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High-Cadence and High-Resolution Halpha Imaging Spectroscopy of a Circular Flare's Remote Ribbon with IBIS

We present an unprecedented high-resolution \ha\ imaging spectroscopic observation of a C4.1 flare taken with IBIS on 2011 October 22. The flare consists of a main circular ribbon that occurred in a parasitic magnetic configuration and a remote ribbon that was observed by the IBIS. Such a circular-ribbon flare with a remote brightening is predicted in 3D fan-spine reconnection but so far has been rarely observed. During the flare impulsive phase, we define "core" and "halo" structures in the observed ribbon. Examining the \ha\ emission spectra averaged in the flare core and halo areas, we find that only those from the flare cores show typical nonthermal electron beam heating characteristics. These characteristics include: broad and centrally reversed emission spectra, excess emission in the red wing with regard to the blue wing (i.e., red asymmetry), and redshifted bisectors of the emission spectra. We also observe rather quick timescales for the heating (30 s) and cooling (14--33 s) in the flare core locations. Therefore, we suggest that the flare cores revealed by IBIS track the sites of electron beam precipitation with exceptional spatial and temporal resolution. The flare cores show two-stage motion (a parallel motion along the ribbon followed by an expansion motion perpendicular to the ribbon) during the two impulsive phases of the flare. Some cores jump quickly (30 \kms) between discrete magnetic elements implying reconnection involving different flux tubes. We observe a very high temporal correlation ($\gtrsim0.9$) between the integrated \ha\ and HXR emission during the flare impulsive phase. A short time delay (4.6 s) is also found in the \ha\ emission spikes relative to HXR bursts. The ionization timescale of the cool chromosphere and the extra time taken for the electrons to travel to the remote ribbon site may contribute to this delay.