The Spatial Analysis of Magnetic Fields in Sunspots

Abstract

A review of existing methods of determining the strength and direction of magnetic fields in sunspots, as a function of position and depth within the spot shows that these have given fragmentary and somewhat discordant results. In particular, little is known of the direction of the magnetic vector. A new method of determining this quantity is described. By means of a Babinet compensator, polarization fringe systems are produced in the components of a normal Zeeman triplet. From the phase difference of the σ -component fringes, the constants of the elliptical polarization at each point in the spot are determined, and the orientation of the lines of force deduced. The method minimizes the effect of instrumental polarization; the reduction is simple and fairly short. An experimental test on four sunspots has been carried out, using the Oxford 35 m solar tower and 12 m spectrograph with a Babcock grating. Measurements of field direction were made, using the triplet λ 6173 (Fe I ); the associated field strengths were determined from the same line and from λ 6149, a weak line of Fe II . The resulting field-strength variation is in agreement with Broxon's parabolic law, but in contrast with Joy and Nicholson's measures of direction, 40 per cent of the flux is found to be confined within a cone of unit solid angle. Comparisons of field-strength at λ 6173 (Fe I ) and λ 6149 (Fe II ) suggest that the gradient in depth does not exceed 1 oersted/km.