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Astron. Astrophys. 357, 1063-1072 (2000)

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On mesogranulation, network formation and supergranulation

M. Rieutord 1,2, T. Roudier 1, J.M. Malherbe 3 and F. Rincon 4

1 Laboratoire d'Astrophysique de Toulouse, Observatoire Midi-Pyrénées, 14 avenue E. Belin, 31400 Toulouse, France
2 Institut Universitaire de France
3 DASOP, Observatoire de Paris, Section de Meudon, 92195 Meudon, France
4 Ecole Normale Supérieure de Lyon, 46 allee d'Italie, 69364 Lyon Cedex 07, France

Received 7 September 1999 / Accepted 3 March 2000

Abstract

We present arguments which show that in all likelihood mesogranulation is not a true scale of solar convection but the combination of the effects of both highly energetic granules, which give birth to strong positive divergences (SPDs) among which we find exploders, and averaging effects of data processing. The important role played by SPDs in horizontal velocity fields appears in the spectra of these fields where the scale [FORMULA]4 Mm is most energetic; we illustrate the effect of averaging with a one-dimensional toy model which shows how two independent non-moving (but evolving) structures can be transformed into a single moving structure when time and space resolution are degraded.

The role of SPDs in the formation of the photospheric network is shown by computing the advection of floating corks by the granular flow. The coincidence of the network bright points distribution and that of the corks is remarkable. We conclude with the possibility that supergranulation is not a proper scale of convection but the result of a large-scale instability of the granular flow, which manifests itself through a correlation of the flows generated by SPDs.

Key words: convection – turbulence – Sun: granulation – Sun: photosphere

Send offprint requests to: M. Rieutord

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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