Dynamics of the solar granulation VI. Time variation of the granular shearflow

Excellent spectrograms can yield observational insight in the dynamics of t he solar surface not yet accessible to numerical simulations. We present re sults of the elaboration of a series of spectrograms taken at the center of the solar disk. Each of the spectrograms includes more than 250 granules, while the series covers a time of 12 min. Our main emphasis is to study the dynamics of the visible solar layers not only as a function of height but also as a function of time. We investigated the temporal and spatial behavior of the turbulent concentr ation at the granular borders and its spreading out into the intergranular space. In the deep photosphere, enhanced turbulence is concentrated predomi nantly near granular borders, while at higher layers the turbulence spreads out over the entire intergranular space. Remarkable is the decay of the tu rbulence with the height in the photosphere. There was no significant varia tion of the turbulence over the 12 min. We also determined the rms turbulent pressure at the granulation layers nea r tau(5000) = 1 The average ratio of turbulent to gas pressure is of the or der of 0.1; values of this size are also discussed in recent theoretical wo rks. In order to take the intermittency into account, we traced the peak to peak variations of the turbulent velocity, which turn out to be approximat e to 4 km sec(-1). The corresponding ratio of turbulent to gas pressure may thus reach locally significant values up to about 0.3. We did not find either a correlation or an anticorrelation between turbulen ce and convective flow, although the turbulence is presumably generated by granular shear flow. We suggest that the intermittent turbulence in the vis ible layers and the convective flow constitutes a dynamical system. This tu rbulence-granulation-dynamical system exhibits a cyclic behavior correspond ing to the dynamical time of the granules, i.e. the growth and decay of the ir velocity profile. The power spectra of the turbulent and granular velocity show a two-compone nt character, which presumably reflects the action of two different process es determining the dynamics of the solar convective boundary layers and abo ve.