Small-Scale Brightenings in the UV Continuum of an M9.1 Solar Flare

We analyze an M9.1 two-ribbon solar flare which occurred on 2004 July 22 using the TRACE white-light and 1700 A images, the RHESSI, and the SOHO/MDI data. We find many small-scale fast-varying brightenings that appeared in the white-light and 1700 A images along the flare ribbons. Some of them underwent rapid motions in weak magnetic field regions. We identify these short-lived brightenings as UV continuum enhancement. Our preliminary result shows that the brightenings are closely related to the HXR emission. They have a lifetime of 30–60 s and a typical size of about 1''–2''. The intensity enhancement is about 150–200 times the mean value of the quiet-Sun. According to previous works, we infer that the 1700 A enhancement may be dominated by the increased emission of 1680 A continuum coming from the temperature minimum region. The impulsive feature in the 1700 A light curves of the small-scale brightenings may be due to the irradiation of the impulsive CIV line intensity caused by the bombardment of non-thermal electron beams.