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

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4. Discussion

We can firstly discuss the values of the RMS intensity fluctuations we have measured. They are not corrected from any resolution effect (no Modulation Transfer Function correction), but can be compared to previous observations of Keller & Koutchmy (1991). We can confirm the slight relative variations of RMS detected in that previous work. At the continuum level, the active region shows an abnormal granulation characterised by a [FORMULA] value slightly smaller than for the quiet region, and at the MgIb1 wing level, [FORMULA] is smaller in the quiet region than in the active one. The behaviour of the RMS of the intensity variations can also be qualitatively compared to numerical predictions of the RMS of the temperature variations. Gadun (1995) found the RMS of the temperature to decrease with increasing altitude in the atmosphere, while our results show an increase of the RMS of the intensity, for both quiet and active regions. Since this result is also valid for subsonically filtered data, p -modes, which are not taken into account in the simulations, cannot be responsible for this.

The spatial intensity correlation we observe between the continuum and the MgIb1 line wing outside the active region can be compared to previous observations and models. The granulation contrast is predicted to show a reversing with increasing height. Following Gadun (1995), the intergranules will be brighter than the granules above a height h [FORMULA] 190 km. As the calculations of Stellmacher (private communication) place the MgI line wing at h [FORMULA] 140 km, our positive correlation is still coherent with that prediction. Recently, Espagnet et al. (1995) have observed at different positions along the profile of the NaD2 line, and have found a coherence of the intensities between the h [FORMULA] 0 km level and heights up to h [FORMULA] 60 to 90 km. However, as pointed out by these authors, previous works (e.g. Komm et al. 1990) have shown a coherence up to h [FORMULA] 170 km, a result which is consistent with ours.

One of the most striking result is certainly the temporal behaviour of the intensity correlation between the continuum and the MgI line wing in the quiet region. Our observations, based on non filtered, subsonically and supersonically filtered time sequences (see Fig. 4) suggest a simple interpretation: the acoustic oscillations induce simultaneous intensity variations at the two levels because they are evanescent waves, whereas the leading of MgI intensity over continuum may be ascribed to falling plasma motions, with which the correlated dark structures we observed in both continuum and magnesium images can be associated, due to their darkness possibly reflecting downdrafts. This could be compared to the numerical simulations performed by Rast (1995), showing that the photosphere is dominated by downflows. However, these simulated spatial scales are different of our observed ones. A simple calculation of the speed of the falling plasma from the observed time offset (see Fig. 4) gives a value of [FORMULA] 1.2 km/s, which is coherent with the speed of downwards granular motions (Stix 1989).

The problem of radiative excess or deficit of magnetic elements has been adressed at the continuum level. We have studied the intensities in the active region, discarding pixels corresponding to the pores. The first result is qualitative: from our correlograms (Fig. 3), it becomes clear that the intensities variations versus height can be separated in two groups: one following the behaviour of the quiet region, and another characterised by higher brightness in the MgIb1 wing, which in turn is related to higher value of the magnetic field. At the continuum level, these magnetic regions corresponds to areas darker than their surroundings (radiative deficit at the continuum level). However, we have checked the contrast of these magnetic elements outside the active region, and surprisingly enough, they show a positive contrast (radiative excess at the continuum level), in agreement with Keller & Koutchmy (1991), which indicates that flux tubes inside an active region have a different behaviour than outside.

The remnant structure of the granulation we observed in the averaged continuum image may indicate a long life phenomenon in the granulation. Similar observation has been obtained by Hirzberger et al. (1997) as well. Considering the granulation at the surface of the sun as a randomly driven phenomenon with a typical lifetime of 5 mn leads to a number of approximately 22 "independent" realisations of the granular pattern during our 109 mn long sequence. Hence, we were expecting the residual RMS after averaging over the whole sequence to be about 0.6% (the RMS of single images divided by the square root of independent realisations). The fact that the observed RMS after averaging is two times greater suggests that the granulation should be described in a more complicated way, e.g. accounting for longer time scale phenomena.

Let us now discuss the behaviour of the horizontal motions of the plasma versus height and magnetic field. The first point is qualitative: the proper motions of the granules in the deep photosphere (h [FORMULA] 0 km) show very clearly the supergranulation structure in the quiet region (see Fig. 8) and outlines clearly the magnetic network seen in the H [FORMULA] line (Fig. 10) which is also coincident with the bright points seen in the MgI line wing (see Baudin et al. 1996). This supergranular structure is not visible in the observed flows at the height of the MgIb1 line wing (h [FORMULA] 140 km, Fig. 9). The proper motions when observed in the active and the quiet regions have a quite different behaviour. In the Q region, the averaged horizontal velocity is much higher at the MgI level (h [FORMULA] 140 km) than at the continuum level (h [FORMULA] 0 km): [FORMULA]  m/s and [FORMULA]  m/s. On the contrary, in the A region, averaged velocities are very similar: [FORMULA]  m/s and [FORMULA]  m/s. Moreover, the correlation coefficients of the components of the velocity vector at the two levels are much higher in the A region. This can be interpreted in a simple manner as a lower value of the parameter [FORMULA] of the plasma (i.e. the ratio of the gas pressure and magnetic pressure) which makes the plasma "frozen" in the magnetic region, specially at the MgI level.

There is a clear tendency for the bright points seen in the MgIb1 images to appear at regions of photospheric flow convergence, which in turn outline the magnetic network (Simon et al. 1988 , Wang et al. 1989). This may suggest that the flux tubes are then pushed and moved about towards the network by the velocity field, where they merge and concentrate.

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© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997