Properties of sunspot moats derived from horizontal motions
Résumé
Context: Sunspots in late phases of evolution are usually surrounded by annular moats, regions where systematic horizontal flows are observed to be directed radially away from the spot. These flows are considered to be a manifestation of the sub-photospheric convection. Aims: The characteristics of moats are derived at two different heights in the solar atmosphere from horizontal motions around sunspots of different sizes, shapes, and phases of evolution. We also study the temporal evolution of moats. Methods: Local correlation tracking is applied to approximately 70-min long time series of white-light and 1600 Å images, acquired by the satellite TRACE, to analyse the horizontal motions of photospheric granules and C IV emission structures in the vicinity of 32 sunspots. Moat regions are defined by means of radially-oriented, outward velocities. Results: Relations between sunspot types and the occurrence, areas, and horizontal velocities of moats in the photosphere and transition region are described. Moats do not show substantial changes during the period of about 12 h. Observed asymmetries in moat shapes and velocities are related to the height in the atmosphere, to sunspot age, and to proper motion. It is suggested that the sub-photospheric convective flows around sunspots may be influenced by the spots' proper motion through the convection zone.