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Multifractal Properties of EUV Intensity Fluctuations and Implications for Impulsive Heating Mechanisms of the Solar Corona
Much work has focused on explaining the mechanism behind the heating of the solar corona. Due to instrument limitations in spatial resolution we cannot study the heating directly. Instead statistical analysis techniques have been applied to study characteristics of the solar emission. In this study we investigated the scaling properties of Extreme Ultraviolet (EUV) intensity fluctuations in an Active Region (AR) from images taken by the Solar Dynamics Observatory (SDO). Four separate regions were studied: the core, a weak emission zone and two distinct core loops. Two complementary methods were used in the analysis. First we calculated the probability distribution functions (PDF) of the increments and followed with the multifractal detrended fluctuation analysis (MF-DFA). The latter takes into account the non-stationary characteristics of the data and eliminates external trends to identify the long range correlations in the time series. Used together they provide characteristic "signatures" of the observed radiation. We distinguished between emission dominated by the corona or the transition region (TR) by considering the lags in pairs of EUV images. While noise is present in all EUV wavebands, it dominates in the hotter channels. Therefore the cooler emission in the 171Å waveband is the more appropriate for these types of analysis. For all regions tested and for both positive and zero lag pixels, we found that at large temporal scales in the approximate fitting range of 15 - 45 min, that the observed time signals are anti-persistent. The pixels with zero lag associated with the TR emission, present stronger anti-correlation compared to those of coronal emission. The signals present varying degrees of multifractality which is a consequence of long-range temporal correlations. The emission in the 335Å waveband has the properties of a multifractal contaminated with noise. The intensity simulated via a phenomenological model for impulsive heating, and the averaged ohmic dissipation in a reduced MHD model for coronal loops, both exhibit multifractal properties and anti-persistence. For the cases studied the PDF of increments differs from those found in the observations. We foresee that the two analysis methods applied in tandem can be effectively used to optimize the parameters in models for coronal heating.