ELFIN’s primary science objective is to measure, for the first time, the angle and energy distribution of precipitating relativistic electrons within and near the loss cone and determine, for the first time, if these bear the characteristic signature Electromagnetic Ion Cyclotron (EMIC) waves, whistler-mode waves, or perhaps other electron scattering mechanisms. For purposes of modeling and predicting Earth’s radiation environment, determining storm-time radiation belt electron loss rates and mechanisms is just as important as determining electron acceleration from the far more expensive, equatorial satellites flying through the radiation belts. Having a second satellite (STAR option) means that ELFIN-B will pass over the same location of the ELFIN-A on a time-scale between 0-90 minutes. From that vantage point, the data will allow us to determine if the precipitation rate has changed spatially (in latitude extent), temporally (in intensity only but not in extent), or both. ELFIN’s instrumentation and dual satellite approach will advance our understanding of dominant wave-loss mechanism of relativistic “killer” electrons and help build better radiation belt models to characterize and predict storm-time radiation belt fluxes.

ELFIN’s secondary science objective is to identify the magnetospheric source location of ionospheric field aligned currents (FACs), in relation to tail boundaries (dipole region, magnetotail, tail boundary). By measuring multiple 100-500 keV ion and 0.5-5 MeV electron isotropy boundaries, ELFIN will adjust mapping models and constrain the location of FAC sources in the magnetosphere. Multiple satellites at the same or different local times provide much better constraints to the mapping than a single satellite, thus ELFIN’s second satellite improves this capability by a factor of two. Given that ionospheric Joule heating (which is driven by FACs) is a significant, perhaps dominant repository of storm and substorm energy, it is imperative to understand and model the flow of that energy in our space weather models. ELFIN’s secondary objective will thus make significant contributions to the area of Magnetosphere-ionosphere coupling and allow us to build better models of the Sun-Earth interaction, moving us closer to the point of predictability.