Relativistic electron precipitation driven by mesoscale transients, inferred from conjugate ground and multi-spacecraft platforms

Precipitation of relativistic electrons into the Earth’s atmosphere regulates the outer radiation belt fluxes and contributes to magnetosphere-atmosphere coupling. One of the main drivers of such precipitation is electron scattering by whistler-mode waves. Such waves typically originate at the equator, where they can resonate with and scatter subrelativistic (tens to a few hundred keV) electrons). However, they can occasionally be ducted by strong density gradients and propagate along field lines far from the equator, reaching middle latitudes; there they can resonate with and scatter relativistic (>500 keV) electrons. Both whistler-mode wave generation and ducting can be driven by equatorial mesoscale (order of one Earth radius) transient structures at the nightside (driven by injections) and the dayside (driven by ultra-low-frequency waves). Here we combine mea surements of whistler-mode waves from ground observatories, relativistic electron precipitation from low-altitude satellites, total electron content maps from GPS receivers, and magnetic field and electron flux from equatorial satellites. We show that the observed relativistic electron precipitation is likely due to mid-latitude whistler-mode waves driven by equatorial transients. These ducted whistler-mode waves are likely absent in statistical models of wave occurrence rate, resulting in an underestimate of relativistic electron precipitation by mesoscale transients. These effects need to be further quantified, parametrized and duly incorporated in future models.