Magnetic evolution of superactive regions

Abstract

Context. It is widely accepted that solar flares are manifestations of magnetic reconnection events taking place in the solar atmosphere. Several aspects of these events remain unclear, although many efforts have been devoted to the investigation of magnetic field configurations at flare occurrence sites. Aims. In this work, we have studied the temporal evolution of some properties of a sample of superactive regions with the aim to single out the most significant for flare activity forecasting. Methods. We have investigated properties of 14 superactive regions, observed between January 1st 2000 and December 31st 2006 with MDI/SOHO instrument and characterized by a particularly intense flare activity during their passage on the solar disk. We have analyzed the temporal evolution of fractal and multifractal properties of photospheric magnetic fields, namely the generalized fractal dimension and the contribution and dimensional diversities, which describe geometrical properties of the magnetic field, as well as the potential unstable volumes of magnetic discontinuities above the studied ARs, which may provide information about the magnetic field configuration in upper layers of the atmosphere. Correlations of these quantities with the flare index, which provides information about the flare activity of a region, have also been estimated. Results. We found that in 50% of our sample the generalized fractal dimension is correlated with the flare index computed over windows of 50 h, while the contribution diversity and the dimensional diversity are anticorrelated with the same index. A clear increase of the potential unstable volume of magnetic discontinuities in the corona is observed before the phases characterized by more frequent and intense flares. We also found that the free energy distribution functions of unstable volumes of the analyzed superactive regions can be fitted with straight lines whose slope is larger than the values found in previous works for less active magnetic regions. Conclusions. The generalized fractal dimension and the potential unstable volume of magnetic discontinuities are the most suitable for statistical investigations of relations with flare activity over longer (50 h) and shorter (few hours) time intervals, respectively.

Pre-Flare Dynamics of Sunspot Groups

“…Flare occurrence may also be expected from highly intermittent magnetic configurations, in other terms from turbulent structures, which can be described by fractal dimension analysis. Abramenko et al (2003) and Georgoulis (2005) reported characteristic variations shortly prior to flares, furthermore, Criscuoli et al (2009) found correlation between the generalized fractal dimension and the flare index. At the same time Georgoulis (2012) pointed out that the fractality and turbulence properties do not distinguish flare-active and flare-quiet active regions.…”

On Flare Predictability Based on Sunspot Group Evolution

“…The maximum horizontal gradient and the length of the neutral line were considered by Cui et al (2006), Jing et al (2006), Huang et al (2010) and Yu et al (2010). The fractal structure was addressed by Criscuoli et al (2009), however, Georgoulis (2012) was sceptical about this approach. In the literature, there are a number of other indicators proposed to measure non-potentiality.…”

Solar Cycle

“…At shorter wavelengths still, episodic solar X-ray flares exhibit a strong solar cycle modulation in occurrence and position across all energy ranges [4]. The Hα flare index is a proxy for the total energy emitted by a flare, expressed as the product of Hα flare intensity and duration in minutes [66], while the X-ray flare index is based on the soft X-ray flux in the 1-8 angstrom range measured by the GOES satellites [16]. Perhaps unsurprisingly, the other major episodic form of solar activity, coronal mass ejections, show a similar occurrence and position variation to flares and sunspots, though there is not one-to-one agreement [29].…”