Using Polar Faculae to Determine the Sun’s High-latitude Rotation Rate. II. Simulations and New Measurements

Abstract In a previous paper, I described a new way of determining the high-latitude solar rotation rate statistically from spacetime maps of polar faculae observed in the 6767 Å continuum by the Michelson Doppler Interferometer on the Solar and Heliospheric Observatory. Now, I have tested the technique by applying it to simulated images whose faculae have known speeds, and I have been able to recover those speeds with an accuracy better than 0.01 km s −1 . Repeated measurements of the Sun’s polar faculae gave the same high-latitude profile as before, but with a slightly faster synodic rotation rate of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>9</mml:mn> <mml:mo>.</mml:mo> <mml:mn>10</mml:mn> <mml:mo>°</mml:mo> <mml:mspace width="0.25em"/> <mml:msup> <mml:mi>day</mml:mi> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> and a rotation period of 39.6 days. Applying this spacetime tracking procedure to magnetic flux elements observed with the Helioseismic Magnetic Imager on the Solar Dynamics Observatory, I obtained a similar rotation profile with a speed of 9.55° day −1 and a synodic rotation period of 37.7 days. These rates are comparable to polar rotation rates, obtained by other techniques, but the new latitude profiles are noticeably flatter than the quartic fits to those prior measurements.