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