Le 14 octobre 2016
De 10h30 à 12h00
Daniele Del Sarto, Université de Lorraine, Institut Jean Lamour, Nancy
Magnetic reconnection, process at the basis of several phenomena observed in natural and laboratory plasmas (e.g., solar flares, magnetic storms in the magnetosphere, disruptions in tokamaks), consists in a global reorganization of the magnetic topology, thanks to which a large amount of potential magnetic energy is transferred to particle kinetic energy in short time intervals. Despite the large scale modification it induces, reconnection is triggered by microscopic effects (e.g., resistivity or finite electron inertia) that become locally important where strong current gradients are developed. However, the smallness of such effects, whose (positive) powers enter in the estimated reconnection rates, questions the validity of the theoretical estimations presently available to account for the reconnection rates inferred from direct measurements. Most of the recent years’ research on this topic focussed then on the role that secondary instabilities and nonlinear and/or kinetic effects (such as the transition to the whistler-mediated reconnection regime) could play on the reconnection rate enhancement.
In this talk, after revising the classical models of stationary (Sweet-Parker’s) and spontaneous reconnection (the tearing-mode theory), and after recalling how the notion of “fast” has changed in reconnection over the years, we show how simple geometric arguments make the growth rate dependent on the current sheet aspect ratio [1,2] and on the magnetic field amplitude relative to the background [3]. We discuss in particular the consequences that this has on the possibility to achieve a Sweet-Parker-like configuration [1,2] and to provide a transition to reconnection rates compatible, e.g., with the sawtooth-crash in tokamaks [3]. We also point out its implications for the existence of a singular solution to the reconnection problem in the quasi-ideal limit (e.g., S-1→ 0 with S-1≠0), the “ideal tearing” model [1], which has been shown to exist also in kinetic regimes [2], and which is triggered once the current sheet attains a critical aspect ratio that depends on the reconnection mechanism considered. We then discuss the role that the relative amplitude of the current sheet’s magnetic field can have on this [3].
[1] F. Pucci, M. Velli, “Reconnection of quasi-singular current sheets: the “ideal” tearing model”, ApJ. Lett., 780, L19 (2013).
[2] D. Del Sarto, F. Pucci, M. Velli, A. Tenerani, ““Ideal” tearing mode and the transition to fast reconnection in weakly collisional MHD and EMHD regimes”, J. Geophys. Res-Space, 121, 1857 (2016).
[3] D. Del Sarto, M. Ottaviani, “Secondary fast reconnecting instability in the sawtooth crash”, arXiv:1603.00276 (2016).