Granular velocities of the Sun from speckle interferometry

We present observations of granular velocities and their relation to the granular intensity pattern from the disc center of the Sun. They were obtained in June 1997 with the Vacuum Tower Telescope at the Observatorio del Teide on Tenerife. High spatial resolution, 0. ′′4–0. ′′5 for the velocities, was achieved with speckle methods applied to two-dimensional narrow-band images in Na D2 from the “Göttingen” Fabry-Perot interferometer. The velocity observations refer to a geometric height of 50– 200 km (above τ5 = 1). Velocity amplitudes of ±2.2 km s−1 are seen. The high velocity regions are small-scale and the upflows coincide frequently with the bright borders of granules or with small-scale brightenings. A statistical analysis reveals only a rough consistency to the -5/3 or -17/3 law of log(power) vs. log(wavenumber) expected for isotropic turbulence (cf. Espagnet et al. 1993). We consider an agreement with such power laws as accidental since the intensity and velocity power spectra found here decrease smoothly from flat ones at low wavenumbers to steep ones at high wavenumbers. The coherence of velocity and intensity fluctuation stays above 0.5 up to horizontal wavenumbers kh ≈ 11 Mm−1(=̂0. ′′8) and the phase difference between intensity and velocity stays stable down to structures of 0. ′′5. While the intensity pattern exhibits a clearly non-Gaussian distribution, the velocity distribution can be represented by a Gaussian with a “macroturbulent” velocity of 0.825 km s−1.