ROTATION OF DOPPLER FEATURES IN THE SOLAR PHOTOSPHERE

The pattern rotation rate for the line-of-sight velocity features in the solar photosphere is determined by cross-correlating Doppler residual coarse arrays from magnetograph observations taken on successive days at the Mount Wilson 150 foot (46 m) tower telescope for the period 1967 January 1 to 1987 March 14. The arrays correlated are the standard Mount Wilson 34 × 34 arrays containing features that remain after the fits for global velocity patterns such as rotation and limbshift have been removed. A correlation averaging technique using long averaging times reduces the noise and permits the study of the velocity correlations which are far weaker than in the corresponding magnetic case. Separate correlations are done for each of the 34 latitude zones spanning the solar disk. From the latitude dependence and the ∼1 day lifetime of the correlation amplitudes, we conclude that the dominant velocity pattern producing the correlation is the supergranulation network, even though it is not resolvable in the coarse arrays. The rotation rate average over the entire period is determined to be ω(φ) = 2.972 - 0.484 sin2 φ - 0.361 sin4 φ μ rad s-1, or ω(φ) = 14.71 - 2.39 sin2 φ- 1.78 sin4 φ deg day-1 sidereal, where φ is the solar latitude. This result is in agreement with an earlier determination of the equatorial rotation of the supergranular cells by Duvall. We find the rate at all latitudes is ∼2% faster than the magnetic and sunspot rates and ∼4% faster than Mount Wilson spectroscopic rate. By intercomparing different portions of our data set, we find no time variation in the rotation rate to within our ∼1% accuracy. Comparing this coarse array determination with Duvall's earlier result indicates that the supergranulation pattern may be a very sensitive indicator of large-scale motions at the top of the solar convection zone. Because this method is largely free from the systematic errors that have plagued the Mount Wilson Doppler measurements, this method could yield additional details of the rotational rate as a function of latitude if a similar but more detailed calculation were carried out using the Mount Wilson raw data instead of the condensed 34 × 34 arrays.