Magnetic Reconnection

10.7 Summary

Theory and numerical simulations of magnetic reconnection processes in the solar corona have been developed for steady 2D reconnection (§ 10.1), bursty 2D reconnection (§ 10.2), and 3D reconnection (§ 10.3). Only steady 2D reconnection models can be formulated analytically (§ 10.1), which provide basic relations for inflow speed, outflow speed, and reconnection rate, but represent oversimplifications for most (if not all) observed flares. A more realistic approach seems to be bursty 2D reconnection models (§ 10.2), which involve the tearing-mode and coalescence instability and can reproduce the sufficiently fast temporal and small spatial scales required by solar flare observations. The sheared magnetic field configurations and the existence or coronal and chromospheric nullpoints, which are now inferred more commonly in solar flares, require ultimately 3D reconnection models, possibly involving nullpoint coalescence, spine reconnection, fan reconnection, and separator reconnection (§ 10.3). Magnetic reconnection operates in two quite distinct physical parameter domains: in the chromosphere during magnetic flux emergence, magnetic flux cancellation, and so-called explosive events (§ 10.4), and under coronal conditions during microflares, flares, and CMEs (§ 10.5). The best known flare/CME models entail magnetic reconnection processes that are driven by a rising filament/prominence, by flux emergence, by converging flows, or by shear motion along the neutral line (§ 10.1). Flare scenarios with a driver perpendicular to the neutral line (rising prominence, flux emergence, convergence flows) are formulated as 2D reconnection models (Kopp-Pneuman 1976; Heyvaerts et al. 1977; Forbes & Priest 1995; Uchida 1980), while scenarios that involve shear along the neutral line (tearing-mode instability, quadrupolar flux transfer, the magnetic breakout model, sheared arcade interactions) require 3D descriptions (Sturrock 1966; Antiochos et al. 1999b; Somov et al. 1998). Ultimately, most of these partial flare models could be unified in a 3D model that includes all driver mechanisms. Observational evidence for magnetic reconnection in flares includes the 3D geometry, reconnection inflows, outflows, detection of shocks, jets, ejected plasmoids, and secondary effects like particle acceleration, conduction fronts, and chromospheric evaporation processes (§ 10.6). Magnetic reconnection not only operates locally in flares, it also organizes the global corona by large-scale restructuring processes.