The Relationship Between Electron Precipitation and the Population of Trapped Electrons in LEO: New Evidence Supporting a Natural Limit to the Flux of Energetic Electrons

Energetic particle precipitation into the atmosphere has been identified as a key loss process for electrons in the Earth’s outer radiation belt region. However, direct measurements of the electron flux precipitating into the atmosphere are challenging from high altitude low inclination spacecraft, such as the Van Allen Probes, due to the small angular size of the loss cone along the orbital path of these high-altitude spacecraft. Here we use data from the Polar Orbiting Environmental Satellites (POES)/Space Environment Monitor in low Earth orbit to assess the relationship between the trapped and precipitating electron flux in integral energy channels >30 keV, >100 keV, and >300 keV. Our results highlight that there is a strong non-linear relationship between the flux of trapped and precipitating electrons, with the ratio of precipitating to trapped flux only becoming significantly enhanced once the trapped flux reaches a critical level. This transition from low to high levels of precipitation is consistent with the theory proposed by Kennel and Petschek (K-P) (1966) https://doi.org/10.1029/JZ071i001p00001, whereby intense chorus waves are excited and trigger pitch angle diffusion, resulting in energetic particle precipitation and limiting the trapped flux. Using electron flux data from POES and chorus wave data from the Van Allen Probes, we further test the observations against predictions from the K-P theory. A particle tracing model is also utilized to illustrate a direct link between the drift paths of injected electrons, the occurrence of chorus waves and the spatial distribution of strong electron precipitation into the atmosphere at different energies.