Relation between energetic electrons, protons, and the thermal plasma sheet population: Plasma sheet recovery events

Abstract

We have investigated thermal, low‐energy suprathermal, and high‐energy protons and electrons in the plasma sheet near local midnight at geocentric distances of 14–20 RE during five different plasma sheet penetrations. The data have been obtained with the Los Alamos National Laboratory/Max‐Planck‐Institut fast plasma analyzer and the Max‐Planck‐Institut/University of Maryland energetic particle sensor system on ISEE 1. It is found that the intensity time profile of suprathermal protons up to ∼150 keV follows closely the intensity profile of the thermal proton population. The protons with energies ≳200 keV exhibit a typical fast rise‐slow decay profile; the decay time constant decreases with increasing energy. When entering the plasma sheet from the lobe the high‐energy protons are delayed in relation to the thermal and low‐energy suprathermal protons. Energetic electrons do not follow the energetic proton profile. They reach their maximum about 30 min after the appearance of the plasma sheet as determined by the thermal electrons. After the energetic electrons have reached their maximum intensity, their intensity time profile follows closely any changes in the electron temperature of the thermal population. The observations are discussed in terms of various models. In particular, acceleration of the ions at a tailward retreating neutral line during recovery can consistently explain the ion observations. The acceleration mechanism for energetic electrons is unclear, and it is argued that they may be accelerated by a stochastic process out of the thermal population.