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Astron. Astrophys. 345, 265-275 (1999)


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Dynamics of the solar granulation

VI. Time variation of the granular shear flow

A. Nesis, R. Hammer, M. Kiefer, H. Schleicher, M. Sigwarth and J. Staiger

Kiepenheuer-Institut für Sonnenphysik, Schöneckstrasse 6, D-79104 Freiburg, Germany (nesis@kis.uni-freiburg.de)

Received 28 September 1998 / Accepted 13 January 1999

Abstract

Excellent spectrograms can yield observational insight in the dynamics of the solar surface not yet accessible to numerical simulations. We present results 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 concentration at the granular borders and its spreading-out into the intergranular space. In the deep photosphere, enhanced turbulence is concentrated predominantly near granular borders, while at higher layers the turbulence spreads out over the entire intergranular space. Remarkable is the decay of the turbulence with the height in the photosphere. There was no significant variation of the turbulence over the 12 min.

We also determined the rms turbulent pressure at the granulation layers near [FORMULA]. The average ratio of turbulent to gas pressure is of the order of 0.1; values of this size are also discussed in recent theoretical works. In order to take the intermittency into account, we traced the peak to peak variations of the turbulent velocity, which turn out to be [FORMULA] 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 turbulence and convective flow, although the turbulence is presumably generated by granular shear flow. We suggest that the intermittent turbulence in the visible layers and the convective flow constitutes a dynamical system. This turbulence-granulation -dynamical system exhibits a cyclic behavior corresponding to the dynamical time of the granules, i.e. the growth and decay of their velocity profile.

The power spectra of the turbulent and granular velocity show a two-component character, which presumably reflects the action of two different processes determining the dynamics of the solar convective boundary layers and above.

Key words: Sun: photosphere – Sun: granulation

Send offprint requests to: A. Nesis

© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999

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