| The GEODESIC sounding rocket encountered hundreds of localized, VLF-wave-filled density depletions in an auroral return current region at altitudes between 900--1000 km. While these are similar to well-studied lower-hybrid "spikelets", which are electrostatic, many of the GEODESIC events exhibited strong VLF magnetic field enhancements as well. In the present study we show that these magnetic field fluctuations can be interpreted as the result of geomagnetic field-aligned electron currents driven by fluctuating electric fields parallel to the geomagnetic field lines. This observation suggests that the electromagnetic wave-filled cavities are signatures of unstable filaments of return current fluctuating at VLF frequencies. We argue that the cavities' spatial dimensions, their location inside the return current region and their total radiated power are consistent with the properties of VLF saucer source regions inferred from earlier satellite observations taken at higher altitudes. |
