Search

We analyze the local dynamics of magnetotail reconnection onset using Magnetospheric Multiscale (MMS) data. In conjunction with MMS, the macroscopic dynamics of this event were captured by a number of other ground and space‐based observatories, as is reported in a companion paper. We find that the local dynamics of the onset were characterized by the rapid thinning of the cross‐tail current sheet below the ion inertial scale, accompanied by the growth of flapping waves and the subsequent onset of electron tearing. Multiple kinetic‐scale magnetic islands were detected coincident with the growth of an initially sub‐Alfvénic, demagnetized tailward ion exhaust. The onset and rapid enhancement of parallel electron inflow at the exhaust boundary was a remote signature of the intensification of reconnection Earthward of the spacecraft. Two secondary reconnection sites are found embedded within the exhaust from a primary X‐line. The primary X‐line was designated as such on the basis that (a) while multiple jet reversals were observed in the current sheet, only one reversal of the electron inflow was observed at the high‐latitude exhaust boundary, (b) the reconnection electric field was roughly five times larger at the primary X‐line than the secondary X‐lines, and (c) energetic electron fluxes increased and transitioned from anti‐field‐aligned to isotropic during the primary X‐line crossing, indicating a change in magnetic topology. The results are consistent with the idea that a primary X‐line mediates the reconnection of lobe magnetic field lines and accelerates electrons more efficiently than its secondary X‐line counterparts. Key Points: Magnetotail reconnection onset was triggered by electron tearing during a solar wind pressure pulseOnset was characterized by the rapid collapse of the current sheet thickness and kinetic‐scale flux rope formationA primary X‐line was established within minutes of the onset [ABSTRACT FROM AUTHOR]

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 2‐hr‐long preconditioning phase. It is demonstrated with data in the (a) upstream solar wind, (b) at the low‐latitude magnetopause, (c) in the high‐latitude polar cap, and (d) 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) and precipitation (observed by Defense Meteorological Satellite Program) 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. Key Points: Magnetotail reconnection onset was observed during a fortuitous multiscale conjunction of the heliophysics observatoriesA transient solar wind pressure pulse triggered thinning and stretching of the cross‐tail current sheet without significant flux loadingA second solar wind pressure pulse caused the thinned current sheet to rapidly collapse and reconnect [ABSTRACT FROM AUTHOR]