Magnetic Reconnection and Dynamos in Laboratory Plasmas

Abstract

Magnetic reconnection is a process of major importance in astrophysics. It is often invoked in the solar corona, but also in the Earth’s magnetosphere and in many other contexts. One of the achievements of YOHKOH has been to provide observational evidence for magnetic reconnection in the solar corona (e.g. Tsuneta et al, 1992). As well as large scale reconnections which may reconfigure the global field, many theories of coronal heating propose that coronal plasma is heated by a superposition of small localised reconnection events (which may be identified with nanoflares; Parker, 1988). One way to quantify the heating by many such small scale events is to use the idea of relaxation theory, first developed by Taylor (1974) to explain the presence of reversed toroidal field in Reverse Field Pinch devices, whereby a plasma tends to minimise its magnetic energy whilst conserving its global magnetic helicity (Heyvaerts and Priest, 1984; Browning, 1988, 1991; Vekstein et al, 1993). This hypothesis has recently been strongly supported by evidence from numerical simulations (Kusano et al, 1994). Helicity conservation has also proved useful in modelling filament eruptions (Rust and Kumar, 1996) and other astrophysical phenomena such as galactic jets (Vekstein et al, 1994).