CROSS-FIELD CURRENT CLOSURE BELOW THE SOLAR PHOTOSPHERE

A simple model is developed to describe how an externally imposed curr ent closes as a function of time below the photosphere. A vertical cur rent density is assumed to turn on at the photospheric boundary. The m odel implies that the subsequent closure of the current in the sub-pho tosphere depends only on the ratio R(A)/R, where R(A) = mu0upsilon(A) is the Alfvenic impedance of the photosphere and R = 1/sigma(P)l is th e resistance corresponding to the conductivity sigma(P) and a characte ristic length l. For R(A)/R much greater than 1, current closure occur s at a front, propagating with the Alfven speed. For R(A)/R much less than 1, current closure is a diffusive process ahead and behind a slow ly propagating Alfvenic front. The first case is the relevant one for the Sun, where R(A)/R approximately 10(8)/upsilon(A), for upsilon(A) i n kilometres per second.