Extremely Collimated Electron Beams In The High Latitude Magnetosphere Observed By Arase

This report describes electron beams observed at high latitudes in the magnetosphere by the Arase satellite. According to fine angular channel measurements, beam electrons are mainly flowing away from the Earth, and are well collimated to within a few degrees. A statistical study indicates that the beams are seen when Arase is located at L-m > similar to 5, and more frequently as the magnetosphere becomes active. The field lines of the beam events are traced back to the typical auroral ovals. The statistical properties of the beams are consistent with the interpretation that ionospheric electrons are accelerated by a parallel electric field of an auroral potential structure and are streaming upward to the high latitude magnetosphere.Plain Language Summary It is well accepted that discrete aurora is generated by precipitating electrons that are accelerated by an upward electric field. As a counterpart, a downward electric field exists in the vicinity of the upward electric field, accelerating ionospheric electrons upward to the magnetosphere. Such electron beams have been already reported by spacecraft observations in low altitudes and also in the magnetosphere. It is known that electron beams observed in the magnetosphere are collimated, but the pitch angle widths of the collimated beams have not been well resolved yet. In this report, we describe results of electron beam observations in the high latitude magnetosphere based on fine angular channel measurements of electrons by the Arase satellite.