This paper analyzes a possible mechanism for ion conic formation in the auroral zone that result from the wave-particle interactions between ions and VLF waves excited due to an instability triggered by precipitating electrons in magnetospheric plasma. This so-called ''fan'' instability does not require the presence of an electron beam or other type 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 instability are calculated, and the wave spectra and amplitudes are shown to be in reasonable good agreement with observations. Also, the calculated energy distribution function for the accelerated ions agrees with experimental data. The analytical results are supported by full numerical solutions of the quasilinear equations. The application of the fan instability to the formation of ion conics is discussed.