‘Ideally’ unstable current sheets and the triggering of fast magnetic reconnection

Abstract

Magnetic reconnection is thought to be the dynamical mechanism underlying many explosive phenomena observed both in space and in the laboratory, though the question of how fast magnetic reconnection is triggered in such high Lundquist (S) number plasmas has remained elusive. It has been well established that reconnection can develop over timescales faster than those predicted traditionally once kinetic scales are reached. It has also been shown that, within the framework of resistive Magnetohydrodynamics (MHD), fast reconnection is achieved for thin enough sheets via the onset of the so-called plasmoid instability. The latter was discovered in studies specifically devoted to the Sweet-Parker current sheet, either as an initial condition or an apparent transient state developing in nonlinear studies. On the other hand, a fast tearing instability can grow on an ideal, i.e., S-independent, timescale (dubbed "ideal" tearing) within current sheets whose aspect ratio scales with the macroscopic Lundquist number as L/a ∼ S 1/3 -much smaller than the Sweet-Parker one -suggesting a new way to approach to the initiation of fast reconnection in collapsing current configurations. Here we present an overview of what we have called "ideal" tearing in resistive MHD, and discuss how the same reasoning can be extended to other plasma models commonly used that include electron inertia and kinetic effects. We then discuss a scenario for the onset of "ideal" fast reconnection via collapsing current sheets and describe a quantitative model for the interpretation of the nonlinear evolution of "ideally" unstable sheets in two dimensions.

Spectral signatures of recursive magnetic field reconnection

“…Far from the neutral line, a standard Kolmogorov spectrum is approached, both in magnetic and kinetic energy, which show a scaling very close to k −5/3 . We have adopted a heuristic method based on the "recursive" reconnection already discussed in Tenerani et al (2015bTenerani et al ( , 2016 to provide a model that predicts magnetic energy density spectral power laws which are consistent with our numerical results. We have extended such model to generic current sheet profiles by introducing the parameter p, where hypothetically 1 ≤ p < ∞; p = 1 is the most natural choice as it corresponds to the Harris profile.…”

Formation and Reconnection of Three-dimensional Current Sheets with a Guide Field in the Solar Corona

“…As the driving increases, the system attempts to process more material and flux through the current sheet and the reconnection rate necessarily increases. Once the rate of material and magnetic flux to process exceeds the current sheet's capacity via the Sweet-Parker type reconnection, basic mass and energy continuity necessitate the rapid onset of nonlinearity and the development of plasmoid-unstable evolution (see also Daughton et al 2011a,b;Pucci & Velli 2014;Tenerani et al 2016). …”

Fast Magnetic Reconnection: “Ideal” Tearing and the Hall Effect

“…From the non-linear evolution point of view, it would be of great interest to follow a collapsing current sheet (initially with 0 guide field), into the Hall regime. The resistive internal singular layer, which in 2 Dimensions becomes the thickness of secondary current sheets in nonlinear evolution (Tenerani et al (2016b)), and where the Hall magnetic field is different from zero, tends to shrink for larger values of the Hall parameter, making it a challenging problem which we hope to address in the near future.…”