Abstract
Abstract. This study employs observations from several sources to determine the location of the polar cap boundary, 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 images 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 magnetosphere are related to the rate of magnetic reconnection occurring at the magnetopause and in the magnetotail, allowing 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 decreases in area correspond to open flux being destroyed at substorm breakup. The polar cap area can continue to decrease 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 boundary, 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 supports this finding.Key words. Ionosphere (plasma convection; polar ionosphere) – Magnetospheric physics (magnetospheric configuration and dynamics)
Highlights
The ability to monitor the location of the polar cap boundary, the boundary between “closed terrestrial” magnetic flux and “open” flux interconnected with the solar wind, allows for the electrodynamics of solar-terrestrial coupling to be investigated, the storage and release of magnetic energy within the magnetosphere during the substorm cycle
Polar Ultraviolet Imager (UVI), the Northern Hemisphere SuperDARN chain, Defense Meteorological Satellite Program (DMSP) F13 and F14, National Oceanographic and Atmospheric Administration (NOAA)-12 and Fast Auroral SnapshoT (FAST) measurements have been combined to estimate the location of the polar cap boundary at all local times, and to track the change in polar cap area through the course of two substorms
The satellite precipitation measurements are interpreted such that high energy (> few keV) particles are located on closed field lines, whereas structured low energy (< 1 keV) particles are of magnetosheath origin and located on open field lines; the poleward boundary of the high energy particles is taken to be a proxy for the open/closed field line boundary (OCB)
Summary
The ability to monitor the location of the polar cap boundary, the boundary between “closed terrestrial” magnetic flux and “open” flux interconnected with the solar wind, allows for the electrodynamics of solar-terrestrial coupling to be investigated, the storage and release of magnetic energy within the magnetosphere during the substorm cycle. The edge between polar rain and harder precipitation associated with the discrete aurora can again be used, though these high energy particles are not necessarily trapped but can be accelerated by the reconnection process ongoing at point e These energized particles are again observed equatorward of the OCB in the closed field line region because field lines convect from the open to closed region through the action of reconnection. A boundary between low spectral width echoes at lower latitudes and high spectral width echoes poleward of this has been identified with the OCB, on both the dayside (Baker et al, 1995) and the nightside (Lester et al, 2001) On the dayside, this boundary can be accompanied by a significant step in backscatter power, with backscatter from the closed field line region having an SNR below the sensitivity of the radar system (Milan and Lester, 2001). This allows for a comparison with our estimate of the simultaneous rate of change of the polar cap area
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