SAR Arcs We Have Seen: Variability of Ring Current – Ionosphere Interactions

Abstract:

Starting in 1987, an all-sky airglow imaging system has operated from a site at the Millstone Hill/Haystack Observatory in Westford, MA. During the ~2½ solar cycles from 1987 to 2014, the most prominent storm-time optical feature observed from a sub-auroral site is a stable auroral red (SAR) arc. The standard use of a SAR arc’s position is to locate the ionospheric footprint of the narrow plasmapause-ring current interaction region where heat conduction from the inner magnetosphere excites emission within the F-layer trough. When mapped from an emission altitude of 400 km to the geomagnetic equatorial plane, SAR arcs from Millstone Hill give the location of the plasmapause at radial distances between 2 to 4.5 earth radii.

 A total of 377 SAR arcs have been observed during the 27 years of imaging at Millstone Hill. A significant number of their morphologies departed from the stability in space and time implied by its name. We have classified these into five categories: longevity, multiplicity, zonal structure, latitudinal inhomogeneity, and tilt with respect to geomagnetic coordinates. In each case, the implications for the inner magnetosphere sources that drive SAR arcs are explored. Collectively, the variable nature of SAR arcs is documented systematically for the first time—an aspect of solar-terrestrial physics not yet addressed in either magnetosphere or ionosphere modeling studies.