MODIFIED LOWER-HYBRID FAN INSTABILITY EXCITED BY PRECIPITATING AURORAL ELECTRONS

This paper analyses a possible mechanism for ion conic formation in th e auroral zone that result from the wave-particle interactions between ions and VLF waves excited due to an instability triggered by precipi tating electrons in magnetospheric plasma. This so-called ''fan'' inst ability does not require the presence of an electron beam or other typ e of positive slope in the electron distribution function, but only an asymmetric high-energy tail on one side of the distribution function. Analytical solutions of the quasi-linear equations describing the ins tability are calculated, and the wave spectra and amplitudes are shown to be in reasonable good agreement with observations. Also, the calcu lated energy distribution function for the accelerated ions agrees wit h experimental data. The analytical results are supported by full nume rical solutions of the quasilinear equations. The application of the f an instability to the formation of ion conics is discussed.