Magnetosphere-Ionosphere Coupling as Reflected in Large-Scale Birkeland Currents

B. J. Anderson

Stresses between the ionosphere and magnetosphere associated with the solar wind
dynamo, resulting magnetospheric convection and plasma pressure distributions
are all transmitted via Birkeland currents. These currents therefore provide
critical insight to the stress-coupling. The Iridium system of more than 70 low
Earth orbiting satellites provides a unique opportunity to monitor the
global-scale Birkeland currents. Even though the present system software limits
the magnetometer data rates and necessitates data accumulation times on the
order of an hour, the system still provides quasi-snapshot views of the currents
on time scales comparable to a LEO orbit period. Storm-time currents display a
variety of phenomena that are not consistent with statistical results obtained
previously including: pronounced dawn-dusk asymmetry associated with the partial
ring current; intense upward/downward currents in the post-midnight sector
having no evening counterpart; and fossil low latitude currents. Intense ion
pressures must be associated with each of these current distributions.
Comparisons with HENA observations are used to test the extent to which the
remotely sensed ion populations can account for the observed systems. The
relationship of these storm-time currents to sub-auroral flows is also assessed
indicating that seasonal conductance effects are a key factor determining
whether the flows are associated with currents.

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Global Aspects of Magnetosphere-Ionosphere Coupling, 2006 Yosemite Workshop, 
Yosemite National Park, CA, USA, 7-10 February 2006