball‐Tail observations and modeling

We investigate Interball‐Tail observations of electron pitch angle distributions after the dipolarization phase of a substorm. For 10 keV electrons we observe beamlike, coniclike, and perpendicularly peaked distributions at L ∼ 11, L ∼ 9, and L ∼ 7, respectively. We examine the efficiency of betatron heating and Fermi acceleration associated with adiabatic transport of the electrons during the substorm dipolarization phase. This dipolarization phase was modeled using transition between different K p levels within a realistic magnetic field model. The calculations reproduce well the evolution of the high‐energy electron flux in the parallel and perpendicular directions. They also reproduce well the pitch angle distribution observed by Interball‐Tail at 10 keV, after the dipolarization phase. It is shown that Fermi acceleration is the leading process, compared to betatron heating. The production of the coniclike distributions is narrowly linked to the existence of a transition region between dipolelike and taillike magnetic fields, at about L ∼ 9.