A new interpretation of VLF chorus

It is suggested that the auroral-zone chorus is generated by an electrostatic loss-cone instability associated with ring current protons. A dispersion relation is derived for the loss-cone instability in a tenuous cold plasma of the upper ionosphere and a much less dense ring current plasma. It is shown that only harmonics of the ion gyrofrequency are excited and that the lowest order of the allowed harmonics is the most strongly excited. The observed dispersion in chorus emissions may be due to the convective instability propagating into regions of greater magnetic field strength. The wave is capable of accelerating electrons to energies of tens of kev, so that the processess responsible for chorus emission may also represent an accelerating mechanism for energetic electrons. It is suggested that the morphology of chorus is controlled by time-varying magnetospheric electric fields and the region of injection of ring-current particles into the magnetosphere.