Measurements of Solar Differential Rotation and Meridional Circulation from Tracking of Photospheric Magnetic Features

Abstract

Abstract Long-lived rotational and meridional flows are important ingredients of the solar cycle. Magnetic field images have typically been used to measure these flows on the solar surface by cross-correlating thin longitudinal strips or square patches across sufficiently long time gaps. Here, I use 1 month of SDO/HMI line-of-sight magnetic field observations, combined with the Southwest Automatic Magnetic Identification Suite magnetic feature-tracking code to measure the motion of individual features in these magnetograms. By controlling for perturbations due to short-lived flows and due to false motions from feature interactions, I effectively isolate the long-lived flows traced by the magnetic features. This allows me to produce high-resolution (2° bins) differential rotation measurements with well-characterized variances and covariances of the fit parameters. I find a sidereal rotational profile of (14.296 ± 0.006) + (−1.847 ± 0.056)sin2 b + (−2.615 ± 0.093) sin4 b, with units of deg day−1, and a large covariance σ BC 2 = −4.87 × 10−3(deg day−1)2. I also produce measurements of the much weaker meridional flow that are broadly consistent with previous results. These measurements exhibit a peak flow of 16.7 ± 0.6 m s−1 at latitude b = 45° but are insufficiently characterized at higher latitudes to ascertain whether the chosen functional form is appropriate. This work demonstrates that measuring the motions of individual features in photospheric magnetograms can produce high-precision results in relatively short time spans, and suggests that high-resolution non-longitudinally averaged photospheric velocity residual measurements could be produced to compare with coronal results and to provide other diagnostics of the solar dynamo.