Temporal Scales of Electron Precipitation Driven by Whistler-Mode Waves

Electron resonant scattering by whistler-mode waves is one of the most important mechanisms responsible for electron precipitation to the Earth's atmosphere. The temporal and spatial scales of such precipitation are dictated by properties of their wave source and background plasma characteristics, which control the efficiency of electron resonant scattering. We investigate these scales with measurements from the two low-altitude Electron Losses and Fields Investigation (ELFIN) CubeSats that move practically along the same orbit, with along-track separations ranging from seconds to tens of minutes. Conjunctions with the equatorial THEMIS mission are also used to aid our interpretation. We compare the variations in energetic electron precipitation at the same L-shells but on successive data collection orbit tracks by the two ELFIN satellites. Variations seen at the smallest inter-satellite separations, those of less than a few seconds, are likely associated with whistler-mode chorus elements or with the scale of chorus wave packets (0.1-1 s in time and similar to 100 km in space at the equator). Variations between precipitation L-shell profiles at intermediate inter-satellite separations, a few seconds to about 1 min, are likely associated with whistler-mode wave power modulations by ultra-low frequency waves, that is, with the wave source region (from a few to tens of seconds to a few minutes in time and similar to 1,000 km in space at the equator). During these two types of variations, consecutive crossings are associated with precipitation L-shell profiles very similar to each other. Therefore the spatial and temporal variations at those scales do not change the net electron loss from the outer radiation belt. Variations at the largest range of inter-satellite separations, several minutes to more than 10 min, are likely associated with mesoscale equatorial plasma structures that are affected by convection (at minutes to tens of minutes temporal variations and asymptotic to[10(3), 10 (4)] km spatial scales). The latter type of variations results in appreciable changes in the precipitation L-shell profiles and can significantly modify the net electron losses during successive tracks. Thus, such mesoscale variations should be included in simulations of the radiation belt dynamics.