FAST observations of upward accelerated electron beams and the downward field-aligned current region

Abstract

Auroral processes result from the exchange of energy and momentum between the magnetosphere and ionosphere, and the current systems arising from this exchange. Field-aligned potential drops in the upward field-aligned current (FAC) region of the auroral zone maintain a required current density in the face of opposing mirror forces. But field-aligned potential drops occur in downward current regions as well, and result in upgoing accelerated electron beams. We investigate the occurrence of upward accelerated electrons with altitude, magnetic local time and season using FAST. By choosing orbits having perigee over the equator, northern-southern hemisphere differences in altitude coverage are largely removed. For an interval near solstice, no electron beams were observed over the summer auroral zone at FAST altitudes, while over the winter hemisphere, 71% of the crossings had upgoing beams, a clear seasonal effect. During the equinox interval the distribution between hemispheres was much more balanced, but the overall occurrence was less (∼46%) than during winter solstice. There is a clear tendency for upgoing accelerated electron beam occurrence at FAST altitudes to maximize when dipole tilt places the field line footpoint deep in the nightside ionosphere, where background ionospheric density is lowest. In other words, as ionospheric density decreases, the potential structure tends to move to lower altitudes.