Observed and modeled response of the dayside/nightside reconnection rates to a solar wind dynamic pressure front
Abstract
The significant effect of solar wind dynamic pressure fronts on the magnetosphere-ionosphere system has been amply demonstrated in recent years. Sudden enhancements in solar wind dynamic pressure, Psw, have been shown to lead to intensification of the auroral electrojets (and Region 1 field-aligned currents), increase in auroral emissions, and significant enhancement in ionospheric convection and the transpolar potential. These observations point to an excited state of the terrestrial system resulting from rapid increases in Psw. One of the most interesting observations associated with sharp enhancements in Psw is the poleward expansion of the auroral oval and the closing of the polar cap. Case studies using Defense Meteorological Satellite Program (DMSP) precipitating particle measurements and Polar Ultra-Violet Imager (UVI) images have demonstrated that sudden Psw increases result in shrinking of the polar cap over a wide range of magnetic local times (MLTs), but particularly on the nightside. This implies a significant reduction of the open flux content of the magnetosphere with implications on the rate of nightside reconnection and magnetospheric dynamics as a whole. We now conduct direct calculations of the dayside/nightside reconnection rates using observed Polar UVI polar cap boundaries and Assimilative Mapping of Ionospheric Electrodynamics (AMIE) potentials for a pressure front on 12 February 2000. Differentiation of the AMIE potential distribution provides the ionospheric electric field parallel to the UVI boundary, which is then estimated in the frame of the moving boundary, yielding the local reconnection rate along the boundary. Integration along the dayside X-line gives the dayside reconnection potential, and along the rest of the boundary the nightside reconnection potential. We show that the dayside rate responds immediately to the compression of the magnetosphere. However, not long after the pressure enhancement the nightside reconnection rate also intensifies, surpassing the rate on the dayside, and thus leading to the previously observed closing of the polar cap. We also compare the results with simulations by the Open Geospace General Circulation Model (OpenGGCM). We find that, even though the reconnection rates also increase in the model after the increase in Psw, the dayside and nightside rates are comparable, leading to steady or even slightly increasing polar cap area. The main source of the discrepancy between data and model seems to be the location of the polar cap boundary in the model, which is at substantially lower magnetic latitudes compared with the data, resulting in a long dayside X-line and thus higher dayside reconnection rate. We explore the physical cause of the discrepancies.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMSM41B2240B
- Keywords:
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- 2784 MAGNETOSPHERIC PHYSICS Solar wind/magnetosphere interactions;
- 2723 MAGNETOSPHERIC PHYSICS Magnetic reconnection;
- 2760 MAGNETOSPHERIC PHYSICS Plasma convection;
- 2736 MAGNETOSPHERIC PHYSICS Magnetosphere/ionosphere interactions