We report a detailed examination of the fine structure inside flare ribbonsand the temporal evolution of this fine structure during the X2.5 solar flarethat occurred on 2004 November 10. We examine elementary bursts of the C IV(1550{\AA}) emission lines seen as local transient brightenings inside theflare ribbons in the ultraviolet (1600{\AA}) images taken with TransitionRegion and Coronal Explorer, and we call them C IV kernels. This flare was alsoobserved in Ha with the Sartorius 18 cm Refractor telescope at Kwasanobservatory, Kyoto University, and in hard X-rays (HXR) with Reuven Ramaty HighEnergy Solar Spectroscopic Imager. Many C IV kernels, whose sizes werecomparable to or less than 2", were found to brighten successively during theevolution of the flare ribbon. The majority of them were well correlated withthe Ha kernels in both space and time, while some of them were associated withthe HXR emission. These kernels were thought to be caused by the precipitationof nonthermal particles at the footpoints of the reconnecting flare loops. Thetime profiles of the C IV kernels showed intermittent bursts, whose peakintensity, duration, and time interval were well described by power-lawdistribution functions. This result is interpreted as evidence for"self-organized criticality" in avalanching behavior in a single flare event,or for fractal current sheets in the impulsive reconnection region.
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