Cessation and restart of reconnection -- observations from the exhaust

Magnetic reconnection in Earth’s magnetotail is inherently intermittent, yet the physical processes governing its cessation and subsequent restart remain poorly understood, largely due to the multiscale nature of the system. In this study, we use high-resolution, multi-point observations from the Magnetospheric Multiscale (MMS) mission to investigate a three-phase event from the terrestrial magnetotail in which reconnection is initially active, subsequently absent for several minutes, and then reinitiates.

The event begins with an off-equatorial, field-aligned ion jet indicative of ongoing reconnection. This jet is replaced by a prolonged quiet interval characterized by a duskward ion flow carried by a hot population, negligible ExB drift, and the absence of conventional reconnection signatures. During this interval, the total plasma plus magnetic pressure increases, and the observations reveal evidence for current sheet thickening followed by thinning. 

The first indication of renewed activity is an injection of energetic field-aligned ions detected off-equatorially, followed by the gradual formation of an equatorial plasma jet and the subsequent arrival of dipolarization fronts. The first dipolarization front clearly separates ions originating from the pre-existing plasma sheet and the lobes, signalling the arrival of magnetic flux tubes that were among the first to reconnect during onset. At the onset of the emerging jet, prior to the arrival of the first dipolarization front, ions briefly become demagnetized and a northward electric field is observed, opposite in sign to the typical Hall electric field expected in the ion diffusion region. These signatures highlight the complex and transient nature of the plasma environment during the evolution of a reconnection outflow jet and point to processes that cannot be fully resolved with the MMS tetrahedron alone.

These observations demonstrate that to understand reconnection intermittency requires simultaneous measurements spanning electron, ion, and magnetohydrodynamic scales. Plasma Observatory, providing coordinated multi-point coverage across these scales, is essential for capturing the coupled evolution of particles, fields, and currents during reconnection cessation and onset—processes that cannot be resolved with present-day multi-spacecraft constellations.