A&A 462, 303

Alexandra Tritschler (Big Bear Solar Observatory, New Jersey Institute of Technology, 40386 North Shore Lane, Big Bear City, CA-92314, U.S. / Kiepenheuer-Institut für Sonnenphysik, Schöneckstrasse 6, 79104 Freiburg, Germany)
Wolfgang Schmidt (Kiepenheuer-Institut für Sonnenphysik, Schöneckstrasse 6, 79104 Freiburg, Germany)
Han Uitenbroek (National Solar Observatory/Sacramento Peak, P.O.~Box 62, Sunspot, NM-88349, U.S.A.)
Sven Wedemeyer-Böhm (Kiepenheuer-Institut für Sonnenphysik, Schöneckstrasse 6, 79104 Freiburg, Germany)

On the fine structure of the solar Ca II K atmosphere

We investigate the evolutionary, dynamical, and morphological properties of the internetwork and network fine structure of the quiet sun at disk centre. The analysis is based on a ~6 h time sequence of narrow-band filtergrams centred on the inner-wing Ca II K2v reversal at 393.3 nm. To examine the temporal evolution of network and internetwork areas separately we employ a double-Gaussian decomposition of the mean intensity distribution. An autocorrelation analysis is performed to determine the respective characteristic time scales. The results for the internetwork are related to predictions derived from numerical simulations of the quiet sun. In order to analyse statistical properties of the fine structure we apply image segmentation techniques. The average evolutionary time scale of the internetwork in our observations is 52 sec, that of the network ranges from 6 min to 68 min with an average of 44 min. Internetwork grains show a tendency to appear on a mesh-like pattern on a spatial scale of 4-5 arcsec. We relate this pattern to the existence of photospheric downdrafts as predicted by convection simulations. The p-mode ridges in the k-nu-diagram extend beyond the acoustic cut-off frequency of ~5 mHz to at least 7.5 mHz, at the high harmonic degrees we observe. The image segmentation shows that typical sizes of network and internetwork grains are very similar and makes clear that their filling factor is highly dependent on spatial and spectral resolution, and the chosen intensity threshold.

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