Detection of Explosive Chromospheric Evaporation in a Two-Ribbon Flare

We present two-dimensional spectral observations of an X1.2 two-ribbon flare in which blue asymmetry of H-alpha profiles are seen at one kernel for more than 1 min. It is interesting that this kernel seems to move along a flare loop since its appearance. While on the footpoints of the loop, the H-alpha profiles are found to be strongly red shifted. We calculate the moving velocity of the kernel by measuring the moving front on TRACE 1600 images and find that it has good temporal correlation with the line-of-sight velocity calculated from the H-alpha profiles using a wing bisect method. Thus the real velocity of the kernel should be the combination of the two kinds of velocities. The maximum velocity of the kernel measured is then estimated to be about 200-300 km/s, which is similar to the chromospheric evaporation velocity calculated from soft X-ray lines. We further compare the H-alpha lines of the kernel with that calculated by the radiative hydrodynamic model of the flare atmosphere and find that both have a strong blue shifted component. Therefore we believe that our results provide the evidence of the explosive evaporation of solar flares predicted by the hydrodynamic model of flare atmosphere.