Decaying post – flare loops system observed by SOHO / CDS and Yohkoh / SXT

The results of an analysis of joint SOHO/CDS and Yohkoh/SXT observations of a decaying post–flare loops system with a rapid time evolution are presented. The loop system was a remnant of a small single loop flare (GOES class C2.9). Using the CDS raster taken in several EUV lines with different formation temperatures and a temperature sensitive line pair Fe XVI 360.8/Si XII 520.7 we confirmed the existence of the vertical stratification in the loop system according to the line formation temperature. The analysis of the SXT data showed a strong decay of the system with time. While the temperature of the hot part of the system (T ' 2.5 MK) decreased only slightly, the total emission measure dropped by more than a factor of four in approximately 103 s. This could be explained by a plasma outflow from the loops with velocity approximately 10 km/s. On the other hand, signs of rapid, probably radiative cooling can be identified in the images obtained from the CDS raster taken in cool lines of O V and O III. Using the density sensitive line pair of Fe XIV 334.2/353.8 and the integrated intensity of Fe XIV 334.2 line we determined the electron densities and emission measures across the top of the loop system. From the results of these measurements, taking all known uncertainties into account, we obtained that the geometrical filling factor at the top of the system in the regions with maximum electron density in the Fe XIV line lies in the interval from ' 0.01 to ' 0.2. A simple theoretical approximation of the energy balance in the post–flare plasma gives a total cooling time ' 750 s.