Chromospheric evaporation in four solar flares observed by Yohkoh

We present an analysis of the Ca XIX soft X-ray spectra of four hares observed by the Yohkoh spacecraft. These spectra show evident blueshifted signatures. Using a two-component fitting technique, we obtain the time variations of the fitting parameters. The peak line-of-sight upflow velocities range from 180 to 320 km s(-1). The durations of upflows with observable velocities (greater than or equal to 100 km s(-1)) are found to be 4-10 minutes. Particularly, for two of the four flares, the emission measure of the blueshifted component is larger than that of the stationary component at early times. This fact agrees qualitatively with the predictions of numerical simulations of chromospheric evaporation. There is a rough correlation between the nonthermal turbulent velocity and the upflow velocity, consistent with the results of Fludra et al. We also find that most Ca XIX resonance profiles show no significant shift of the profile peak, regardless of the blue asymmetry which sometimes appears. The possible causes of this phenomenon are discussed.