THE SOLAR CORONA AS A MINIMUM ENERGY SYSTEM

This paper is an exploration of the possibility that the large-scale e quilibrium of plasma and magnetic fields in the solar corona is a mini mum energy state. Support for this conjecture is sought by considering the simplest form of that equilibrium in a dipole solar field, as sug gested by the observed structure of the corona at times of minimum sol ar activity. Approximate, axisymmetric solutions to the MHD equations are constructed to include both a magnetically closed, hydrostatic reg ion and a magnetically open region where plasma flows along field line s in the form of a transonic, thermally-driven wind. Sequences of such solutions are obtained for various degrees of magnetic field opening, and the total energy of each solution is computed, including contribu tions from both the plasma and magnetic field. It is shown that along a sequence of increasingly closed coronal magnetic field, the total en ergy curve is a non-monotonic function of the parameter measuring the degree of magnetic held opening, with a minimum occurring at moderate field opening. For reasonable choices of model parameters (coronal tem perature, base density, base magnetic field strength, etc.), the morph ology of the minimum energy solution resembles the observed quiet, sol ar minimum corona. The exact location energy minimum along a given seq uence depends rather sensitively on some of the adopted parameter valu es. It is nevertheless argued that the existence of an energy minimum along the sequences of solutions should remain a robust property of mo re realistic coronal wind models that incorporate the basic characteri stics of the equilibrium corona the presence of both open and closed m agnetic regions.