Solar impulsive soft X-ray brightenings and their connection with footpoint hard X-ray emission sources

We have investigated 46 solar flares showing impulsive soft X-ray brightenings in footpoints. To obtain their main observational characteristics we used the Yohkoh/SXT images. The results are presented in the form of normalized histograms showing the distribution of the characteristics. For 18 flares it has been possible to make a quantitative comparison between the footpoint response in soft X-ray images and in hard X-ray images. The latter have been derived from the Yohkoh/HXT and reconstructed using the MEM procedure. The evident correlation between these two kinds of response, as well as between the deposited energies, strongly supports a common physical cause, namely non-thermal electron beams. We present evidence that in the impulsive phase the chromospheric evaporation is driven mainly by low-energy non-thermal electrons. We show that the low-energy cut-off energy in hard X-ray photon fluxes may vary from flare to flare and may even vary during a given flare.