Magnetic reconnection: MHD theory and modelling

Abstract

In this review we focus on the fundamental theory of magnetohydrodynamic reconnection, together with applications to understanding a wide range of dynamic processes in the solar corona, such as flares, jets, coronal mass ejections, the solar wind and coronal heating. We summarise only briefly the related topics of collisionless reconnection, non-thermal particle acceleration, and reconnection in systems other than the corona. We introduce several preliminary topics that are necessary before the subtleties of reconnection can be fully described: these include null points (Sects. 2.1–2.2), other topological and geometrical features such as separatrices, separators and quasi-separatrix layers (Sects. 2.3, 2.6), the conservation of magnetic flux and field lines (Sect. 3), and magnetic helicity (Sect. 4.6). Formation of current sheets in two- and three-dimensional fields is reviewed in Sect. 5. These set the scene for a discussion of the definition and properties of reconnection in three dimensions that covers the conditions for reconnection, the failure of the concept of a flux velocity, the nature of diffusion, and the differences between two-dimensional and three-dimensional reconnection (Sect. 4). Classical 2D models are briefly presented, including magnetic annihilation (Sect. 6), slow and fast regimes of steady reconnection (Sect. 7), and non-steady reconnection such as the tearing mode (Sect. 8). Then three routes to fast reconnection in a collisional or collisionless medium are described (Sect. 9). The remainder of the review is dedicated to our current understanding of how magnetic reconnection operates in three dimensions and in complex magnetic fields such as that of the Sun’s corona. In Sects. 10–12, 14.1 the different regimes of reconnection that are possible in three dimensions are summarised, including at a null point, separator, quasi-separator or a braid. The role of 3D reconnection in solar flares (Sect. 13) is reviewed, as well as in coronal heating (Sect. 14), and the release of the solar wind (Sect. 15.2). Extensions including the role of reconnection in the magnetosphere (Sect. 15.3), the link between reconnection and turbulence (Sect. 16), and the role of reconnection in particle acceleration (Sect. 17) are briefly mentioned.

Application of historic datasets to understanding open solar flux and the 20th-century grand solar maximum. 2. Solar observations

“…Potentially, this provides an explanation of the factor two difference between in-situ OSF values and those from photospheric modelling (Lockwood et al, 2022). One reason why much of this streamer belt OSF has been missed in PFSS modelling of the corona (at all phases of the solar cycle) is likely to be the assumption that the corona is currentfree, an assumption that is directly contradicted by the fact that magnetic reconnection is commonly observed in the corona (see review by Pontin and Priest, 2022). Additional open flux in nonpotential models arises through inflation of the magnetic field by coronal electric currents.…”

The Location and Angle Distribution of Magnetic Reconnection in the Solar Corona

“…The challenge in studying the onset angle or height of magnetic reconnection is that MHD models of active regions are typically only easily able to identify the global consequences of reconnection, or to state whether or not reconnection has occurred. They are not easily able to identify the location of reconnection, unless the topology contains special regions, such as null points or separatrix and quasiseparatrix layers (Aulanier et al 2006;Démoulin 2006;Baker et al 2009;Baumann & Galsgaard 2013;Wyper & Pontin 2014;Boozer 2019;Pontin & Priest 2022), where strong current sheets develop following even minimal stressing. In models with simpler topologies, identifying magnetic field lines that reconnect is very difficult because (1) once reconnected, the identity of the field line is altered, and (2) field lines move in response to applied forces, so even in the absence of reconnection, uniquely identifying a given field line requires advecting the seed point of the field line-tracing algorithm with the velocity flow.…”

Filament Leg–Leg Reconnection as a Source of Prominent Supra-arcade Downflows

“…Eruptive solar flares are long recognized to be consequences of magnetic reconnection of oppositely oriented field lines into new flux rope field lines and flare loops, as described by the standard solar flare model in two dimensions (2D; Carmichael 1964;Sturrock 1966;Hirayama 1974;Kopp & Pneuman 1976), later extended to three dimensions (3D; Aulanier et al 2012Aulanier et al , 2013Janvier et al 2013Janvier et al , 2015; see also the reviews of Li et al (2021a), Pontin & Priest (2022), and Kazachenko et al (2022). However, the observations of flares and eruptions revealed some problems with this picture.…”