A generation mechanism of narrow band hiss emissions above one half the electron cyclotron frequency in the outer magnetosphere

Abstract

A mechanism of narrow band VLF hiss emissions above the local half cyclotron frequency observed in the outer magnetosphere by satellites is proposed. The proposed mechanism is a kind of Harris type electrostatic instability for whistler mode waves with short wavelengths and large wave normal angles in a bi‐Maxwellian hot plasma. The free energy of the instability originates from the anisotropy of the plasma whose temperature is as low as several electron volts. This wave mode has both an electrostatic and an electromagnetic nature. The growth rate of this instability and the frequency limit, below which the waves grow, are strictly dependent on the anisotropy factor A of the plasma. A type of narrow band hiss is considered to be generated in a region where A is maximum. When this hiss propagates from the source, the band of frequency becomes narrower because the upper limit frequency decreases due to a decrease of A. Extremely narrow band hiss observed by the satellite OGO 5 can be explained by a combination of the above instability and propagation effects.