On the power spectrum of solar surface flows

摘要

Context. The surface of the Sun provides us with a uniqueand very detailed view of turbulent stellar convection. Studying itsdynamics can therefore help us make significant progress in stellarconvection modelling. Many features of solar surface turbulence likethe supergranulation are still poorly understood. Aims. The aim of this work is to give new observationalconstraints on these flows by determining the horizontal scaledependence of the velocity and intensity fields, as represented bytheir power spectra, and to offer some theoretical guidelines tointerpret these spectra. Methods. We use long time-series of images taken by the SolarOptical Telescope (SOT) on board the Hinode satellite; we reconstructboth horizontal (by granule tracking) and vertical (by Doppler effect)velocity fields in a field-of-view of Mm2.The dynamics in the subgranulation range can be investigated withunprecedented precision thanks to the absence of seeing effects and theuse of the modulation transfer function of SOT for correcting thespectra. Results. At small subgranulation scales down to 0.4Mm thespectral density of kinetic energy associated with vertical motionsexhibits a k-10/3-like power law, while the intensity fluctuation spectrum follows either a k-17/3 or a k-3-likepower law at the two continuum levels investigated (525 and 450nmrespectively). We discuss the possible physical origin of thesescalings and interpret the combined presence of k-17/3 and k-10/3power laws for the intensity and vertical velocity as a signature ofbuoyancy-driven turbulent dynamics in a strongly thermally diffusiveregime. In the mesogranulation range and up to a scale of 25Mm,we find that the amplitude of the vertical velocity field decreaseslike with the horizontal scale .This behaviour corresponds to a k2spectral power law. Still in the 2.5-10Mm mesoscale range, wefind that intensity fluctuations in the blue continuum also follow a k2power law. In passing we show that granule tracking cannot samplescales below 2.5Mm. We finally further confirm the presence of asignificant supergranulation energy peak at 30Mm in thehorizontal velocity power spectrum and show that the emergence of apore erases this spectral peak. We tentatively estimate the scaleheight of the vertical velocity field in the supergranulation range andfind 1Mm; this value suggests that supergranulation flows areshallow. Key words: convection - turbulence - sun: photosphere