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The thick-target model predicts that in flare foot points, we should observelowering of HXR sources' altitude with increasing energy. The foot point of HXRsources result from the direct interaction of non-thermal electron beams withplasma in the lower part of the solar atmosphere, where the density increasesrapidly. Therefore, we can estimate the plasma density distribution along thenon-thermal electron beam directly from the observations of the altitude-energyrelation obtained for the HXR foot point sources. The relation's shape isdensity-dependent and is also determined by the power-law distribution ofnon-thermal electrons. Additionally, during the impulsive phase theseparameters may change dramatically. Thus, the interpretation of observed HXRfoot point sources' altitudes is not straightforward and needs detailednumerical modelling of the electron precipitation process. The numerical modelwas calculated using the hydrodynamic 1D model with an application of theFokker-Planck formalism for non-thermal beam precipitation. HXR data fromRHESSI were used to trace chromospheric density changes during a non-thermalemission burst, in detail. We have found that the amount of mass thatevaporated from the chromosphere is in good agreement with the ranges obtainedfrom hydrodynamical modelling of a flaring loop, and from an analysis ofobserved emission measure in the loop top, and with specific scaling laws.Consistency between the obtained values shows that HXR images may provide animportant constraint for models - a mass of plasma that evaporated due to anon-thermal electron beam depositing energy in the chromosphere. High-energy,non-thermal sources' (above 20 keV in this case) positions fit the columndensity changes obtained from the hydrodynamical model perfectly. Densitychanges seem to be less affected by the electrons' spectral index.

Plasma dynamics in the flaring loop observed by RHESSI