Solar-Cycle Variations of the Auroral Return-Current

Details Solar-Cycle Variations of the Auroral Return-Current Solar-Cycle Variations of the Auroral Return-Current

May 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sm41b05d&link_type=abstract

American Geophysical Union, Spring Meeting 2001, abstract #SM41B-05

Physics

2704 Auroral Phenomena (2407), 2708 Current Systems (2409), 2736 Magnetosphere/Ionosphere Interactions

Scientific paper

We report on a study of upward electron beams measure by FAST in the post-midnight auroral zone. Ionospheric electrons are the dominant carrier of downward current in the auroral return-current regions. Previous studies of upward electron beams have measured the occurrence of upward electron beams in the midnight auroral zone. Near solar minimum, it was found that upward electron beams were present 71% of the time over the winter hemisphere, near solstice. At the same time, the summer hemisphere was void of upward electron beams at FAST altitude. During solar minimum, at equinox, it was found that the maximum occurrence of electron beams was when the dipole tilt of the earth was most anti-sunward. These results suggest that when the ionospheric is tenuous, electrons in the upward current region are accelerated make the flux needed to carry the imposed parallel current In this study, we examine the role of the solar cycle on the presence upward electron beams. During solar maximum, the fluxes of EUV and the ionospheric density are higher. Despite the higher magnetospheric activity of solar max, we only observe upward electron beams 28% of the time. This reduction in the presence of electron beams suggests that the higher ionospheric densities of solar maximum provide sufficient density to carry the imposed auroral currents. Thus, an upward acceleration mechanism does not manifest itself and the solar maximum return-current is carried by low-energy electrons. Observations of electron density in the plasma sheet, at geosynchronous orbit, show a solar cycle dependence with the minimum density near solar minimum and maximum density near solar maximum. This relationship is consistent with an ionospheric source of large numbers of low-energy electrons carrying auroral return-currents near solar maximum and more tenuous accelerated electrons carrying the return-current during solar minimum.

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