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.

Recent advances in reconstructing Earth’s magnetic field and associated currents by utilizing data mining of in situ magnetometer observations in the magnetosphere have proven remarkably accurate at reproducing observed ion diffusion regions. We investigate the effect of placing regions of localized resistivity in global simulations of the magnetosphere at specific locations inspired by the data mining results for the substorm occurring on July 6, 2017. When explicit resistivity is included, the simulation forms an x-line at the same time and location as the MMS observation of an ion diffusion region at 15:35 UT on that day. Without this explicit resistivity, reconnection forms later in the substorm and far too close to Earth ($\gtrsim-15R_E$), a common problem with global simulations of Earth’s magnetosphere. A consequence of reconnection taking place farther down the tail due to localized resistivity is that the reconnection outflows transport magnetic flux Earthward and thus prevent the current sheet from thinning enough for reconnection to take place nearer Earth. As these flows rebound tailward from the inner magnetosphere, they can temporarily and locally (in the dawn-dusk direction) stretch the magnetic field allowing for small scale x-lines to form in the near Earth region. Due to the narrow cross-tail extent of these x-lines ($\lesssim5R_E$) and their short lifespan ($\lesssim5$min), they would be difficult to observe with in situ measurements. Future work will explore time-dependent resistivity using 5 minute cadence data mining reconstructions.