Statistical Study of Electron Bunching in Auroral Langmuir Waves

Abstract

AbstractLangmuir waves are a fundamental consequence of electron beam‐plasma interactions in space physics. Wave‐particle correlator experiments have proven to be an effective way to probe the physics of Langmuir wave growth, damping, and particle trapping, but previous experiments were limited mainly to special cases. The CHARM‐II rocket flown into active nighttime aurora included a correlator and encountered Langmuir waves along much of its trajectory. Careful consideration of the data yields 57 events in which significant correlations were detected between electrons and Langmuir wave phases. The majority of the selected correlations indicated energy exchange slightly dominant over particle trapping. Examination of the component of the correlation resulting from energy exchange reveals a striking relationship between electron beam dynamics and the nature of the wave‐particle correlation: Whenever the beam flux at the measured electron energy was increasing with time, the phase of the resistive component of the electron bunching implied energy transfer from the beam particles to the wave field, and when the electron beam flux was decreasing, the reverse occurred. Two possible explanations for this effect are the time variation in the interaction of a given energy with the waves in a beam, which is time‐varying due to dispersion effects, and short‐term enhancements in wave growth or damping on a time‐varying beam due to time‐of‐flight effects on the particle distribution. The latter mechanism is investigated and found plausible using a numerical test‐particle simulation.