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Astron. Astrophys. 344, L29-L32 (1999)

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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 [FORMULA]=-171 m s-1 for granules, and a mean velocity [FORMULA]=+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 [FORMULA]=-93 m s-1 and [FORMULA]=+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).

[FIGURE] Fig. 2. Scatter plot of center line intensities and velocities derived from C I 538.03 nm frames. Upflows have negative velocity to respect to the mean that is set to zero. In the left panel we report the values of intensities and velocities associated to pixels belonging to granules (G). In the right panel we report values relative to pixels belonging to granular cell boundaries (CB).

[FIGURE] Fig. 3. Scatter plot of center line intensities and velocities as derived from Fe I 557.61 nm frames. The meaning of the panels is the same as in Fig. 2.

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.

[FIGURE] Fig. 4. Distribution of granule intensity fluctuations with respect to the mean granule intensity, for the C line core (upper panel ) and Fe line core (lower panel ).

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© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999