  We analyze a magnetotail reconnection onset event on 3 July 2017 that was
observed under otherwise quiescent magnetospheric conditions by a fortuitous
conjunction of six 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 and that the current sheet thinning continued even as
the magnetotail and solar wind pressures decreased. We suggest that field line
curvature induced scattering (observed by Magnetospheric Multiscale (MMS)) and
precipitation (observed by Defense Meteorological Satellite Program (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.
