Long lasting energetic proton precipitation in the inner magnetosphere after substorms

Abstract

Data of polar‐orbiting low‐altitude NOAA spacecraft are used to study the precipitation of energetic (30–80 keV) protons, several degrees equatorward of the energetic proton isotropic boundary. This precipitation, to be called low‐latitude proton precipitation (LLPP), is observed at all local time sectors. The LLPP precipitating flux is found to be generally smaller than the trapped flux. The LLPP particle flux significantly increases during intense substorms and then decays with a long characteristic timescale of about 9 hours. Therefore, it may also be found during very quiet times. The long decay and the observed loss cone anisotropy imply a moderate pitch angle diffusion of energetic protons. During quiet magnetic conditions following the disturbance the latitudinal position of the LLPP region moves slowly to higher latitudes. The equatorial boundary of the precipitation is found to be at lower latitudes in the night sector than in the dayside sector. Mapping to the equatorial plane along magnetic field lines showed that the equatorial boundary of the precipitation at the nightside is situated on a closer drift shell than that at the dayside.