Abstract
AbstractThe open magnetic flux content of the magnetosphere varies during substorms as a result of dayside and nightside reconnection. The open flux can be calculated from the area of the polar cap, delineated by the open‐closed field line boundary (OCB). This study presents a superposed epoch analysis of the location of the OCB and the change in the magnetic flux content in individual nightside MLT sectors during substorm growth, expansion, and recovery phases. Far ultraviolet (FUV) observations from the IMAGE satellite are used to derive a proxy of the OCB location. In the hour prior to substorm onset, the total nightside flux content increases by up to 0.12 GWb on average, resulting in an equatorward expansion of the OCB. Following substorm onset, the OCB contracts toward the pole as the open magnetic flux content decreases by up to 0.14 GWb on average, but the rate of decrease of the total nightside open flux content differs by 5–66% between the three IMAGE far ultraviolet instruments. The OCB does not contract poleward uniformly in all nightside magnetic local time (MLT) sectors after substorm onset. Close to the substorm onset MLT sector, the OCB contracts immediately following substorm onset; however, the OCB in more dawnward and duskward MLT sectors continues to expand equatorward for up to 120 minutes after substorm onset. Despite the continued increase in flux in these sectors after substorm onset, the total nightside flux content decreases immediately at substorm onset, indicating that the nightside reconnection rate exceeds the dayside rate following substorm onset.
Highlights
The Dungey (1961) Cycle describes the interaction between the Earth's magnetosphere and the interplanetary magnetic field (IMF) frozen into the solar wind
We show the results of the superposed epoch analysis of the open‐closed field line boundary (OCB) location for substorms with onset latitudes between 64° and 66°
Using the OCB location to estimate the total nightside flux content calculated over 12 magnetic local time (MLT) sectors, we have shown that, on average, the total nightside flux content increases before and decreases immediately after substorm onsets identified by Frey et al (2004) consistent with dayside reconnection being dominant before onset and nightside reconnection being dominant after onset
Summary
The Dungey (1961) Cycle describes the interaction between the Earth's magnetosphere and the interplanetary magnetic field (IMF) frozen into the solar wind. In a simplified case where the IMF has a strongly southward orientation, magnetic reconnection at the dayside magnetopause leads to the opening of closed magnetic field lines of Earth's magnetosphere. The foot points of the open magnetic field lines are in the polar cap, the region encircled by the auroral oval. The open magnetic field lines convect through the lobes to the nightside magnetosphere, and their footpoints cross the polar cap. Magnetic reconnection in the magnetotail acts to close open magnetic field lines. The newly closed magnetic field lines convect sunward, restoring the structure of the dayside magnetosphere. The open flux content of the polar caps, FPC, varies
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