VERY IMPULSIVE SOLAR-FLARES OBSERVED WITH THE YOHKOH SPACECRAFT

Using X-ray data from two instruments aboard the Yohkoh spacecraft, launched in 1991 August to study solar high-energy phenomena, some 38 extremely impulsive flares of X-ray importance greater than C1 have been identified and studied. Electron temperatures of these flares, derived from Yohkoh Bent Crystal Spectrometer data, appear to decline immediately after the intensity maximum is attained, implying that energy input into the flaring plasma is reduced or possibly ceases after this time. 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)) within a loop structure. With mass loss along field lines eliminated also, 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 similar 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), corresponding to a sphere of only 1790 km in diameter.