Context. Hard X-rays (HXRs) contain the most direct information about the non-thermal electron population in solar flares. The approximation of the HXR emission mechanism (bremsstrahlung), known as the thick-target model, is well developed. It allows one to diagnose the physical conditions within a flaring structure. The thick-target model predicts that in flare foot points, we should observe lowering of HXR sources' altitude with increasing energy.
Aims: 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. However, the relation is not only density-dependent. Its shape is also determined by the power-law distribution of non-thermal electrons. Additionally, during the impulsive phase, the plasma density and a degree of ionisation within foot points may change dramatically due to heating and chromospheric evaporation. For this reason, the interpretation of observed HXR foot point sources' altitudes is not straightforward and needs detailed numerical modelling of the electron precipitation process.
Methods: We present the results of numerical modelling of one well-observed solar flare. We used HXR observations obtained by RHESSI. The numerical model was calculated using the hydrodynamic 1D model with an application of the Fokker-Planck formalism for non-thermal beam precipitation.
Results: HXR data 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 the range of 2.7 × 1013 − 4.0 × 1014 g. This is in good agreement with the ranges obtained from hydrodynamical modelling of a flaring loop (2.3 × 1013 − 3.3 × 1013 g), and from an analysis of observed emission measure in the loop top (3.9 × 1013 − 5.3 × 1013 g). Additionally, we used specific scaling laws which gave another estimation of the evaporated mass around 2 × 1014 g.
Conclusions: 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. The obtained results significantly improve our understanding of non-thermal electron beam precipitation and allow us to refine the energy balance in solar flare foot points during the impulsive phase.