Variations in the polar cap area during two substorm cycles

TLDR: In this article, the authors employed observations from several sources to determine the location of the polar cap bound-ary, or open/closed field line boundary, at all local times, allowing the amount of open flux in the magnetosphere to be quantified.

Abstract: This study employs observations from several sources to determine the location of the polar cap bound- ary, or open/closed field line boundary, at all local times, allowing the amount of open flux in the magnetosphere to be quantified. These data sources include global auroral im- ages from the Ultraviolet Imager (UVI) instrument on board the Polar spacecraft, SuperDARN HF radar measurements of the convection flow, and low altitude particle measurements from Defense Meteorological Satellite Program (DMSP) and National Oceanographic and Atmospheric Administration (NOAA) satellites, and the Fast Auroral SnapshoT (FAST) spacecraft. Changes in the open flux content of the mag- netosphere are related to the rate of magnetic reconnection occurring at the magnetopause and in the magnetotail, al- lowing us to estimate the day- and nightside reconnection voltages during two substorm cycles. Specifically, increases in the polar cap area are found to be consistent with open flux being created when the IMF is oriented southwards and low-latitude magnetopause reconnection is ongoing, and de- creases in area correspond to open flux being destroyed at substorm breakup. The polar cap area can continue to de- crease for 100 min following the onset of substorm breakup, continuing even after substorm-associated auroral features have died away. An estimate of the dayside reconnection voltage, determined from plasma drift measurements in the ionosphere, indicates that reconnection can take place at all local times along the dayside portion of the polar cap bound- ary, and hence presumably across the majority of the dayside magnetopause. The observation of ionospheric signatures of bursty reconnection over a wide extent of local times sup- ports this finding.