Extreme-ultraviolet flare loop emissions in an eruptive event

The TRACE/BBSO joint campaign on 27 September 1998 observed an eruptive flare event which lasted for half an hour. The observation covered several ultraviolet (UV) and extreme-ultraviolet (EUV) lines and Hα center and off-band emissions with very high spatial resolution. We find the EUV emissions in different stages of the flare display different characteristics. (1) During the 'pre-flare' phase, when the SXR output was weak, we observed simultaneous impulsive HXR peak at 25-100 keV and strong EUV emission. (2) In the impulsive phase, when Hα, UV and SXR emissions were rising to the maxima, the EUV emission was very weak. (3) During the main phase, when SXR emission was decaying, a peak in the EUV emission was observed which was substantially delayed by 7 min compared to emissions from other wavelengths. Based on our observations, we propose that the 'pre-flare' phase in this event was a separate energy release process rather than a mere pre-cursor of the flare, and it is likely that the 'pre-flare' EUV emission was due to weak in situ heating of low-lying coronal loops. The mechanism of the EUV emission in the main phase is investigated. It is suggested that the delayed EUV emission may come from cooling of SXR loops.