On the power spectrum of solar surface flows

Abstract

The aim of this work is to give new observational constraints on solar surface flows by determining the horizontal scale dependence of the velocity and intensity fields, as represented by their power spectra, and to offer some theoretical guidelines to interpret these spectra. We use long time series of images taken by SOT/Hinode and reconstruct both horizontal (by granule tracking) and vertical (by Doppler effect) velocity fields in a field of view 75x75Mm^2. At small sub-granulation scales, the kinetic energy spectral density associated with vertical motions exhibits a k^{-10/3}-like spectrum, while the intensity fluctuation spectrum follows a k^{-3} or k^{-17/3}-like spectrum at the two continuum levels investigated (525 and 450 nm respectively). We discuss the physical origin of these scalings and argue that they provide a direct observational signature of buoyancy-driven turbulent dynamics in a strongly thermally diffusive regime. In the mesogranulation range and up to a scale of 25Mm, we find that the vertical velocity field amplitude decreases like L^{-3/2} with the horizontal scale L. This behaviour corresponds to a k^2 spectral power law. Still in the mesoscale range, [2.5, 10]Mm, we find that intensity fluctuations in the blue continuum follow a power law in k^2. We show that granule tracking cannot sample scales below 2.5Mm. We locate the supergranulation energy peak around 30Mm and show that the emergence of a pore erases this spectral peak. Thanks to a scale height estimate, we find that supergranular flows are shallow. (abridged abstract).