Extremely field-aligned cool electrons in the dayside outer magnetosphere

For 200days in 2016 while Time History of Events and Macroscale Interactions during Substorms D (THEMIS-D) was in the dayside, equatorial magnetosphere, its electron energy coverage was modified such that the first 15 energy steps covered the range of 1-30eV and 16 steps covered energies to 30keV. These measurements were free of backgrounds from photoelectrons, secondaries, or ionospheric plasma plumes. Three energy bands of electrons were observed: cold electrons having energies below 1eV (plasmaspheric plumes measured by the spacecraft potential); cool electrons, defined as electrons having energies of 1-25eV; and hot electrons having energies of 25eV to 30keV. The cool electron fluxes at fixed radial distances varied by an order of magnitude from one orbit to the next. These fluxes often increased with increasing radial distance, suggesting an external source. They were extremely field aligned, having pitch angle ratios (flux at 0-20 degrees and 160-180 degrees divided by the flux at 80-100 degrees) greater than 100. Evidence is presented that they resulted from cusp electrons moving from open to closed magnetospheric field lines due to their ExB/B-2 drift. They constituted the majority of the electron energy density at such times and places. They were not associated with magnetopause reconnection because they were not observed at the magnetopause, but they were observed as far as 3 R-E inside of it. Their occurrence probability in the outer magnetosphere was similar to 50% in June and similar to 10% in September, suggesting a dayside source attributed to the tilt of the northern cusp toward the Sun during the summer.