The Impact of Microscopic Magnetic Reconnection on Pre-Flare Energy Storage

Abstract

It is widely accepted that magnetic reconnection releases a large amount of energy during solar flares. Studies of reconnection usually assume that the length scale over which the global (macroscopic) magnetic field reverses is identical to the thickness of the reconnection site. However, in spatially extended high-Lundquist number plasmas such as the solar corona, this scenario is untenable; the reconnection site is microscopic and embedded inside the macroscopic current set up by global fields. We use numerical simulations and scaling arguments to show that embedded effects on reconnection could have a profound influence on energy storage before a flare. From largescale high-Lundquist number resistive magnetohydrodynamics simulations of reconnection with a diffusion region on a much smaller scale than the macroscopic current sheet, we find that the generation of secondary islands is governed by the local magnetic field immediately upstream of the diffusion region rather than the (potentially much larger) global field. This diminishes the production of secondary islands and leads to a thicker diffusion region than those predicted using the global field strength. Such considerations are crucial for understanding the onset of solar eruptions and how energy accumulates before such eruptions. We argue that if reconnection with secondary islands is fast, the energy storage times before an eruption are too small to explain observations. If reconnection with secondary islands remains slow, embedded effects cause the diffusion region to begin far wider than kinetic scales, so energy storage before a flare can occur while collisional (Sweet-Parker) reconnection with secondary islands proceeds.

Magnetic Reconnection: Recursive Current Sheet Collapse Triggered by “Ideal” Tearing

“…The results therefore have different origin, and for the IT case the presence of flows modifies the scaling exponent α c according to eq. (5): incidentally, the latter expression is in excellent agreement with the aspect ratios at which plasmoids are observed to be ejected in the (Cassak & Drake 2009) Figure 4. Length L 1 /L (green), width a 1 /L (magenta), and inverse aspect ratio a 1 /L 1 (blue) of the first secondary current sheet vs. time (run 1).…”

“…2 shows a temporal sequence of recursive plasmoid formation, taking place at the center, near the flow stagnation point of the original instability. Previously, such recursive reconnection has been modeled as a succession of unstable SP layers (Loureiro et al 2005;Daughton et al 2009;Cassak & Drake 2009;Huang & Battacharjee 2010;Uzdensky et al 2010;Loureiro et al 2012). Our simulations show instead that it is driven by onset of the IT mode, triggered by the dynamical lengthening of sheets to the local critical threshold, in a way similar to that discussed in section 4.1.…”

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

“…n * 3, 2 and β = 3/8, 0.8. This heuristic argument then leads to values for the number of plasmoids consistent with that found directly in numerical simulations (Daughton et al 2009;Cassak and Drake 2009;Huang and Bhattacharjee 2010).…”

Secondary fast reconnecting instability in the sawtooth crash

“…2 for the collisional (Eqs. (8)(9)(10)(11) with ∆ ′ given by (12)) and non-collisional regime (Eqs. (33-36)).…”