TEMPORAL CORRELATION OF SOLAR HARD X-RAY-BURSTS WITH CHROMOSPHERIC EVAPORATION

During the impulsive phase of many solar flares, blueshifted emission wings are observed on the soft X-ray spectral lines of highly excited ions that are produced in the flare plasma. This emission has been com monly interpreted as chromospheric evaporation of material from the fo otpoints of coronal loops by non-thermal particle beams, although the question of whether the bulk of the energy is carried by electrons or ions (protons) has been the subject of much debate. The precise tempor al relationship between the onsets of the blueshifted emission and the hard X-ray bursts is particularly important in resolving the mechanis m of energy transfer to the hot plasma in the impulsive phase. A sampl e of flares observed with the Bragg Crystal Spectrometer (BCS) on Yohk oh has been analysed for blueshifted emission and the results compared with hard X-ray light curves obtained with the Burst and Transient So urce Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO). I n some flares, the blueshifted emission precedes the onset of the hard X-rays by up to 100 s. There is no evidence for a temporal correlatio n between the maximum energy input to the hard X-ray bursts and the ma ximum blueshifted intensity. These results lend support to those model s favouring protons as the dominant energy carrier in the impulsive ph ase of hares and are inconsistent with the hypothesis that the bulk of the energy resides in electron beams, although some other energy inpu t, while unlikely, cannot be completely eliminated.