COMPARISONS BETWEEN THEORY AND OBSERVATION OF ACTIVE-REGION TILTS

Active regions in the Sun are generally tilted relative to the azimuth al direction, with the leading side being closer to the equator than t he following side. This tilt is known to increase with latitude. Recen tly, theoretical calculations of the dynamics of emerging, initially t oroidal active-region flux tubes have been done, showing that the obse rved tilts can be explained by the Coriolis force acting on a divergin g flow field in emerging flux loops. The calculations of Fan, Fisher, & McClymont predict that alpha infinity PHI1/4B0-5/4 sin theta, where alpha is the tilt angle of the active region, B0 is the magnetic field strength of the active-region flux tube near the base of the convecti on zone, and PHI is the amount of magnetic flux in the tube. We compar e these theoretical predictions with the behavior of a sample of 24,70 1 sunspot groups observed at Mount Wilson over a period of 68 yr, usin g the polarity separation distance d as a proxy for PHI. Our major fin dings are: 1. The mean tilt of sunspot groups is an increasing functio n of both latitude theta and polarity separation d. If the data are fi t to functions of the form alphaBAR = A(y) d(y) sin theta, where alpha BAR is the mean tilt, then we find 0.1 < y < 0.5. The theoretically pr edicted value of y = 0.25 seems to be consistent with the observations , for which we find A0.25 = 6.69 +/- 0.27, with d given in Mn and alph aBAR given in degrees. 2. When we compare the average tilts of large s unspot groups (d > 50 Mm) to the detailed numerical simulations of Fan , Fisher, & McClymont assuming PHI almost-equal-to 10(22) Mx, we find best agreement if B0 almost-equal-to 20-30 kG. 3. We find that the rms deviation DELTAalpha of spot-group tilts away from the mean tiltt beh avior obeys the approximate relation DELTAalpha almost-equal-to 10-deg rees x (d/100)-3/4 (where DELTAalpha is measured in degrees and d is m easured in Mm) and does not seem to depend on latitude. The lack of a latitude dependence suggests that the range of B0 for emerging flux lo ops is narrow. We propose that the source of the rms deviations is buf feting by convective motions, and we show how convection could produce a dependence of DELTAalpha on d similar to that observed.