Longitudinal Variations of Energetic Electrons Scattered by NWC Transmitter: DEMETER Observations

Low-Earth orbit (LEO) satellites often observe wisp-like energy spectra, characterized by lower energies at higher L values of energetic electrons, resulting from the scattering effect of the narrow-band artificial transmitter signals. Typically, the flux levels of wisp energy spectra increase as they approach the South Atlantic Anomaly (SAA). Here, we report, for the first time, the longitudinal variations of wisp positions in the energy-L coordinate based on observations from the DEMETER satellite. The L-values of wisps decrease at the longitudes <180° and increase at the longitudes >180°. By considering the satellite's orbital characteristics, we found that the longitudinal variations of wisp are essentially the MLT-dependent radial transport of electrons. This finding indicates that the electron drift paths can be modified within a single drift period. We propose a duskward convection electric field as a potential mechanism to explain the longitudinal variations of wisps. By assuming a uniform duskward convection electric field with an amplitude of ∼1.5 mV/m, our simulations can reasonably reproduce the longitudinal variations of wisps observed in the southern hemisphere on 21 March 2009. Our results support the existence of a convection electric field in the inner belt and suggest that this electric field significantly contributes to the variations in energies and L values of energetic electrons within one drift period.