We exploit a rare joint set of high-resolution, very high cadence TRACE UV images and high-resolution magnetograms from SOHO MDI to investigate the dynamical properties of flare ribbons in a GOES M1 class flare from NOAA active region 9236 on 2000 November 23 at 23:28 UT. Assuming that flare ribbons locate the chromospheric footpoints of magnetic field lines reconnecting in the corona and that magnetic flux is conserved, we measure the magnetic reconnection rate (in maxwells per second) by overlaying the ribbons on co-registered magnetograms and using intensity-based binary masks to track the magnetic flux swept over by the evolving ribbons, and by assumption swept up in the reconnection. In the event observed, the ribbons did not separate with time but remained stationary while they brightened, lengthened, and faded in place. Thus, the ribbons may be akin to hard X-ray flare kernels moving antiparallel to each other, which others interpret as caused by strong photospheric shear. The derived reconnection rate is noisy, with little correlation between adjacent 1.4 s samples; the peak rate for pixels summed over the ribbon is ~5 × 1018 Mx s-1; the average rise-phase rate is 10 times lower. The "local" rates for adjacent pixels added to the ribbon at adjacent times show correlations with 1600 ? band intensities, supporting the reconnection interpretation. For simple assumptions about geometry, the reconnection appears fast (Vin ≥ 0.01VA). The peak reconnection rates, along with estimates of the current-sheet length scale suggested by measured quantities, imply peak electric fields of order 40 V cm-1. We discuss caveats to these results.
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