A study of the contrast of sunspots from photometric images

The thermal contrast , and the umbra-penumbraA_u/A_p, were calculated for 63 sunspots of various sizes and morphologies. Contrary to the assumptions of the PSI model, andA_u/A_p were found to be quite variable. The values of ranged from 0.1807 to 0.4266;A_u/A_p ranged from 0.0089 to 0.4899. The values of andA_u/A_p correlated well (r = 0.6018;p<0.005) and the regression for andA_u/A_p was obtained: = (0.220 ± 0.016) + (0.340 ± O.06)A_u/A_p. The values of andA_u/A_p were then compared with complexity ratings, magnetic field strength, time, and . The quantities andA_u/A_p were found to be independent of the complexity, magnetic field strength, and time factors. The correlation between andA_u/A_p lead to the proposed division of into an umbral thermal contrast _u, and a penumbral thermal contrast _p. These values were calculated from the photometric data: _u = 0.57 ± 0.01 and _p = 0.26 ± 0.006.