Quasi-electrostatic whistler mode wave excitation by linear scattering of EM whistler mode waves from magnetic field-aligned density irregularities

Abstract

Recent observations by Starks et al. (2008) from multiple spacecraft suggest that the actual nighttime intensity of VLF transmitter signals in the radiation belts is approximately 20 dB below the level that is assumed in the model developed by Abel and Thorne (1998) and approximately 10 dB below model values during the day. In the present work, we discuss one experimentally established mechanism which might be responsible for some of this intensity discrepancy, linear mode coupling as electromagnetic whistler mode waves propagate through regions containing small-scale (2-100 m) magnetic field-aligned plasma density irregularities. The scattering process excites quasi-electrostatic whistler mode waves, which represents a power loss for the input waves. Although the distribution and amplitude of the irregularities is not well known at present, we construct plausible models in order to use numerical simulations to determine the characteristics of the mode coupling mechanism and the conditions under which the input VLF waves can lose significant power to the excited quasi-electrostatic whistler mode waves. For short propagation paths of approximately 15 km, the full-wave model predicts power losses ranging from -3 dB (25% probability) to -7 dB (2% probability). For longer propagation paths of approximately 150 km, the full-wave model predicts power losses ranging from -4 dB (25% probability) to over -10 dB (2% probability). We conclude that for the irregularity models investigated, the mode coupling mechanism can result in significant power loss for VLF electromagnetic whistler mode waves.