Temperature and density estimates of extreme-ultraviolet flare ribbons derived from trace diffraction patterns

We report on Transition Region And Coronal Explorer 171 observations of the GOES X20 class flare on 2001April2 that shows EUV flare ribbons with intense diffraction patterns. Between the 11th to 14th order, the diffraction patterns of the compact flare ribbon are dispersed into two sources. The two sources are identified as emission from the Fe IX line at 171.1 and the combined emission from Fe X lines at 174.5, 175.3, and 177.2 . The prominent emission of the Fe IX line indicates that the EUV-emitting ribbon has a strong temperature component near the lower end of the 171 temperature response (∼ 0.6-1.5MK). Fitting the observation with an isothermal model, the derived temperature is around 0.65MK. However, the low sensitivity of the 171 filter to high-temperature plasma does not provide estimates of the emission measure for temperatures above 1.5MK. Using the derived temperature of 0.65MK, the observed 171 flux gives a density of the EUV ribbon of 3 × 10¹¹cm^-3. This density is much lower than the density of the hard X-ray producing region (∼ 10¹³ to 10¹⁴cm^-3) suggesting that the EUV sources, though closely related spatially, lie at higher altitudes.