A cyclotron resonance model of VLF chorus emissions detected during electron microburst precipitation

Abstract

VLF chorus, consisting of narrowband rising frequency emissions, has often been observed in association with microburst electron precipitation. We present the first simultaneous rocket observations of these two phenomena, with emphasis on understanding the source of the VLF emissions. The rocket experiment was launched on May 6, 1993, from Poker Flat, Alaska (L = 5.6). In this work, the observed 1–4 kHz chorus emissions are interpreted in terms of a cyclotron resonance interaction. The frequency range of the risers and the observed electron energy range agree with those required for this interaction. Using a criterion derived from the conservation of energy during an interaction, it is shown that a cold plasma cyclotron resonance interaction can produce the lower‐frequency portions of the observed chorus risers, from ≈1000 Hz to ≈2500 Hz, while a warm plasma model is required to produce frequencies > 2500 Hz. The warm plasma model assumes a two‐component plasma, with an isotropic cold component and a bi‐Maxwellian warm component. The effect of the warm component is to change the wave dispersion relation, allowing the production of the higher‐frequency risers. A portion of the anisotropy required to produce the high‐frequency emissions can also be provided by a loss cone distribution. The chorus source is estimated from this cyclotron resonance theory to be located near the equatorial plane.