The velocity structure of moving magnetic feature pairs around sunspots: support for the U-loop model

TL;DR

This study analyzes the velocity structure of moving magnetic feature pairs around sunspots, providing observational support for the U-loop model by examining Doppler shifts, velocities, and lifetimes across different active regions.

Contribution

The paper offers new observational evidence supporting the U-loop model of MMF pairs, linking their velocities and lifetimes to sunspot evolution stages.

Findings

MMF pairs with opposite polarity support downflows of 50-100 m/s.

Doppler shift differences between MMF pair elements vary with active region maturity.

MMF properties, including lifetime, depend on the evolution stage of the parent sunspot.

Abstract

Using data recorded by the Michelson Doppler Imager (MDI) instrument on the Solar and Heliospheric Observatory (SOHO), we have traced 123 pairs of opposite magnetic polarity moving magnetic features (MMFs) in three active regions NOAA ARs 8375, 0330 and 9575. At the time of observation, AR 8375 was young, AR 0330 mature, and AR 9575 decaying. The vertical velocity indicates that the elements of MMF pairs with polarity opposite to that of the sunspot support a downflow of around 50-100 m s$^{-1}$. The average Doppler shift difference between negative and positive elements of an MMF pair is about 150 m s$^{-1}$ in AR 8375, 100 m s$^{-1}$ in AR 0330, and 20 m s$^{-1}$ in AR 9575. These observational results are in agreement with the model that MMF pairs are part of a U-loop emanating from the sunspot's magnetic canopy. According to this model the downflow is caused by the Evershed flow returning below the solar surface. For AR 8375, the horizontal velocity of MMFs ranges from 0.1 km s$^{-1}$ to 0.7 km s$^{-1}$, and on average, the velocity of an MMF pair decreases significantly (from 0.6 km s$^{-1}$ to 0.35 km s$^{-1}$) with increasing distance from the MMF's birth place. This result suggests that the change in MMF flow speed does not reflect the radial structure of the moat flow, but rather is intrinsic to the evolution of the MMF pairs. This result is also in agreement with the U-loop model of MMF pairs. We also find that properties of MMF pairs, most strikingly the lifetime, depend on the evolution stages of the parent sunspot. The mean lifetimes of MMF pairs in ARs 9575 and 0330 are 0.7 hours and 1.6 hours, respectively, which is considerably shorter than the 4 hours lifetime previously found for AR 8375.