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Abstract Onset of reconnection in the magnetotail requires its current sheet (CS) to thin down to the thermal ion gyroradius (or thinner) to demagnetize ions (or even electrons) and to provide their Landau dissipation. However, in isotropic plasma models of the tail the ion‐scale CSs inflate too rapidly with the distance from Earth to remain ion‐scale beyond 20 Earth's radii, where most X‐lines are observed. A key to solving this problem was recently found due to the discovery of “overstretched” thin CSs (OTCSs): If an ion‐scale CS is embedded into a much thicker CS with even a weak field‐aligned ion anisotropy, its current density iso‐contours can be stretched far beyond the magnetic field lines. Here we investigate onset of reconnection in OTCS with their scales and features closer to the observed geometry and evolution of Earth's magnetotail: extension beyond 100 ion inertial lengths, magnetic flux accumulation, dipole field effects and weak external driving. 2‐D particle‐in‐cell (PIC) simulations with open boundaries show that OTCSs help explain the observed X‐line location in the magnetotail. The reconnection electric field strongly exceeds both the external driving field and the slow convection electric field caused by the latter. The magnetic topology change (onset of reconnection proper) is preceded by divergent plasma flows suggesting that the latter are produced by the ion tearing plasma motions. OTCS are also shown to form in isotropic CS after an even shorter driving period, but their transient nature may question universality of this onset scenario. 

Onset of reconnection in the tail requires the current sheet thickness to be of the order of the ion thermal gyroradius or smaller. However, existing isotropic plasma models cannot explain the formation of such thin sheets at distances where the X‐lines are typically observed. Here we reproduce such thin and long sheets in particle‐in‐cell simulations using a new model of their equilibria with weakly anisotropic ion species assuming quasi‐adiabatic ion dynamics, which substantially modifies the current density. It is found that anisotropy/agyrotropy contributions to the force balance in such equilibria are comparable to the pressure gradient in spite of weak ion anisotropy. New equilibria whose current distributions are substantially overstretched compared to the magnetic field lines are found to be stable in spite of the fact that they are substantially longer than isotropic sheets with similar thickness. 

Abstract The formation, development, and impact of slow shocks in the upstream regions of reconnecting current layers are explored. Slow shocks have been documented in the upstream regions of magnetohydrodynamic (MHD) simulations of magnetic reconnection as well as in similar simulations with the kglobal kinetic macroscale simulation model. They are therefore a candidate mechanism for preheating the plasma that is injected into the current layers that facilitate magnetic energy release in solar flares. Of particular interest is their potential role in producing the hot thermal component of electrons in flares. During multi-island reconnection, the formation and merging of flux ropes in the reconnecting current layer drives plasma flows and pressure disturbances in the upstream region. These pressure disturbances steepen into slow shocks that propagate along the reconnecting component of the magnetic field and satisfy the expected Rankine–Hugoniot jump conditions. Plasma heating arises from both compression across the shock and the parallel electric field that develops to maintain charge neutrality in a kinetic system. Shocks are weaker at lower plasma β , where shock steepening is slow. While these upstream slow shocks are intrinsic to the dynamics of multi-island reconnection, their contribution to electron heating remains relatively minor compared with that from Fermi reflection and the parallel electric fields that bound the reconnection outflow.