Complexity of magnetic structures: Flares and cycle phase dependence

Observatoire Midi-Pyrénées, 57 avenue d'Azereix, BP 826, 65008 Tarbes Cedex, France

Corresponding author: meunier@bagn.obs-mip.fr

Received: 2 July 2003
Accepted: 17 February 2004

Abstract

A long data set of MDI/SOHO full-disk magnetograms is analyzed to provide a large sample of active regions. The objective is to study the variations of the fractal dimension of these structures on different time scales: in relation to their activity level (spots, flares) and during the solar cycle. First, the variations of the fractal dimension of the structure during solar flares are studied using a very large sample. Variations are found to be of very small amplitude. The complexity of the structures is slightly larger after the flare when considering regions close to the flare (2-3°), while a decrease seems to be observed at larger scales (up to 10°), although this should be confirmed using other approaches. The results obtained by Meunier ([CITE]) concerning the fractal dimension versus the size of the structures for various magnetic thresholds are confirmed on a much larger sample, with a peculiar behavior seen around 550-800 Mm². This scale is observed to increase from cycle minimum to cycle maximum. In addition, I observe that the fractal dimension of active regions reaches a plateau above 3000 Mm² and that structures of a given size are more complex when they have spots. The fractal dimension is strongly variable during the solar cycle, with the sign of the variation depending on their size and on their flux content (spot or no spot) and with an amplitude up to 0.17 between the ascending phase and cycle maximum.