MEASUREMENTS OF SHORT-WAVELENGTH VLF BURSTS IN THE AURORAL IONOSPHERE- A CASE FOR ELECTROMAGNETIC MODE CONVERSION

Intense, localized bursts of VLF emissions were measured on the Univer sity of California at Berkeley Alaska '93 auroral sounding rocket. A w avelength determination for these waves was obtained using cross-spect ral analysis on high time resolution electric field measurements from dipole antennas and an onboard burst memory system. The observation of an antenna null in the cross-spectral response reveals very short wav elength emissions with wavelengths less than 5.5 meters. The applicabi lity of a collapse theory for these emissions is put in question since these examples are not well correlated with electron density depletio ns. In addition, these waves do not appear to be associated with a par ticular type of electron distribution which may have been responsible for their generation. The presence of electromagnetic modes in the VLF frequency range in the vicinity of many of these electrostatic bursts , along with continual electron density turbulence of 1-5%, may provid e evidence for a linear mode coupling mechanism in which electrostatic VLF waves are produced by the scattering of incident electromagnetic whistlers from electron density irregularities.