What are the Boundaries of Solar Granules?

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Scientific paper

This investigation is based on a series of spectrograms of extraordinary spatial resolution taken with the vacuum tower telescope(VTT) at Iza\ na (Tenerife) in 1990. The quantitative analysis of these spectrograms revealed an asymmetrical character of the granular flow (non-Benard like convection). The intensity maximum and the maximum of the upward line-of-sight velocity do not coincide. In most cases the maximum of the velocity lies near the border of the granule and falls rapidly to the adjacent intergranular lane(from 1.5 to 0.2kmsec(-1) over 200km), but moderately towards the other intergranular lane. In some granules the position with zero velocity coincides with the position of highest intensity, whereas maxima of velocities with different signs lie at their border, thus reflecting a typical velocity profile of a rotating eddy. The low correlation(of less than 0.5) between intensity and Doppler velocity fluctuations along the spectrograph slit reflects the asymmetric character of the solar granular flow. Concerning the border of granules we find that bright regions often exhibit downward, instead of the expected upward velocity. Moreover, by investigating the broadening of a non-magnetically sensitive line, we were able to localize regions with enhanced turbulence within the intergranular space. We find that these regions do not always cover the whole intergranular lane, but are concentrated at the border of the granules, especially where the steep decrease of the velocity takes place. On the basis of these findings we suggest that a typical granule consists of a region of high intensity and low turbulence in its interior and a region of high turbulence and moderate intensity at its border. In other words, we surmise that regions of enhanced turbulence outline the borders of granules. Using our time series of spectrograms, which were taken every 15sec over a total of 5min, we followed the dynamics of these properties and the evolution of the steep intensity and velocity changes along the slit. These changes are connected with shear instabilities and turbulence production. The findings from non-active regions will be compared with those from active regions based on magnetically sensitive lines.

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