Convection and Electrodynamic Signatures in the Vicinity of a Sun-Aligned Arc: Results from the Polar Acceleration Regions and Convection Study (Polar Arcs)

Abstract

An experimental campaign designed to study high-latitude auroral arcs was conducted in Sondre Stromfjord, Greenland, on February 26, 1987. The Polar Acceleration Regions and Convection Study (Polar ARCS) consisted of a coordinated set of ground-based, airborne, and sounding rocket measurements of a weak, sun-aligned arc system within the duskside polar cap. A rocket-borne barium release experiment, two DMSP satellite overflights, all-sky photography, and incoherent scatter radar measurements provided information on the large-scale plasma convection over the polar cap region while a second rocket instrumented with a DC magnetometer, Langmuir and electric field probes, and an electron spectrometer provided measurements of small-scale electrodynamics. The large-scale data indicate that small, sun-aligned precipitation events formed within a region of antisunward convection between the duskside auroral oval and a large sun-aligned arc further poleward. This convection signature, used to assess the relationship of the sun-aligned arc to the large-scale magnetospheric configuration, is found to be consistent with either a model in which the arc formed on open field lines on die dusk side of a bifurcated polar cap or on closed field lines threading an expanded low-latitude boundary layer, but not a model in which the polar cap arc field lines map to an expanded plasma sheet. The antisunward convection signature may also be explained by a model in which the polar cap arc formed on long field lines recently reconnected through a highly skewed plasma sheet. The small-scale measurements indicate the rocket passed through three narrow (< 20 km) regions of low-energy (< 100 eV) electron precipitation in which the electric and magnetic field perturbations were well correlated. These precipitation events are shown to be associated with regions of downward Poynting flux and small-scale upward and downward field-aligned currents of 1-2 u.A/m 2 . The paired field-aligned currents are associated with velocity shears (higher and lower speed streams) embedded in the region of antisunward flow.

Polar cap precursor of nightside auroral oval intensifications using polar cap arcs

“…As summarized in Weiss et al . [], there have been models for polar cap arcs that map arcs through open field lines linking the mantle and magnetosheath, or through closed field lines threading a bifurcated or expanded plasma sheet, or through closed field lines connecting to the low latitude boundary layer (LLBL). Carlson and Cowley [] focused on subvisual intensity (hundreds of Rayleigh (R)) arcs, which are present 50% of the time when the IMF is northward, unlike the rarely occurring and intense (>kR) theta aurora.…”

Polar Cap Science: MIT Coupling and Energy Deposition

“…Sun-aligned arcs are auroral forms that stretch roughly antisunwards across the polar cap, although there are many variations in morphology (Valladares & Carlson, 1991;Zhu et al, 1997). Strong mesoscale flows are thought to exist within and around these arcs (Weiss et al, 1993;Koustov et al, 2008). Sun-aligned arcs are primarily observed under IMF Bz north conditions (Rairden & Mende, 1989;Valladares et al, 1994).…”