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

Context.Moving magnetic feature (MMF) pairs are among the most significant fine-scale structures around sunspots. Several models have been proposed to interpret the origin and evolution of MMF pairs. These models provide important clues to understanding MMF pairs. Aims.We present an analysis of the velocity structure of MMF pairs in order to put further constrains on the MMF models. Methods.Using continuum images, longitudinal magnetograms and Dopplergrams, 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. Results.The vertical velocity, measured from MDI Dopplergrams for the three active regions, indicates that the elements of MMF pairs with polarity opposite to that of the sunspot support a downflow (Doppler redshift) 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 where 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. In contrast, the decrease of the average velocity is far less obvious from 0.5 km s-1 to 0.4 km s-1 with increasing distance from the sunspot. 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 h and 1.6 h, respectively, which is considerably shorter than the 4 h lifetime previously found for AR 8375