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Astronomy and Astrophysics



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Context . Several studies have investigated the fractal and multifractal nature of magnetic features in the solar photosphere and its variation with the solar magnetic activity cycle.

Aims . Here we extend those studies by examining the fractal geometry of bright magnetic features at higher atmospheric levels, specifically in the solar chromosphere.We analyze structures identified in CaIIK images obtainedwith the Precision Solar Photometric Met Telescopes (PSPTs) at Osservatorio Astronomico di Roma (OAR) andMauna Loa Solar Observatory (MLSO).

Methods. Fractal dimension estimates depend on the estimator employed, the quality of the images, and the structure identification techniques used.We examine both real and simulated data and employ two different perimeter-area estimators in order to understand the sensitivity of the deduced fractal properties to pixelation and image quality.

Results. The fractal dimension of bright “magnetic” features in CaIIK images ranges between values of 1.2 and 1.7 for small and large structures respectively. This size dependency largely reflects the importance of image pixelization in the measurement of small objects. The fractal dimension of chromospheric features does not show any clear systematic variation with time over the period Concl examined, the descending phase of solar cycle 23.

Conclusions . These conclusions, and the analysis of both real and synthetic images on which they are based, are important in the interpretation of previously reported results.