A CYCLOTRON-RESONANCE MODEL OF VLF CHORUS EMISSIONS DETECTED DURING ELECTRON MICROBURST PRECIPITATION

VLF chorus, consisting of narrowband rising frequency emissions, has o ften been observed in association with microburst electron precipitati on. We present the first simultaneous rocket observations of these two phenomena, with emphasis on understanding the source of the VLF emiss ions. The rocket experiment was launched on May 6, 1993, from Poker Fl at, Alaska (L = 5.6). In this work, the observed 1-4 kHz chorus emissi ons 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 deri ved from the conservation of energy during an interaction, it is shown that a cold plasma cyclotron resonance interaction can produce the lo wer-frequency portions of the observed chorus risers, from approximate to 1000 Hz to approximate to 2500 Hz, while a warm plasma model is re quired to produce frequencies > 2500 Hz. The warm plasma model assumes a two-component plasma, with an isotropic cold component and a bi-Max wellian warm component. The effect of the warm component is to change the wave dispersion relation, allowing the production of the higher-fr equency risers. A portion of the anisotropy required to produce the hi gh-frequency emissions can also be provided by a loss cone distributio n. The chorus source is estimated from this cyclotron resonance theory to be located near the equatorial plane.