4. Vertical flow characteristics and granule dimension
It is well established that photospheric features observed in white light images are associated with photospheric velocity field. Nevertheless, this simple picture thickens when we cross the over-shooting region (cf. Nesis et al. 1997, Espagnet et al. 1995). In the previous section we showed the transition from a velocity-intensity negative correlation to a positive one when we go up in height. Moreover we can expect a further dependence of velocity and temperature (intensity) fluctuations on granule dimension, and this dependence can change with height (Steffen et al., 1989). Some experimental evidence derives from the analysis of spectrograms (Wiehr & Kneer, 1988) or from the analysis of WL images (Hirzberger et al., 1997). To study the characteristics of vertical flow in regard to the granule dimension we plot mean velocities and intensities with respect to the area of associated "zero" level granules. We use over 400 identified granules whose sizes are in the range 0.4-1.3 arcsec.
As it is shown in Fig. 5 (lower panels), we observe that the upflow velocities (negative) of the granules increase with granule size both for C and Fe lines.
Instead, the intensity fluctuations of the granules show different behaviors at the two considered levels (Fig. 5, upper panels). In fact, for thermodynamic reasons, we expect that temperature fluctuations of granules have different sign in the lower and upper layers. Near the basis of the photosphere granules are hotter (brighter) with respect to the surroundings, and the fluctuations increase with their dimensions. Large elements result brighter than smaller ones because they extend deeper into the convection zone, and have less radiative loss in the horizontal direction (Steffen et al., 1989). In the upper photosphere (overshooting region) fluctuations change in sign, a super-cooling effect and a smaller radiative losses produce a negative correlation between temperature (i.e. intensity) and granule sizes.
We have presented preliminary results showing that the dynamical and thermodynamic properties of the observed photospheric structures depend on their vertical position and horizontal scale. This is due to the convective overshooting into stable atmospheric layers.
We are extending this analysis to a larger sample of images obtained in a recent observation campaign with the IPM. With the new data base, including also images obtained in the Na D I and Mg I lines, we will explore more carefully the height dependence, the magnetic field effects and the time evolution of photospheric structures.
© European Southern Observatory (ESO) 1999
Online publication: March 18, 1999