Abstract
Abstract We investigated the evolution of a C3.3 impulsive flare and its preflare activity, which occurred in NOAA Active Region 10923 on 2006 November 12, using Hinode X-Ray Telescope (XRT) data. For an extensive investigation, we also used GOES X-ray flux, TRACE 171 Å, and SOHO MDI data. Examining the time-series of the XRT and TRACE images, we can identify the following evolutionary sequences: (1) There were three bundles of loops along the sheared polarity inversion line forming a sigmoidal structure during the preflare phase. (2) Preflare brightening occurred between two upper-loop bundles, and they consequently formed one larger bundle. (3) The main flare occurred near the location where this new loop bundle and the third bundle met together. (4) As a result, a single stacked loop structure was formed. This morphological evolution of the X-ray loops is quite consistent with a tether-cutting model involving a single-bipole explosion. Our result shows that the preflare and the main flare in this event are a two-step reconnection process, which strongly suggests that the preflare activity plays an important role in triggering the main flare.
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