On the extent of the moat flow in axisymmetric sunspots

TL;DR

This study quantitatively compares the extent of moat flows in sunspots of different sizes using high-resolution solar observations, revealing that moat flow extent varies independently of sunspot size and is influenced by local magnetic and activity conditions.

Contribution

It provides the first detailed comparison of moat flow extent across sunspots of varying sizes using multiple observational methods and flow analysis techniques.

Findings

Moat flow is present in all studied sunspots with outward flow and magnetic feature advection.

The extent of the moat flow varies and does not strongly correlate with sunspot size.

Moat flow shows asymmetry and is influenced by sunspot tilt, topology, and activity level.

Abstract

Unipolar, axisymmetric sunspots are figuratively called "theoretician's sunspots" because their simplicity supposedly makes them more suitable for theoretical descriptions or numerical models. On 2013 November 18, a very large specimen (active region NOAA 11899) crossed the central meridian of the Sun. The moat flow associated with this very large spot is quantitatively compared to that of a medium and a small sunspot to determine the extend of the moat flow in different environments. We employ continuum images and magnetograms of the Helioseismic and Magnetic Imager (HMI) as well as extreme ultraviolet (EUV) images at $\lambda$160 nm of the Atmospheric Imaging Assembly (AIA), both on board the Solar Dynamics Observatory (SDO), to measure horizontal proper motions with Local Correlation Tracking (LCT) and flux transport velocities with the Differential Affine Velocity Estimator (DAVE). We compute time-averaged flow maps ($\pm$6 hours around meridian passage) and radial averages of photometric, magnetic and flow properties. Flow fields of a small- and a medium-sized axisymmetric sunspot provide the context for interpreting the results. All sunspots show the outward moat flow and the advection of moving magnetic features (MMFs). However, the extent of the moat flow varies from spot to spot but a correlation of flow properties with size is tenuous, if at all present. The moat flow is asymmetric and predominantly in the east-west direction, whereby deviations are related to the tilt angle of the sunspot group as well as to the topology and activity level of the trailing plage.