VERY IMPULSIVE SOLAR-FLARES OBSERVED WITH THE YOHKOH SPACECRAFT

Using X-ray data from two instruments aboard the Yohkoh spacecraft, la unched in 1991 August to study solar high-energy phenomena, some 38 ex tremely impulsive flares of X-ray importance greater than C1 have been identified and studied. Electron temperatures of these flares, derive d from Yohkoh Bent Crystal Spectrometer data, appear to decline immedi ately after the intensity maximum is attained, implying that energy in put into the flaring plasma is reduced or possibly ceases after this t ime. Images of these flares with the high-resolution Yohkoh Soft X-ray Telescope show that, contrary to expectation if thermal conduction is a significant cooling mechanism, the emitting regions consist of tiny pointlike sources (volumes less than or equal to 3 x 10(24) cm(3)) wi thin a loop structure. With mass loss along field lines eliminated als o, from a previous study, the chief energy-loss mechanism is likely to be radiation, and if so a lower limit for the electron density of sim ilar to 10(12) cm(-3) is imposed. This is only compatible with values of the emission measure if the flare volume is 3 x 10(24) cm(3), corre sponding to a sphere of only 1790 km in diameter.