Is stellar granulation turbulence?
Å. Nordlund 1, 2,
H.C. Spruit 3,
H.-G. Ludwig 2, 3 and
R. Trampedach 4
Received 27 March 1997 / Accepted 10 July 1997
We show that power spectra of granulation images or velocity fields cannot be compared meaningfully with spectra from theoretical models based on turbulent cascades. The small scale power in these images is due almost entirely to the sharp edges between granules and intergranular lanes, not to turbulence in the usual sense. This is demonstrated with a number of experiments with result from numerical simulations, and with simpler synthetic data with power spectra similar to that of granulation. The reason for the seemingly laminar behavior of the granulation flow, in spite of the high Reynolds numbers involved, is the influence of stratification on the local ratio of turbulence to bulk flow. The rapid expansion of upflows, their deep origin and near-adiabatic stratification lead to low levels of turbulence in the overturning fluid at the surface. Higher levels of turbulence are expected in the converging flows near downdrafts, but mostly at scales that are below current observational resolution limits, and contributing relatively little to the total convective flux and to spectral line broadening.
Key words: convection Sun: granulation stars: atmospheres
Send offprint requests to: H.C. Spruit
© European Southern Observatory (ESO) 1997
Online publication: March 24, 1998