On the brightness and velocity structure of solar granulation

Abstract

Abstract.A 45 min time series of two-dimensional spectra has been obtained with the Vacuum Tower Telescope at the Observatorio del Teide in Izaña, Tenerife. Scans over the non-magnetic Fe i 5576 Å line of a quiet granular field at disk center were taken simultaneously with a time series of broad band images. From the spectra intensity and velocity maps have been calculated at different line-depths. From the white light images granular shapes have been computed by means of an automatic image segmentation algorithm. A statistical analysis of the intensity and velocity distribution in the detected granular shapes has been carried out. Intensities and velocities are well correlated at low photospheric levels. In the higher photosphere the intensity pattern dissolves whereas the velocities show almost no variation within the probed height interval. The intensity excess of small granules dissolves at lower heights than that of larger ones. The intensity and velocity distribution within the granules depends on the granular size. In smaller structures the maximum intensities and velocities are located close to the granular barycenters whereas for larger granules the maxima are shifted towards the granular boundaries. The width of the transition zone between granules and intergranular lanes is independent on the granular intensity and is constant at approximately 0. 5. The time evolution of the granular pattern shows a clear dependence of the lifetime of structures on the spatial wavenumber. The e-folding times of the temporal coherences decrease according to a power law with an exponent of β = 3/2 which is incompatible with the Kolmogorov energy spectrum of homogeneous and isotropic turbulence and might be taken as a hint against the overall turbulent character of granular motions.

Anisotropy and dynamics of photospheric velocity patterns: 2D power and coherence analyses

“…1) originates largely in the first 120 km above τ 500 = 1. So, our findings of a change in dynamics and in topology of the velocity field above these layers is in line with the disappearance of the granular intensity structures with height in the photosphere (Espagnet et al 1995;Krieg et al 1999;Berrilli et al 2002;Hirzberger 2002).…”

“…Concerning granular intensity and velocity fluctuations they found that for structures larger than one arcsec the intensity fluctuations vanish rapidly with photospheric height, whereas the flow velocities stay almost constant up to 250 km. In another paper Hirzberger (2002) investigated statistically the intensity and velocity patterns at different photospheric levels. Applying new and innovative concepts from fractal geometry statistics to the analysis of granular patterns he found that the overall granular shapes are significantly elongated.…”

“…Applying new and innovative concepts from fractal geometry statistics to the analysis of granular patterns he found that the overall granular shapes are significantly elongated. Furthermore Hirzberger (2002) showed that the temporal coherence decreased according to a power-law with an exponent that is incompatible with the Kolmogorov energy spectrum of homogeneous and isotropic turbulence. He interpreted this as observational evidence against an overall turbulent character of granular motions.…”

Astrophysics in 2002

“…The same results, however, were also claimed to be consistent with a completely random and changing flow pattern that does not require any self-organization of the granular flows (Rast 2002). Other studies find that the temporal coherence time of granulation is consistent with a turbulent Kolmogorov spectrum (Hirzberger 2002;Abramenko et al 2001Abramenko et al , 2002, that some recurrently fragmenting granules survive by means of their descendants for more than 3 hr (Müller et al 2001), but that only large granules with size scales of տ2500 km and lifetimes of տ0.5 hr represent reliable tracers of the surface velocity field (Rieutord et al 2001). It was reassuring to hear that supergranulation flows measured in white light could also be recovered by simultaneous measurements in UV with TRACE (Krijger et al 2002).…”

The Small-scale Structure of Photospheric Convection Retrieved by a Deconvolution Technique Applied to Hinode/SP Data

“…However, so far observational results do not support downward motions faster than upflows. Hirzberger (2002) investigated the velocity field of the photosphere and reported almost the same magnitude of 1.5 km/s for both, upflows and downflows. Kostik & Khomenko (2007) used a bisector analysis to extract the height structure of the velocity field from the asymmetric shape of an absorption line and also found that both upward and downward motions have approximately the same amplitude at heights from 0 km to 500 km.…”