The Sun's rotation rate in its equatorial plane

Abstract

We investigate the Sun's rotation rate in its equatorial plane, as a function of depth, using Big Bear data for the years 1986, 1988, 1989 and 1990. One novel feature of this investigation is that we have used an iterative inversion technique, based upon the regularized least-squares method. Such methods generally have one or more adjustable parameters, and the details of the inferred rotation profile depend upon the values chosen for those parameters. We find that the iterative technique produces results that are much less sensitive to the values of the parameters, which may be seen as one advantage of our procedure. Our results exhibit various features, including a secular decrease in the rotation rate beneath the convection zone, a locally enhanced rotation rate near 0.9 R_⊙ (R_⊙ being the photospheric radius), and a local minimum near 0.6 R_⊙. Not only are these features apparently fairly robust from year to year, they have all variously been seen in inversions of other datasets. However, we demonstrate that some caution should be exercised in interpreting such features in the Sun's equatorial rotation rate.