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Astron. Astrophys. 326, 842-850 (1997)

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1. Introduction

A large part of the solar interior energy is transmitted by convection towards the photospheric level, where a small part is stored in concentrated magnetic fields. In the upper atmosphere, the quasi laminar convective flow compete with the more turbulent motion of eddies of different scales. The phenomenon of granulation is currently being intensively studied (see the conferences of last years: Stenflo 1990 , Rutten & Schrijver 1994) so the subject does not need an extensive introduction. However, some important results relevant to our study may be pointed out. In the past, spectrographs and scanning methods with pin hole photometer as well as raster scans with multichannel systems were used to collect precise data. Classical results were reported by Frazier (1970 , 1971) who considered both the case of the quiet sun and of plages from precise spectrographic photo-electric measurements. The influence of magnetic phenomena was clearly shown at photospheric levels: a small brightening in the continuum, related to small magnetic regions called "flux tubes". Later, the photographic experiment SOUP yielded other classical results (Title et al. 1989) showing the influence of 5 mn oscillations on the granulation. The important role of the mesogranulation was outlined in this work, using local correlation technics (November 1986), and also in other works as Koutchmy & Lebecq (1986) using spatial variations of intensity. The problem of granulation is theoretically studied with numerical simulations of the convection to help the interpretation of the observations, as for instance by Chan et al. (1991), or in the most recent work of Gadun (1995). These works predict the evolution of the temperature and of the vertical and horizontal velocities with height. However, these predictions are usually based on models of the granulation which do not include oscillatory motions nor waves. Furthermore, the influence of the magnetic field should be considered, in particular to separate the problems of mass flows due to pure thermal instability from those of magneto-convection. This is specially important since it is well known that the magnetic field is concentrated and the magnetic pressure cannot be neglected in the regions of flux tubes.

A statistical analysis based on well calibrated data, leading to the estimation of the relevant parameters of the granulation, is a valuable approach to tackle these problems. Such analysis must be performed using new observational tools like precise and linear 2D imagers and narrow pass-band spectral filters. Indeed, an important aspect of the measurements is the photometric accuracy (Koutchmy 1994). The use of CCD devices provide photometrically reliable data. However, another important photometric problem is the mixing of the contribution of the lines emission with the contribution of the true continuum formed in the deepest layers. In the optical blue region, the line blocking effect is of the order of 10% in a [FORMULA] 10nm bandwith, and in network elements, the line brightening is typically of 30% to 50%. Thus, a true continuum window, which means a very narrow region in the optical domain, must be used, implying the use of a very narrow passband filter. Finally, to fully consider the problems connected with the influence of the magnetic field on convection and the occurence of magnetic elements, the use of imagers is needed in order to discriminate the magnetic regions from the non magnetic ones. It could appear that very high spatial resolution observations is preferable. We do not cover this aspect in this work. We think it is difficult, if not impossible, to simultaneously consider very high spatial resolution images and the removal or the analysis of several phenomena like 5 mn intensity oscillations, meso-granulation and the network cells boundaries which cannot be ignored when the photospheric convection is studied. We will focus this paper on specific parameters related to the granulation field in magnetic and non magnetic regions, at two levels in the solar atmosphere, leaving to another work results pertaining to oscillations at different levels, as well as results on the dynamical properties of higher layers in the chromosphere.

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© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997