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 University of California at Berkeley Alaska '93 auroral sounding rocket. A wavelength determination for these waves was obtained using cross‐spectral 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 wavelength emissions with wavelengths less than 5.5 meters. The applicability of a collapse theory for these emissions is put in question since these examples are not well correlated with electron density depletions. In addition, these waves do not appear to be associated with a particular 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 provide 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.