Velocities and divergences as a function of supergranule size

Free access article

Issue		A&A Volume 461, Number 3, January III 2007


Page(s)		1141 - 1147
Section		The Sun
DOI		http://dx.doi.org/10.1051/0004-6361:20065625

A&A 461, 1141-1147 (2007)
DOI: 10.1051/0004-6361:20065625

Velocities and divergences as a function of supergranule size

N. Meunier¹, R. Tkaczuk², Th. Roudier¹, and M. Rieutord²

¹ Laboratoire d'Astrophysique de l'Observatoire Midi-Pyrénées, Université Paul Sabatier Toulouse III, CNRS, 57 avenue d'Azereix, BP 826, 65008 Tarbes Cedex, France
e-mail: [nadege.meunier;thierry.roudier]@ast.obs-mip.fr
² Laboratoire d'Astrophysique de l'Observatoire Midi-Pyrénées, Université Paul Sabatier Toulouse III, CNRS, 14 avenue Édouard Belin, 31400 Toulouse, France
e-mail: [ruben.tkaczuk;michel.rieutord]@ast.obs-mip.fr

(Received 17 May 2006 / Accepted 23 September 2006)

Abstract
Context.The origin of supergranulation is not understood yet and many scenarios, which range from large-scale deep convection to large-scale instabilities of surface granular flows, are possible.
Aims.We characterize the velocities and divergences in supergranulation cells as a function of their size.
Methods. Using local correlation tracking, we determine the horizontal flow fields from MDI intensity maps and derive the divergences. The smoothed divergences are used to determine the cells for various spatial smoothings, in particular at the supergranular scale.
Results. We find evidence of intermittency in the supergranular range and a correlation between the size of supergranules and the strength of the diverging flow. We also show that the relation between rms velocities and scale (the supergranule radius R) can be represented by a law $V_{\rm rms}\sim R^{0.66}$ .
Conclusions. The results issued from our data point towards a scenario where supergranulation is a surface phenomenon of the sun, probably the consequence of a large-scale instability triggered by strong positive correlated rising flows.

Key words: Sun: granulation -- Sun: photosphere -- Sun: general

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