Abstract
Following the St. Patrick’s Day (17 March) geomagnetic storm of 2013,
the interplanetary magnetic field had near-zero clock angle for almost
two days. Throughout this period multiple cusp-aligned auroral arcs
formed in the polar regions; we present observations of, and provide a
new explanation for, this poorly-understood phenomenon. The arcs were
observed by auroral imagers onboard satellites of the Defense
Meteorological Satellite Program (DMSP). Ionospheric flow measurements
and observations of energetic particles from the same satellites show
that the arcs were produced by inverted-V precipitation associated with
upward field-aligned currents at shears in the convection pattern. The
large-scale convection pattern revealed by the Super Dual Auroral Radar
Network (SuperDARN) and the corresponding FAC pattern observed by the
Active Magnetosphere and Planetary Electrodynamics Response Experiment
(AMPERE) suggest that dual-lobe reconnection was ongoing to produce
significant closure of the magnetosphere. However, we propose that once
the magnetosphere became nearly closed complicated lobe reconnection
geometries arose that produced interleaving of regions of open and
closed magnetic flux and spatial and temporal structure in the
convection pattern that evolved on timescales shorter than the orbital
period of the DMSP spacecraft.