SAR arcs we have seen: Evidence for variability in stable auroral red arcs

Since 1987, an all‐sky airglow imaging system has operated from a site at the Millstone Hill/Haystack Observatory in Westford, MA. During the ~2.5 solar cycles from 1987 to 2014, many studies using all‐sky images, in conjunction with incoherent scatter radar and satellite data, described subauroral, ionospheric disturbances observed during individual geomagnetic storms. The most prominent storm time optical feature from a subauroral 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 314 SAR arcs have been observed during the 27 years of imaging at Millstone Hill. We find no single morphology for all SAR arcs, but rather patterns that occasionally depart from stability in space and time. 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. While individual SAR arc variability characteristics have been noted in previous studies, here we describe for the first time all five types from the same site—an aspect not yet addressed in either magnetosphere or ionosphere modeling studies.