EVOLUTION OF 2 SMALL SOLAR-FLARES

Data from the YOHKOH satellite have been analysed for two small flares (GOES class C) of total duration of 10 and 60 minutes. Upflows in S X V, Ca XIX and Fe XXV lines were compared and the presence of a range o f upflow velocities was found. Emission from flare loop footpoints cor responding to plasma moving with a typical velocity of 200-400 km s(-1 ) is seen in soft X-ray images. In one of these events (23 June 1992), which occurred in sheared loops being part of a sparse magnetic arcad e, with initial energy release taking place near one of the footpoints , a large proportion of upflowing plasma was seen at least 1 minute be fore the first peak in hard X-rays. In the second event (13 July 1992) , the increase of soft X-ray emission began more than 3 minutes before , and weak mass upflows one minute before the rapid increase of temper ature and the onset of the detectable hard X-ray emission. This event was probably triggered by emerging magnetic flux and accompanied by he ating and restructuring of two nearby magnetic loops. In both events t he emission measure of upflowing plasma is present simultaneously to, and is very well correlated in time with the hard X-ray flux in the 14 -23 keV band. Differential emission measure in the temperature range 5 - 60 10(6) K was derived from S XV, Ca XIX and Fe XXV line and contin uum fluxes, and from images in two broad band soft X-ray filters, and used to analyse the thermal contribution to the hard X-ray emission. A non-thermal component of the hard X-ray emission is found at the peak of the 23 June 1992 flare. The hard X-ray emission in the 13 July 199 2 flare is primarily thermal, however, a possibility of an enhanced ta il of the electron energy distribution above 14 keV is also indicated. The chromospheric evaporation in these flares was driven both by elec tron beams and thermal conduction, with conduction predominating durin g most of the rise phase of the 13 July 1992 flare. In both events, th e soft X-ray emission measure at flare maximum was a few times 10(48) cm(-3) with an electron temperature 19 and 24 10(6)K; the estimated lo wer limit of the electron density is similar to 10(11) cm(-3). The bro adening of Ca XIX spectral lines in the decay phase of these flares in dicates persisting random motions with a velocity of 60 km s(-1), whic h is very similar to the non-thermal broadening observed previously by SMM in M and X class flares.