Flares, CMEs and explosive events: the “ideal” tearing mode and fast reconnection triggering in magnetized plasmas, from fluid to kinetic scales

Fulvia Pucci (National Institute for Natural Sciences, NINS)


Astrophysical plasmas are characterized by large Lundquist numbers, meaning that on large scales the effect of collisions is negligible, and that the magnetic field can not be dissipated because frozen-in conditions apply. Magnetic reconnection is thought to be the mechanism responsible for flares, CMEs and explosive events in astrophysical as well as in laboratory plasmas. One of the main questions is what triggers magnetic reconnection and how this mechanism capable to account for fast magnetic energy conversion to kinetic and thermal energies of particles. Previous models of reconnection failed to reproduce the fast dynamics of magnetic reconnection: either the mechanism is too slow to explain to the reconnection rate, or the plasmoid instability disrupts the sheets before they can even form. By examining current sheets with thicknesses scaling as different powers of the Lundquist number number S, in our work Pucci and Velli 2014 we showed that the growth rate of the tearing mode increases rapidly in thinning sheets, and, once the thickness reaches a scaling a/L∝S^{-1/3} the time-scale for the instability to develop becomes of the order of the Alfvén time and independent of the resistivity. 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 final turbulent disruption of the sheet itself. This has fundamental implications for magnetically driven reconnection throughout the corona, and in particular for coronal heating and the triggering of coronal mass ejections (Shibata and Tanuma 2001). A similar critical aspect ratio scaling holds for general magnetic field equilibria, for example encompassing multiple current layers occurring in solar as well as laboratory plasmas contexts (Contopoulos 2007, Dalhburg and Karpen 1995, Ono et al. 1997). I will also talk about our latest work on the fractal reconnection scenario and reconnection in partial ionized plasmas, in collaboration with K. Shibata-san and Dr. A. Singh-san. Finally I will briefly introduce collisionless reconnection for planetary magnetospheres, the main observed features and my contribution to the study of energization in the earth’s magnetotail, a project carried out in collaboration with the MRX (magnetic reconnection experiment) group in Princeton.

host contact: Shinsuke Takasao