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Evolution of Hard X-ray Sources and Ultraviolet Solar Flare Ribbons for a Confined Eruption of a Magnetic Flux Rope

We study the magnetic field structures of hard X-ray sources and flare ribbons of the M1.1 flare in active region NOAA 10767 on 2005 May 27. We have found in a nonlinear force-free field extrapolation, over the same polarity inversion line, a small pre-eruptive magnetic flux rope located next to sheared magnetic arcades. Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Transition Region and Coronal Explorer (TRACE) observed this confined flare in the X-ray bands and ultraviolet (UV) 1600 Å\ bands, respectively. In this event magnetic reconnection occurred at several locations. It first started at the location of the pre-eruptive flux rope. Then, the observations indicate that magnetic reconnection occurred between the pre-eruptive magnetic flux rope and the sheared magnetic arcades more than 10 minutes before the flare peak. It implied the formation of the larger flux rope, as observed with TRACE. Next, hard X-ray (HXR) sources appeared at the footpoints of this larger flux rope at the peak of the flare. The associated high-energy particles may have been accelerated below the flux rope, in or around a reconnection region. Still, the close spatial association between the HXR sources and the flux rope footpoints favors an acceleration within the flux rope. Finally, a topological analysis of a large solar region including the active regions NOAA 10766 and 10767 shows the existence of large-scale Quasi-Separatrix Layers (QSLs) before the eruption of the flux rope. No enhanced emission was found at these QSLs during the flare, but the UV flare ribbons stopped at the border of the closest large-scale QSL.