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Astron. Astrophys. 330, 1136-1144 (1998)

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1. Introduction

The solar surface is subject to convective and oscillatory motions, the properties of which are fundamental to understanding the physics of solar surface evolution. The convective motions which concentrate, sweep and spread magnetic fields over the solar surface, are the basic elements which contribute to magnetic activity in the quiet sun. A study of their physical properties needs to address many issues, such as the g-mode detection (Andersen et al. 1994), the solar background irradiance (Rabello Soarres et al. 1997), the magnetic flux tube concentration, the photospheric network formation, the measurement of the diffusion coefficients and the motions of magnetic flux tubes (cf. Coronal heating). At the present time, three different convective scales have been identified (or are suspected to exist) on the solar surface. The best known is solar granulation, followed by mesogranulation (November et al. 1981, 1982) and supergranulation (Leighton et al. 1962). A rough description of the macro-properties of such cells can be given by their mean size, velocity and lifetime. The solar granulation, for example, is characterized by a mean size of 1000 km, a Doppler velocity of 1 km/s at the disk center (i.e. vertical motions), a radial expansion of 1.6 to 2.6 km/s (Brandt et al. 1991) and a lifetime of 10 to 16 min. Meanwhile, the supergranulation is generally defined by a chromospheric network (or Doppler data), with a scale of 20 to 50 Mm, a vertical velocity of 50 m/s (Küveler, 1983), a horizontal flow of 500 m/s and a lifetime of around 20 hours (Simon and Leigton, 1964). Finally, the mesogranulation has a typical scale of 5-10 Mm, a vertical velocity of 60 to 200 m/s (rms) and a horizontal velocity of 450 m/s (Deubner et al. 1989). Its lifetime has not as of yet been accurately determined. In the first determination by November et al. (1981), these mesogranules persisted for at least 2 hours but in other studies they seem to survive for at least 3 h (Muller et al 1992) which is much longer than the lifetime of exploding granules. Recently, a 1/e lifetime of 5 to 6 hours was found by Brandt et al. 1994 for mesoscale flows from a 4.5 hours image sequence. A previous determination by Darvann (1991) estimates the 1/e lifetime to lie between 3.5 to 7 hours with a mean of 4.3 hours. At the present time, the convective nature of the mesogranulation is still being discussed by different authors (Straus and Bonaccini 1997, Straus et al. 1992, Deubner 1989, Wang 1989). Although at the low photospheric levels, mesoscales appear as an extention of the granulation size without further distinction from granulation, they can be identified in the higher photosphere as internal gravity waves in the solar atmosphere (Straus and Bonaccini 1997). Various approaches to mesoscale measurement are necessary to understand their physical origin. The overlapping of mesogranulation and granulation and their respective evolutions (Darvann 1991) are the major difficulties in determining the mesoscale lifetime by indirect methods like the Local Correlation Technique (LCT). A long time sequence (of several hours) with subarcsecond spatial resolution, a large field of view and a quantitative mesocell definition are required for an accurate lifetime evaluation.

The present investigation, using a 6.7 hour solar granulation time sequence obtained at the Pic du Midi Observatory, attempts to determine the lifetime of these mesocells. This is achieved by a direct measurement, following each cell from birth to death, and by the standard time correlation coefficient method.

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© European Southern Observatory (ESO) 1998

Online publication: January 27, 1998