Radiation belt electron precipitation by man-made VLF transmissions

Abstract

Enhancements of drift-loss cone fluxes in the inner radiation belt have been observed to coincide with the geographic location of the powerful VLF transmitter NWC. In this paper we expand upon the earlier study to examine the occurrence frequency of drift-loss cone enhancements observed above transmitters and the intensity of the flux enhancements and to demonstrate the linkage to transmitter operation. Our study has confirmed the strong dependence that these enhancements have upon nighttime ionospheric conditions. No enhancements were observed during daytime periods, consistent with the increased ionospheric absorption. We have also confirmed the persistent occurrence of the wisp features east of the NWC transmitter. The enhancements are initially observed within a few degrees west of NWC and are present in 95% of the nighttime orbital data east of the transmitter for time periods when the transmitter is broadcasting. No enhancements are observed when NWC is not broadcasting. This provides conclusive evidence of the linkage between these drift-loss cone electron flux enhancements and transmissions from NWC. When contrasted with periods when NWC is nonoperational, there are typically ∼430 times more 100-260 keV resonant electrons present in the drift-loss cone across L = 1.67-1.9 owing to NWC transmissions. There are almost no wisp-like enhancements produced by the transmitter NPM, despite its low-latitude location and relatively high output power. The lack of any wisp enhancement for L < 1.6 suggests that nonducted propagation is an inefficient mechanism for scattering electrons, which explains the lower cutoff in L of the NWC-generated wisps and the lack of NPM-generated wisps. Copyright 2008 by the American Geophysical Union.