Observation of High‐Energy Electrons Precipitated by NWC Transmitter From PROBA‐V Low‐Earth Orbit Satellite. Issue 16 (13th August 2020) - Cunningham, G. S.
Abstract: The very low‐frequency transmitter in the Northwest Cape of Australia (NWC) has previously been observed to pitch‐angle scatter electrons with energies from 30–400 keV, creating enhanced fluxes measured by low‐Earth orbiting (LEO) satellites. Here we use observations from the Energetic Particle Telescope on PROBA‐V. We compare the measured flux, as a function of local magnetic field strength, when the NWC transmitter is "on" versus "off, " and find enhanced fluxes only when NWC is "on" and located on the nightside. The enhanced fluxes occur in the population gradually transitioning from "permanently trapped" to "quasi‐trapped." We show that electrons up to 800 keV, substantially higher energy than previously studied, are scattered by resonant interactions with NWC to produce enhanced fluxes. The enhanced fluxes appear at multiple L‐shells for each energy channel, consistent with resonance conditions at distinct wave normal angles, that indicate ducted interactions at L < 1.55 and unducted interactions at L > 1.65. Plain Language Summary: Radio waves with a low enough frequency can reflect off Earth's surface as well as the ionosphere, a layer of charged particles above Earth's atmosphere created by the Sun's ionization of the upper atmosphere. These waves bounce back and forth in the Earth‐ionosphere waveguide, enabling AM (Amplitude Modulation) radio to be heard at large distances, for example. Transmitters with even lower frequencies than AM radio, in the tens of kiloHertz range, are used to communicate with submarines just below the ocean's surface that can be located on the other side of the world. The ionosphere is not a perfect reflector, though, and some of the transmitter wave power escapes into Earth's magnetosphere, a region where charged particles like electrons can be trapped for many months on magnetic field lines. The radio waves can scatter the trapped electrons into Earth's atmosphere, where they rapidly lose energy and no longer pose a threat to space‐based assets. Understanding how naval transmitters scatter trapped electrons is needed to help us explain why electrons stay trapped for as long as they do, and enable us to predict radiation damage to the growing number of satellites that orbit Earth with low altitudes, which limits the satellite's useful lifetime. Key Points: The ground‐based transmitter in northwest Australia scatters electrons into the drift loss cone The Energetic Particle Telescope on the PROBA‐V satellite sees enhanced fluxes at 500–800 keV from low‐Earth orbit The enhanced fluxes occur at energy‐dependent L‐shells that are consistent with the resonance condition for ducted and nonducted waves