Freja observatons of correlated small‐scale density depletions and enhanced lower hybrid waves

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Abstract
Localized density depletions filled with electrostatic waves in the lower hybrid frequency range are commonly observed by the wave instrument on the Freja satellite. We refer to these phenomena by the phenomenological name Lower Hybrid Cavities (LHCs). Typically, the amplitude of the density depletion is a few per cent of the ambient plasma density, and its width is around 50 m. The structures are identified at all magnetic latitudes we have searched (60–75 degrees), and at all local times at the satellite altitude (around 1,700 km). Clear examples are found in regions with fairly low or moderate wave activity, but not where the highest wave amplitudes are encountered. We do not investigate the detailed small‐scale correlation between LHCs and ion heating in this letter, but note that up to now, we have no LHC observations in intense ion heating regions.