Scaling and Long Term Correlation Properties of EUV Intensity Fluctuations and Implications for Impulsive Heating Mechanisms of the Solar Corona
Scaling properties of the stochastic component of EUV intensity fluctuations from AIA/SDO observations show long-term correlations and can carry information about the energetics of coronal loops. Power spectra indicate that the stochastic time series are nonstationary. Thus we apply the method of detrended fluctuation analysis (DFA), which was designed to determine the true scaling properties of a signal. It can identify the long-term correlations in noisy and nonstationary time series after accounting for external influences. The scaling exponents encountered in the solar fluctuation functions indicate long-time correlations of the series. We study to what degree the properties may correspond to those of fractional Brownian motion (fBm) or fractional Gaussian noise (fGn) processes. Analysis of a non-flaring active region (AR) indicates that the EUV emission in the hot 131 Å (Fe XXI), hot 94 Å (Fe XVIII) and 335 Å intensity bands has different properties from the warm emission in the 211, 193 and 171 Å bands. Further differences are found in the quiet vs AR core regions. The intensity values satisfy probability distribution functions (pdf)s corresponding to superposed lognormal and Gaussian functions. The pdfs of the increments are Gaussian. The properties of the data can be reproduced by a physically motivated phenomenological model for impulsive heating with added noise. We propose that DFA, complemented with the identification of the pdfs, can be a useful tool to constrain more realistic models of coronal heating.