Presentation #105.03 in the session Multi-scale physics of Earth’s magnetotail.
We analyze a magnetotail reconnection onset event that was observed under otherwise quiescent magnetospheric conditions by a fortuitous conjunction of many space and ground-based observatories. The study investigates the large-scale coupling of the solar wind–magnetosphere system that precipitated the onset of the magnetotail reconnection, focusing on the processes that thinned and stretched the cross-tail current layer in the absence of significant flux loading during a two-hour long preconditioning phase. It is demonstrated with data in the (1) upstream solar wind, (2) at the low-latitude magnetopause, (3) in the high-latitude polar cap, and (4) in the magnetotail that the typical picture of solar wind-driven current sheet thinning via flux loading does not appear relevant for this particular event. We find that the current sheet thinning was, instead, initiated by a transient solar wind pressure pulse. We seek to understand how the current sheet thinning continued as the magnetotail and solar wind pressures decreased. We suggest that field line curvature induced scattering (observed by MMS) and precipitation (observed by DMSP) of high-energy thermal protons may have evacuated plasma sheet thermal energy, which may require a thinning of the plasma sheet to preserve pressure equilibrium with the solar wind.