Power-law Indices of EUV Intensity Power Spectrum in Flaring Coronal Active Regions

Abstract

Solar intensity power spectra are usually characterised by coloured noise, with the spectral energy following a segmented power-law function of frequency, $S(f)\propto f^{-\alpha}$, over different frequency ranges. Typically, the power-law index exceeds 1 in the low-frequency part ($\alpha_\mathrm{lf}$) and is around 0 at high frequencies ($\alpha_\mathrm{hf}$). This work investigates the spatial and temporal evolution of the power-law indices of coronal EUV intensity power spectra in flare-hosting active regions. The spatial distribution of the power-law index in the low-frequency domain ($\alpha_\mathrm{lf}$) closely mirrors EUV intensity images, indicating that $\alpha_\mathrm{lf}$ can reveal the dynamics of coronal plasma structures. Temporally, $\alpha_\mathrm{lf}$ remains stable in quiescent active regions, but it exhibits significant variability before the flare onset. Motivated by this behaviour, we analysed 14 flare events, quantifying the temporal variation of the indices $\alpha_\mathrm{lf}$ and $\alpha_\mathrm{hf}$ as potential flare precursors. In all flare events considered, notable deviations of $\alpha_\mathrm{lf}$ beyond a defined threshold consistently occurred at the flare site within a few minutes before the flare. In some cases, the change in the value of $\alpha_\mathrm{lf} - \alpha_\mathrm{hf}$ was detected within 30--90\, minutes before the flare. This proof-of-concept study suggests that the temporal variation of the power-law indices in coronal EUV intensity power spectra could potentially serve as short-term precursors of solar flares, which needs to be validated on a larger flare sample.