Lower hybrid ion heating cavities in the auroral ionosphere

Summary form only given. Lower hybrid cavities (LHCs) are localized, density-depleted regions of enhanced VLF wave amplitude found within regions of VLF hiss. They can cause ion heating to the level of a few eV in the direction transverse to the geomagnetic field B0. Statistical studies indicate that the cavities are cylindrical or ellipsoidal in shape, are aligned with B/sub 0/, and have diameters of order 20-50 m. There are many outstanding questions surrounding the formation of LHCs, including the origin of their density depletion, their B/sub 0/ aligned length, and the detailed relation between wave fields, plasma density and ion acceleration. The GEODESIC sounding rocket, launched in early 2000, brings some new information to bear on these problems. Over 100 LHCs were encountered near the leading edge of an auroral substorm expansion. A Suprathermal Ion Imager (SII) newly developed for the flight provides 2D ion distribution function images from 0-20 eV with a time resolution of 11 ms, sufficient to resolve core and tail ion distributions at the edges and interiors of individual LHCs cavities. The measurements show tail heating to 1-2 eV superimposed on an unaffected core distribution. Magnetic field observations on GEODESIC indicate a significant enhancement of 0-10 kHz magnetic field fluctuations inside LHCs, in contrast to all previous attempts to detect such fields.