Trapped electron distributions on open magnetic field lines

In a previous paper on the September 8, 1978, magnetopause crossing by the ISEE spacecraft, observations of trapped-like electron distributions coexisting with streaming ion distributions were reported. These observations were made for particles in the energy range >20 keV, inside the magnetopause, and near a possible reconnection event. In this paper we have investigated the idea that the electrons could be trapped by magnetic mirroring from the field in the magnetosheath. The loss cone angle predicted from the instantaneously measured magnetic field and the observed maximum field in the magnetosheath compares very favorably with the observed loss cone angles. The measured loss cone is always slightly wider than the predicted one, which can result from superimposing the magnetospheric source distribution onto the loss cone, but follows the magnetic field strength variations as the predicted value does. We argue that the ions are not similarly reflected because they fail to meet the adiabatic condition during the passage through the magnetopause. Having thus resolved the conflicting predictions about field line topology based on electrons and ions, we can be much more confident that the September 8, 1978, crossing of the magnetopause by ISEE is indeed a reconnection event