Energetic electron precipitation during a magnetospheric substorm and its relationship to wave particle interaction

Energetic electron precipitation was observed via X ray measurements from three simultaneously flown balloons in a region conjugate to the location of the geosynchronous satellite GEOS 2. Data from the particle, wave, and field experiments on this satellite were used to investigate the relationship between magnetospheric processes in the equatorial plane and the electron precipitation. A large part of the electron precipitation during a substorm on July 3, 1979, was accompanied by VLF waves at whistler frequencies and had a temporal structure grossly similar to the intensity variation of the equatorial electron fluxes. Some precipitation also occurred in the absence of VLF waves. During the wave associated precipitation the equatorial electrons were anisotropically distributed with maximum fluxes perpendicular to the geomagnetic field. The anisotropy was larger than the critical anisotropy and thus enabled wave growth. The growth rate itself was determined numerically from the observations and turned out to have a frequency dependence similar to that of the observed waves. The interaction of the electrons with the waves led to strong pitch angle diffusion. The temporal variations of the electron flux in the loss cone were similar to those of the electron precipitation observed with a balloon near the conjugate point to GEOS 2. It is concluded that most of the electron precipitation during this substorm was caused by wave particle interaction