Phase Correlation of Electrons and Langmuir Waves

Abstract

Multiple spacecraft observations have confirmed the ubiquitous nature of Langmuir waves in the presence of auroral electrons. The electrons show variations consistent with bunching at or near the plasma frequency. Linear analysis of the interaction of a finite Gaussian packet of Langmuir waves shows that there are two components to the perturbation to the electron distribution function, one in-phase (or 180° out-of-phase) with respect to the wave electric field called the resistive component and one which is 90° (or 270°) out-of-phase with respect to the electric field. For small wave packets, the resistive perturbation dominates. For longer wave packets, a non-linear analysis is appropriate which suggests that the electrons become trapped and the reactive phase dominates. Rocket observations have measured both components. The UI observations differ from those of the UC Berkeley observations in that a purely reactive phase bunching was observed as compared to a predominantly resistive perturbation. The resistive phase results of the UC Berkeley group were interpreted as arising from a short wave packet. The UI observations of the reactive phase can be explained by either a long, coherent train of Langmuir waves or that the narrower velocity response of the UI detectors made it possible to capture only one side of the reactive component of the perturbed distribution function for a short wave packet in the linear regime. Future wave-particle correlator experiments should be able to resolve these questions by providing more examples with better velocity space coverage.