Extremely field‐aligned cool electrons in the dayside outer magnetosphere

• Published in: Geophysical research letters Vol. 44; no. 1; pp. 44 - 51
• Main Authors: Mozer, F. S., Agapitov, O. A., Angelopoulos, V., Hull, A., Larson, D., Lejosne, S., McFadden, J. P.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16-01-2017
• Subjects: Alignment; Attitude (inclination); Cusps; Density; Distance; Drift; Electron energy; Electron flux; Electrons; Energy; Energy bands; Energy measurement; field‐aligned electrons; Flux density; Fluxes; Geophysics; History; Hot electrons; Interactions; Ionospheric plasma; Lines; Magnetic fields; Magnetopause; Magnetopause reconnection; Magnetosphere; Magnetospheres; Photoelectrons; Pitch (inclination); Plumes; Probability theory; Ratios; Spacecraft; Stellar mass; Summer; Sun
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2016GL072054

For 200 days 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 1–30 eV and 16 steps covered energies to 30 keV. 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 1 eV (plasmaspheric plumes measured by the spacecraft potential); cool electrons, defined as electrons having energies of 1–25 eV; and hot electrons having energies of 25 eV to 30 keV. 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° and 160–180° divided by the flux at 80–100°) greater than 100. Evidence is presented that they resulted from cusp electrons moving from open to closed magnetospheric field lines due to their E × B/B2 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 RE inside of it. Their occurrence probability in the outer magnetosphere was ~50% in June and ~10% in September, suggesting a dayside source attributed to the tilt of the northern cusp toward the Sun during the summer.
Key Points
The first measurements of 1–30 eV electrons with energy resolution less than 2 eV have been made
These electrons are strongly field aligned (ratio of parallel to perpendicular fluxes >100) in the dayside outer magnetosphere
The correlation of these electrons with E × B/B2 suggests that they were convected on to closed magnetospheric field lines from the cusp