Reconnection Onset in Overstretched Thin Current Sheets: PIC Simulations

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.