Evolution of the flow field in decaying active regions

Abstract

Context. Fully fledged sunspots are known to be surrounded by a radial outflow called the moat flow. Aims. We investigate the evolution of the horizontal flow field around sunspots during their decay, that is, from fully fledged to naked spots, after they loose the penumbra, to the remnant region after the spot has fully dissolved. Methods. We analysed the extension and horizontal velocity of the flow field around eight sunspots using SDO/HMI Doppler maps. By assuming a radially symmetrical flow field, the applied analysis method determines the radial dependence of the azimuthally averaged flow field. For comparison, we studied the flow in supergranules using the same technique. Results. All investigated, fully fledged sunspots are surrounded by a flow field whose horizontal velocity profile decreases continuously from 881 m s−1 at 1.1 Mm off the spot boundary, down to 199 m s−1 at a mean distance of 11.9 Mm to that boundary, in agreement with other studies. Once the penumbra is fully dissolved, however, the velocity profile of the flow changes: The horizontal velocity increases with increasing distance to the spot boundary until a maximum value of about 398 m s−1 is reached. Then, the horizontal velocity decreases for farther distances to the spot boundary. In supergranules, the horizontal velocity increases with increasing distance to their centre up to a mean maximum velocity of 355 m s−1. For larger distances, the horizontal velocity decreases. We thus find that the velocity profile of naked sunspots resembles that of supergranular flows. The evolution of the flow field around individual sunspots is influenced by the way the sunspot decays and by the interplay with the surrounding flow areas. Conclusions. Observations of the flow around eight decaying sunspots suggest that as long as penumbrae are present, sunspots with their moat cell are embedded in network cells. The disappearance of the penumbra (and consequently the moat flow) and the competing surrounding supergranular cells, both have a significant role in the evolution of the flow field: The moat cell transforms into a supergranule, which hosts the remaining naked spot.