3. Height dependence of intensity and velocity fields
There are good reasons to think that the granules present convective characteristics, if we consider the convection in terms of upflows and downflows. The upward moving fluid diverges and overturns as it rises into lower density layers. The ascending fluid that reaches the surface radiates its energy and produces higher density material that is pulled down by gravity (cf. Stein & Nordlund, 1998).
To study the velocity and intensity fluctuations associated to WL cell boundaries and granules, we superimpose these "zero" level features to C and Fe velocity and line center intensity frames.
We observe that the relation between the vertical velocity and the brightness is reversed passing from the lower photosphere (Fig. 2) to the upper photosphere (Fig. 3). In the lower layers we calculate a mean velocity =-171 m s-1 for granules, and a mean velocity =+57 m s-1, for cell boundaries. The rms is approximately the same and it is of the order of 300 m s-1. In the upper layers we found =-93 m s-1 and =+81 m s-1, the rms is about 190 m s-1. The observed peak-to-peak amplitude of the velocity is of about 2 km s-1 in the lower layers, and it is of about 1 km s-1 in the upper layers. This result is related to the decrease of the velocity amplitudes with height (cf. Stein & Nordlund 1998).
The distribution of mean granular intensities (Fig. 4) shows an asymmetric shape, for lower layers, that disappears when we go up of about 300 km. A similar asymmetry is reported, for maximum granular intensities in WL images, by Hirzberger et al. (1997). In the upper layers the intensity of granules spreads, because of diverging horizontal motions, producing a more uniform bright distribution.
© European Southern Observatory (ESO) 1999
Online publication: March 18, 1999