Dynamics of the solar granulation

Abstract

We study the evolution of the granulation dynamics from the observational point of view. Based on series of excellent spectrograms taken at the VTT, Observatorio del Teide (Tenerife), in 1999, we calculated temporal – spatial maps of the Doppler velocity, line width, and intensity in order to track the dynamical behavior of these observables at different positions along the spectrograph slit. The Doppler velocity map reveals a granular dynamical time – the characteristic time associated with the decay of the Doppler velocity – of approximately 2 min, while the line width map does not show any characteristic time scale but rather a strong intermittence. The intensity map reveals the life time of the granulation as it is given in the literature. The granular dynamical time is practically equal to the value determined from spectrograms taken at the solar minimum 1994; so the dynamical time does not show any change over the solar cycle. The stochastic properties of the Doppler velocity and intensity data samples are studied (i) by means of their statistical moments and (ii) theoretically using presupposed model distributions. For the latter we estimated the distributions' parameters by means of the maximum likelihood method. The histograms of the Doppler velocity variations point to an asymmetric model distribution, while the histograms of the intensity variations infer a symmetric one. The intensity variations can be described well by a Gaussian probability density function, while the Doppler velocity variations are described by the double exponential (Gumbel) distribution, an asymmetric probability function. A remarkable result of the statistical analysis based on both series of observations in 1994 and 1999 is the unambiguous lack of flows with large velocity amplitudes within the intergranular space.