A NEW DETERMINATION OF THE SOLAR ROTATION RATE

We use "stackplot" displays to compare observations of the photospheric magnetic field during sunspot cycle 21 with simulations based on the flux-transport model. Adopting nominal rates of diffusion, differential rotation, and meridional flow, we obtain slanted patterns similar to those of the observed field, even when the sources of flux are assigned random longitudes in the model. At low latitudes, the slopes of the nearly vertical patterns of simulated field are sensitive to the rotation rate used in the calculation, and insensitive to the rates of diffusion and flow during much of the sunspot cycle. Good agreement between the observed and simulated patterns requires a synodic equatorial rotation period of 26.75 +/- 0.05 days, which is within the limits obtained by Komm et al., but significantly less than the traditional 26.90 day value of Snodgrass and Newton & Nunn. Below 55-degrees latitude, we obtain the synodic rotation rate omega(theta) = 13.46 - 2.7 cos2 theta + 1.2 cos4 theta - 3.2 cos6 theta deg day-1, where theta is colatitude.