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

Journal Article · Fri Feb 10 04:00:00 EST 2017 · Astrophysical Journal

DOI:https://doi.org/10.3847/1538-4357/836/1/10· OSTI ID:22869476

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)sin{sup 2} b + (−2.615 ± 0.093) sin{sup 4} b, with units of deg day{sup −1}, and a large covariance σ {sub BC} {sup 2} = −4.87 × 10{sup −3}(deg day{sup −1}){sup 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{sup −1} at latitude b = 45° but are insufficiently characterized at higher latitudes to ascertain whether the chosen functional form 2cos bsin b 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.

• OSTI ID: 22869476
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 836; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
DISTURBANCES
MAGNETIC FIELDS
PERTURBATION THEORY
PHOTOSPHERE
RESOLUTION
ROTATION
SOLAR CYCLE
SUN
VELOCITY