Wisp-Like Energy Spectrum of Precipitating Electrons Observed by DEMETER Satellite

Abstract

Very-low-frequency (VLF) signals used 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, resulting in pitch angle diffusion of trapped electrons. The energy-L spectrum of quasi-trapped electrons (in the drift loss cone scattered by North West Cape (NWC) transmitted signals in the inner radiation belt is called “wisp,” characterized by narrow spectral peaks of enhanced fluxes. 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 drift into the bounce loss cone (BLC). The transmitter-induced BLC precipitation with a wisp structure has been considered uncommon. In this study, we report the direct and clear observational evidence of transmitter-induced precipitating wisps which are commonly observed at the edge of the northern hemisphere precipitation regions (the regions conjugated to the SAA). Moreover, we systematically analyze the dependence of these electron fluxes on L-shell, electron energies and geomagnetic activities, using long-term measurements from the DEMETER satellite. The intensities and positions of the precipitating wisps in the energy-L spectrum are highly correlated with the quasi-trapped wisps. The visible wisp structure in the precipitating electrons can only be detected when the quasi-trapped electron fluxes exceed a certain threshold ∼10³ cm⁻²ster⁻¹s⁻¹ MeV⁻¹. The overall variation in precipitating electron fluxes follows the trend observed in trapped electron fluxes. These results provide new insights into the quantitative scattering effects of NWC transmitter signals on energetic electrons.