Effects of plasma density irregularities on the pitch angle scattering of radiation belt electrons by signals from ground based VLF transmitters

Recent DEMETER spacecraft observations show that VLF signals from the NPM transmitter in Hawaii often strongly excite quasi-electrostatic whistler mode waves as the NPM signals propagate upward through plasma density irregularities. As a result of the NPM wave energy loss to the quasi-electrostatic waves, the transmitter signals will arrive at the radiation belts with less intensity than predicted by present models of VLF wave propagation and will produce less pitch angle scattering of energetic electrons than presently believed. This type of wave energy loss may be partially responsible for the pervasive wave intensity deficit for VLF transmitter signals in the plasmasphere recently noted by Starks et al. (2008).