Adv. Space Res. Vol. 6 No. 6. pp. 7~. 1Y86 (t27~i~1I~ ~ri SftUO Printed in Great Britain. AU rights reser’ed. Cop~rlUht © COSPAR ROLE OF MAGNETIC RECONNECTION IN SOLAR FLARES E. R. Priest Department ofz4pplied Mathematics, North Haugh, St Andresvs KY/ó 9SS, Scotland ABSTRACT The behaviour of the magnetic field in the solar atmosphere is governed by the equations of MHD rather than the electromagnetism of wires. In particular, the roles of magnetic reconnection are: to create small flares; to trigger large flares; and to release magnetic energy in large flares. Observational evidence for reconnection is reviewed together with recent theory and observation of (post-) flare loops, which enable one to deduce the re- connection electric field from the motion of either the flare loops or Ha ribbons. Also, an account is given of a new unified theory for fast steady—state reconnection which includes the Sweet—Parker, Petschek and Sonnerup—like models as special cases. 1. INTRODUCTION During and since the Solar !‘laximum Year there has been a great advance in our understanding of the observations and theory of magnetic reconnection in flares, as we shall indicate in this review. The behaviour of the magnetic field in the solar atmosphere is For the most part governed by the two basic equations of MHD, dv p - ~p + x (1) ~~Vx(vxB)÷nc72B (2) namely the equation of motion, which indicates how the plasma is accelerated by a plasma pressure gradient and a magnetic force, and the induction equation, which shows that the magnetic field changes in time due to plasma motions and magnetic diffusion. Two points should be noted about these equations. Firstly, the nonlinear terms imply that there is a much more subtle coupling between the plasma and the magnetic field than found with simply the electromagnetism of wires. Also the physics is quite different. For wires the electric field E is a primary variable and drives an electric current jnc, (3) whereas the magnetic field (8) is secondary and may be deduced from Amperes Law (V x B z ~.i,j). For MHD, by contrast, the plasma velocity (v) and magnetic field (B) are primary and determined by (1) and (2), while j and E are secondary and may be calculated if necessary from jnVxB/~.i and Ee-vxB+j/o, (4) in which the i/o term is usually negligible. Therefore, it is usually misleading and naive to talk about’~currentsdriven by electric fields along circuits in the solar atmosphere! SecOndly, the ratio of the first to the second term on the right of (2) is the magnetic Reynoldsnumber, - L RnV~ (5) 73