Mesoscale dynamics on the Sun's surface from HINODE observations

¹ Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 57 avenue d'Azereix, 65000 Tarbes, France e-mail: roudier@ast.obs-mip.fr
² Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 avenue Edouard Belin, 31400 Toulouse, France
³ Laboratoire de Modélisation et d'Imagerie en Géosciences, Université de Pau et des Pays de l'Adour, CNRS, Avenue de l'Université, 64013 Pau Cedex, France
⁴ LESIA, Observatoire de Paris, Section de Meudon, 92195 Meudon, France
⁵ LAOG, CNRS, Université Joseph Fourier, BP 43, 38041 Grenoble Cedex, France
⁶ Lockheed Martin Advance Technology Center, Palo Alto, CA, USA

Received: 7 October 2008
Accepted: 16 December 2008

Abstract

Context.

Aims. The interactions of velocity scales on the Sun's surface, from granulation to supergranulation are still not understood, nor are their interaction with magnetic fields. We thus aim at giving a better description of dynamics in the mesoscale range which lies between the two scales mentioned above.

Methods. We analyse a 48 h high-resolution time sequence of the quiet Sun photosphere at the disk center obtained with the Solar Optical Telescope onboard Hinode. The observations, which have a field of view of 100´´  100´´, typically contain four supergranules. We monitor in detail the motion and evolution of granules as well as those of the radial magnetic field.

Results. This analysis allows us to better characterize Trees of Fragmenting Granules issued from repeated fragmentation of granules, especially their lifetime statistics. Using floating corks advected by measured velocity fields, we show their crucial role in the advection of the magnetic field and in the build up of the network. Finally, thanks to the long duration of the time series, we estimate that the turbulent diffusion coefficient induced by horizontal motion is approximately 430 km² s^-1.

Conclusions. These results demonstrate that the long living families contribute to the formation of the magnetic network and suggest that supergranulation could be an emergent length scale building up as small magnetic elements are advected and concentrated by TFG flows. Our estimate for the magnetic diffusion associated with this horizontal motion might provide a useful input for mean-field dynamo models.