On the structure and energetics of large scale hydromagnetic disturbances in the solar photosphere

Abstract

It is suggested that the solar differential rotation is maintained against frictional dissipation by large scale quasi-horizontal hydromagnetic disturbances, which possess tilted structures similar to upper level troughs in the westerlies of the earth's atmosphere. On the sun, these tilts would be directed from the equator toward the east limb in each hemisphere. Horizontal Reynolds stresses in these disturbances thus convert eddy kinetic energy into kinetic energy of the zonal flow, while horizontal Maxwell stresses convert the mean zonal kinetic energy into “eddy” magnetic energy. In order for the disturbances as a whole to feed kinetic energy into the differential rotation, the horizontal eddy magnetic fields must be somewhat less than 7 gauss at the level considered. Measurements of the large scale patterns of the line of sight component of the magnetic field, made by Howard, show clearly the required tilt in the patterns toward the east limb in each hemisphere. These measurements also show that the patterns do have a very large horizontal scale, possessing a longitudinal wave number of about 6, and a latitudinal extent of about 60°. The magnitudes of these fields appear to be compatible with horizontal eddy magnetic fields less than 7 gauss. The braking action by the symmetric steady component of the horizontal Maxwell stress appears from the observations to be small compared with that due to the eddy component.