Using Polar Faculae to Determine the Sun's High-Latitude Rotation Rate. II: Simulations and New Measurements

TL;DR

This paper introduces a new statistical method to measure the Sun's high-latitude rotation rate using polar faculae, validated through simulations and applied to observational data, yielding consistent and refined rotation profiles.

Contribution

The paper presents a novel space-time tracking technique for polar faculae that improves measurement accuracy and provides new insights into the Sun's high-latitude rotation profile.

Findings

Simulation tests recover known faculae speeds with >0.01 km/s accuracy.

Repeated measurements show a high-latitude rotation rate of ~9.10°/day.

Application to SDO/HMI data yields a rotation rate of ~9.55°/day.

Abstract

In a previous paper, I described a new way of determining the high-latitude solar rotation rate statistically from space-time maps of polar faculae observed in the 6767 \r{A} continuum by the Michelson Doppler Interferometer (MDI) on the Solar and Heliospheric Observatory (SOHO) Sheeley (2024). 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 9.$^{\circ}$10 day$^{-1}$ and a rotation period of 39.6 days. Applying this space-time tracking procedure to magnetic flux elements observed with the Helioseismic Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO), I obtained a similar rotation profile with a speed of 9.$^{\circ}$55 day$^{-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.