OBSERVATION OF ELECTRON BUNCHING DURING LANDAU GROWTH AND DAMPING

A high-frequency wave-particle correlator instrument has been flown for the first time on a recent auroral sounding rocket. The direct observation of electron bunching showed that large-amplitude Langmuir waves were resonant with low-energy, field-aligned electrons, roughly corresponding to expectations. The observed electron bunching, however, appeared to be in phase (or 180-degrees out of phase) with the electric field signal rather than the wave potential, indicating that an energy exchange was occurring. An elementary energy transfer analysis shows that Landau growth or damping could account for the electron bunching. A Monte Carlo test particle simulation supports this model. The simulation shows a perturbation of the electron distribution function in phase with the electric field for roughly half of a bounce period after the particles interact with the wave, followed by particle trapping in the wave potential. The latter effect was not observed, for which we offer two interpretations. Either the particle trapping did not occur due to finite frequency bandwidths and changing wavelengths, or the physical size of the wave packet was too small to establish particle trapping.