Space Physics Seminar fall-2015

Fast tearing: the transition to kinetic effects

By examining sheets with thicknesses scaling as different powers of the magnetic Reynolds number S, Pucci & Velli (2014) showed that the growth rate of the tearing mode increases as current sheets thin and, once the thickness reaches a scaling a/L=S1/3, the time scale for the instability to develop becomes of the order of the Alfvén time. That means that a fast instability sets in well before Sweet-Parker type current sheets can form. In addition, such an instability produces many islands in the sheet, leading to a fast nonlinear evolution and most probably a turbulent disruption of the sheet itself (Tenerani et al. 2015). This has fundamental implications for magnetically driven reconnection throughout the corona, and in particular for coronal heating and the triggering of coronal mass ejections. At high Lundquist numbers scales get smaller and the transition to kinetic physics becomes important: herewe consider first the Hall effect, showing that we can define a trigger relation as in the resistive case, and then the effect of electron skin depth reconnection. In particular we present a linear study aiming to show how an ”ideal tearing mode” is achieved when kinetic effects are included, including scaling laws for sheet aspect ratios and growth rates. We show that, taking into account this effect an ”ideal” growth rate where ??A ? 1 still defines the trigger condition for fast reconnection. These results provide a step towards identifying the proper conditions for reconnection to effectively occur in lab and space plasmas.