MAGNETIC-FIELD INTENSIFICATION AND FORMATION OF FIELD-ALIGNED CURRENT IN A NONUNIFORM MAGNETIC-FIELD

A study is made of the induction of a direct field-aligned current due to Alfven waves excited through an initial-boundary condition for an incompressible fluid with high conductivity in a non-uniform magnetic field. A simple but exact case is considered in which the direction of inhomogeneity of the magnetic field is perpendicular to that of the shear fluid motion at the boundary, and the fluid moves across the magnetic field. The boundary does not satisfy the frozen-flux condition, but has a large Lundquist number. The conductivity of the fluid held between the two boundaries is high, and the frozen-flux condition is approximately satisfied there. A preflare stage of the sun is investigated, and a reasonable magnetic field intensity and d.c. field-aligned current in the magnetic arcade are obtained for a given horizontal shear motion that is antisymmetric at both boundaries of the arcade foot points, taking account of the observed horizontal fluid motion.