Ionospheric Plasma Density Gradients Associated With Night‐Side Energetic Electron Precipitation.

Energetic electron precipitation from the equatorial magnetosphere into the atmosphere plays an important role in magnetosphere‐ionosphere coupling: precipitating electrons alter ionospheric properties, whereas ionospheric outflows modify equatorial plasma conditions affecting electromagnetic wave generation and energetic electron scattering. However, ionospheric measurements cannot be directly related to wave and energetic electron properties measured by high‐altitude, near‐equatorial spacecraft, due to large mapping uncertainties. We aim to resolve this by projecting low‐altitude measurements of energetic electron precipitation by ELFIN CubeSats onto total electron content (TEC) maps serving as a proxy for ionospheric density structures. We examine three types of precipitation on the nightside: precipitation of <200 keV electrons in the plasma sheet, bursty precipitation of <500 keV electrons by whistler‐mode waves, and relativistic (>500 keV) electron precipitation by EMIC waves. All three types of precipitation show distinct features in TEC horizontal gradients, and we discuss possible implications of these features. Plain Language Summary: Bursty precipitation of energetic electrons, via pitch‐angle scattering by whistler‐mode waves from the magnetosphere to the ionosphere, is an important factor in the global magnetosphere‐ionosphere coupling. It induces local modifications of ionospheric density and chemical composition. A recurrent problem in the investigation of this process is the presence of large uncertainties in the field‐line mapping between ionospheric density structures and high altitude satellites measuring electron fluxes in the magnetosphere. In the present study, such uncertainties are significantly reduced by making use of precipitating electron fluxes recorded by ELFIN CubeSats at low altitudes (450 km) just above the ionosphere and comparing them with maps of the corresponding ionospheric density structures. We identify three different types of electron precipitation on the nightside, corresponding to low, moderate, and high energy precipitating electrons. We show that each type of the precipitation is characterized by particular plasma density gradients in the ionosphere, suggesting a key role of wave ducting by plasma density gradients in fostering the precipitation of 300–500 keV electrons by whistler‐mode waves, and the potential importance of midnight plasma injections in generating EMIC waves that can further precipitate 0.5–2 MeV electrons far away from the plasmasphere. Key Points: Total electron content (TEC) maps are examined during energetic electron precipitation at the conjugate low altitudeNight‐side whistler‐mode wave driven precipitation is often associated with local horizontal TEC gradientsNight‐side EMIC wave driven precipitation is often poleward of the TEC minima associated with the plasmapause projection [ABSTRACT FROM AUTHOR]