Simulation Study on "Wisp" Electron Spectra Generated by NWC Very Low Frequency Transmitter Signals

Abstract

Very low frequency signals emitted by worldwide spread ground-based man-made transmitters, which primarily propagate within Earth-ionospheric waveguides, are used for submarine communication. A portion of these signals penetrates the ionosphere and leaks into the magnetosphere when the ionospheric electron density decrease on the nightside due to the attenuated sunlit. VLF transmitter signals in the magnetosphere can scatter electrons in the inner radiation belt at energies of 100s keV into the drift loss cone through cyclotron resonance, which is an important loss mechanism for electrons in the inner radiation belt, and also playing an important role in transferring energy and mass from magnetosphere to ionosphere. Electrons scattered by transmitter signals exhibit “wisp” signature in <i>L</i>-<i>E</i><sub><i>k</i></sub> spectrum, satisfying the first-order cyclotron resonance relationship between electrons and the transmitter signals. The “wisp” spectrum can be clearly observed by Low Earth Orbit satellites, offering opportunities to study wave-particle interactions in near-Earth space. In this study, using the Drift-Diffusion-Source model, we reproduce the “wisp” spectrum formed by scattering effects of NWC transmitter signals observed by DEMETER satellite on March 19, 2009. Our simulation results suggest that the equatorial pitch angle of electrons observed by DEMETER varies with the longitude, resulting in distinctions in the observed “wisp” spectrum along different longitudes. Specifically, as the satellite approaches South Atlantic Anomaly (SAA) region, both the energy range and flux level of the observed “wisp” spectrum gradually increase. When using the wave normal angle model (the central wave normal angle is 60°) and the background electron density model from previous studies, the energy range of the simulated “wisp” spectra is higher than the observations. Adjusting the central wave normal angle to 40° or increasing the background density by a factor of 1.3, the simulated results agree well with the observations. Our results clarify the scattering effect of NWC transmitter signals on electrons in the radiation belt, and underscore the importance of analyzing the formation of “wisp” spectrum for understanding wave-particle interactions in near-earth space. Additionally, the Drift-Diffusion-Source model can be used to study wave-particle interactions in the inner radiation belt, providing an important basis for developing radiation belt remediation technology.