Energy dependent pitch angle distributions of auroral primary electrons

Abstract

The acceleration of auroral primary electrons by dc parallel electric field is often questioned on the ground that the pitch angle distribution of the primary electrons shows a complex behavior; the relatively low energy electrons are found to be field-aligned while the relatively energetic ones are found to be “isotropized” over all pitch angles directed into the atmosphere. In the central regions of arcs or potential structures the field-aligned and the “isotropized” components are found simultaneously. On the other hand, near the arc edges, where electrons have relatively low energies, the field-aligned distributions prevail. It is shown here that double-layer/parallel-electric field acceleration and the subsequent electron-beam plasma interactions involving Cerenkov and anomalous cyclotron resonances yield pitch angle distributions as observed from rockets and satellites. It is highlighted that the electron acceleration by weak parallel electric fields forming a runaway electron tail is limited to a critical parallel energy determined by the anomalous cyclotron resonance. On the other hand, with acceleration by a localized parallel electric field, like that in a double layer, such a limitation does not occur. The beam-plasma interactions after the acceleration by a strong double layer reshape the electron distribution function. During the early stage of the interaction an extended plateau forms. The electrons with parallel energies above the critical energy undergo anomalous cyclotron resonance and thereby their parallel energy is partly transferred to their perpendicular energy. This causes the plateau to shrink towards the critical energy, and also a pitch angle scattering of the electrons leading towards isotropization. The extent of isotropization depends on the time that the electrons spend in the interaction zone. The electrons having parallel energies less than the critical energy are likely to remain field aligned.