Study of the precipitation wisp scattered by NWC transmitter signals: observed by DEMETER satellite.

The 19.8 kHz very‐low‐frequency (VLF) signals emitted from ground‐based North West Cape (NWC) transmitter for submarine communication can penetrate the ionosphere and leak into the magnetosphere. These signals interact with hundreds of keV electrons in the inner magnetosphere through cyclotron resonance, leading to the pitch angle diffusion of trapped electrons. Previous studies use the term “wisp” to describe the enhancement of quasi-trapped electron scattered by NWC transmitter signals at L = 1.4-1.8, which shows decreasing energy with increasing L. These quasi-trapped electrons drift eastward and can be clearly observed by Low-Earth-Orbit satellites until they precipitate into the South Atlantic Anomaly (SAA) region, where they are lost into the atmosphere. In this study, we report the ‘wisp’ structure in the untrapped electrons and systematically analyze the dependence of these electron fluxes on satellite positions, electron energies, L-shell, and geomagnetic activities using long-term measurements from the DEMETER satellite. The ‘wisp’ structure in untrapped electrons was observed at the edge of the northern hemisphere precipitation region (the regions conjugated to the SAA), with the flux level approximately 102-103 cm−2ster−1s−1MeV−1. The intensity and position of the untrapped ‘wisp’ in the energy spectrum are highly correlated with the quasi-trapped ‘wisp’.  The visible ‘wisp’ structure in the untrapped electrons can only be detected when the quasi-trapped electrons, scattered by the NWC signal, exceed a certain threshold (i.e., greater than 103 cm−2ster−1s−1MeV−1). The overall variation in untrapped electron fluxes follows the trend observed in trapped electron fluxes. These results provide helpful information regarding the quantitative scattering effects of NWC transmitter signals on energetic electrons.