Lower hybrid cavities in the inner magnetosphere

Abstract

We present observations of lower hybrid cavities from altitudes above 10,000 km, using data from Viking and Cluster satellites. Lower hybrid cavities (LHCs) are narrow (ion gyroradius scale) density depletions with enhanced amplitude of waves in the lower hybrid frequency range, previously reported below 1750 km altitude by the Freja satellite and several sounding rockets. Detailed analysis of a Cluster event shows the same kind of rotating wave structure as previously observed on sounding rockets, with a change of sense of rotation at the lower hybrid frequency. The scale size of the structures are of the same order as the ion gyro radius, in similarity with the ionospheric observations. There are too few Cluster events yet to enable any occurrence statistics, but the distribution of the Viking LHCs are consistent with Freja results from the topside ionosphere.