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Abstract Reconstruction of the magnetospheric magnetic field using swarms of virtual spacecraft provided by data mining confirms seminal in situ evidence (Angelopoulos et al., 2008,https://doi.org/10.1126/science.1160495) that on 26 February 2008 an X‐line emerged in the region between two distant Time History of Events and Macroscale Interactions during Substorms probes at the time of the substorm activation in the magnetotail. It also shows that the X‐line formation was preceded by rapid current decay that happened 15 min earlier. The current was built up earthward of the pre‐existing X‐line formed prior to the previous substorm activation 45 min before. The most pronounced effect of the tail reconfiguration at the moments of two substorm activations and the current disruption is the rapid earthward redistribution of the magnetic flux. Comparison of low‐altitude mapping of the magnetotail structure with all‐sky imager data shows that these rapid reconfigurations might be triggered by plasma flows whose source was farther from the Earth than the resolved X‐lines. 

Abstract So far, the structure and dynamics of the terrestrial magnetotail have largely been investigated within ( is the Earth's radius) because of the lack of in‐situ observations beyond that distance, as well as limitations of earlier empirical models. Here we reconstruct the global structure of the cislunar tail within using 2011–2023 data from the ARTEMIS mission, the 2010 THEMIS‐ARTEMIS transition orbits and other missions, such as IMP‐8 and Geotail. The reconstruction is made using a new‐generation data mining‐based empirical algorithm with minimal ad hoc assumptions on the structure of the equatorial current sheet and its evolution during storms and substorms. It is found that the cislunar tail has a regular structure, stable in the substorm growth phase and regularly changing during the expansion and recovery phases. Substorms likely involve the formation of an X‐line around and magnetic flux accumulation earthward of it, which persists in the growth phase and flattens out after the substorm onset. Consistent with earlier reconstructions of the shorter tail region, the present ones reveal the near‐Earth dipolarization of the magnetic field earthward of , which is a part of the whole cislunar tail flux redistribution. Thin current sheets embedded into a thicker current halo extend up to 40 and may exist at lunar distances, consistent with in‐situ ARTEMIS observations, near O‐lines of plasmoids/flux ropes. 

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.