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In the present work we study the C8 flare occurred on September 26, 2000 at19:49 UT and observed by the SOHO/SUMER spectrometer from the beginning of theimpulsive phase to well beyond the disappearance in the X-rays. The emissionfirst decayed progressively through equilibrium states until the plasma reached2-3 MK. Then, a series of cooler lines, i.e. Ca x, Ca vii, Ne vi, O iv and Siiii (formed in the temperature range log T = 4.3 - 6.3 under equilibriumconditions), are emitted at the same time and all evolve in a similar way. Herewe show that the simultaneous emission of lines with such a different formationtemperature is due to thermal instability occurring in the flaring plasma assoon as it has cooled below ~ 2 MK. We can qualitatively reproduce the relativestart time of the light curves of each line in the correct order with a simple(and standard) model of a single flaring loop. The agreement with the observedlight curves is greatly improved, and a slower evolution of the line emissionis predicted, if we assume that the model loop consists of an ensemble ofsubloops or strands heated at slightly different times. Our analysis can beuseful for flare observations with SDO/EVE.

Post-flare UV light curves explained with thermal instability of loop plasma