  The timing of 503 solar flares observed simultaneously in hard X-rays, soft
X-rays and H-alpha is analyzed. We investigated the start and the peak time
differences in different wavelengths, as well as the differences between the
end of the hard X-ray emission and the maximum of the soft X-ray and H-alpha
emission. In more than 90% of the analyzed events, a thermal preheating seen in
soft X-rays is present prior to the impulsive flare phase. On average, the soft
X-ray emission starts 3 min before the hard X-ray and the H-alpha emission. No
correlation between the duration of the preheating phase and the importance of
the subsequent flare is found. Furthermore, the duration of the preheating
phase does not differ for impulsive and gradual flares. For at least half of
the events, the end of the nonthermal emission coincides well with the maximum
of the thermal emission, consistent with the beam-driven evaporation model. On
the other hand, for about 25% of the events there is strong evidence for
prolonged evaporation beyond the end of the hard X-rays. For these events, the
presence of an additional energy transport mechanism, most probably thermal
conduction, seems to play an important role.
