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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. 

Abstract This short article highlights unsolved problems of magnetic reconnection in collisionless plasma. Advanced in-situ plasma measurements and simulations have enabled scientists to gain a novel understanding of magnetic reconnection. Nevertheless, outstanding questions remain concerning the complex dynamics and structures in the diffusion region, cross-scale and regional couplings, the onset of magnetic reconnection, and the details of particle energization. We discuss future directions for magnetic reconnection research, including new observations, new simulations, and interdisciplinary approaches. 

Abstract Recent multi-point measurements, in particular from the Magnetospheric Multiscale (MMS) spacecraft, have advanced the understanding of micro-scale aspects of magnetic reconnection. In addition, the MMS mission, as part of the Heliospheric System Observatory, combined with recent advances in global magnetospheric modeling, have furthered the understanding of meso- and global-scale structure and consequences of reconnection. Magnetic reconnection at the dayside magnetopause and in the magnetotail are the drivers of the global Dungey cycle, a classical picture of global magnetospheric circulation. Some recent advances in the global structure and consequences of reconnection that are addressed here include a detailed understanding of the location and steadiness of reconnection at the dayside magnetopause, the importance of multiple plasma sources in the global circulation, and reconnection consequences in the magnetotail. These advances notwithstanding, there are important questions about global reconnection that remain. These questions focus on how multiple reconnection and reconnection variability fit into and complicate the Dungey Cycle picture of global magnetospheric circulation. 

Abstract Statistical and case studies, as well as data‐mining reconstructions suggest that the magnetotail current in the substorm growth phase has a multiscale structure with a thin ion‐scale current sheet embedded into a much thicker sheet. This multiscale structure may be critically important for the tail stability and onset conditions for magnetospheric substorms. The observed thin current sheets are found to be too long to be explained by the models with isotropic plasmas. At the same time, plasma observations reveal only weak field‐aligned anisotropy of the ion species, whereas the anisotropic electron contribution is insufficient to explain the force balance discrepancy. Here we elaborate a self‐consistent equilibrium theory of multiscale current sheets, which differs from conventional isotropic models by weak ion anisotropy outside the sheet and agyrotropy caused by quasi‐adiabatic ion orbits inside the sheet. It is shown that, in spite of weak anisotropy, the current density perturbation may be quite strong and localized on the scale of the figure‐of‐eight ion orbits. The magnetic field, current and plasma density in the limit of weak field‐aligned ion anisotropy and strong current sheet embedding, when the ion scale thin current sheet is nested in a much thicker Harris‐like current sheet, are investigated and presented in an analytical form making it possible to describe the multiscale equilibrium in sharply stretched 2D magnetic field configurations and to use it in kinetic simulations and stability analysis. 

Abstract Mining of substorm magnetic field data reveals the formation of two X‐lines preceded by the flux accumulation at the tailward end of a thin current sheet (TCS). Three‐dimensional particle‐in‐cell simulations guided by these pre‐onset reconnection features are performed, taking also into account weak external driving, negative charging of TCS and domination of electrons as current carriers. Simulations reveal an interesting multiscale picture. On the global scale, they show the formation of two X‐lines, with stronger magnetic field variations and inhomogeneous electric fields found closer to Earth. The X‐line appearance is preceded by the formation of two diverging electron outflow regions embedded into a single diverging ion outflow pattern and transforming into faster electron‐scale reconnection jets after the onset. Distributions of the agyrotropy parameters suggest that reconnection is provided by ion and then electron demagnetization. The bulk flow and agyrotropy distributions are consistent with MMS observations.