  Soft X-ray spectra 3.33 A - 6.15 A from the RESIK instrument on CORONAS-F
constitute a unique database for the study of the physical conditions of solar
flare plasmas, enabling the calculation of differential emission measures. The
two RESIK channels for the shortest wavelengths overlap with the lower end of
RHESSI spectral energy range, which is located around 3 keV, making it possible
to compare both data sets. We aim to compare observations from RESIK and RHESSI
spectrometers and cross-correlate these instruments. Observations are compared
with synthetic spectra calculated based on the results of one-dimensional
hydrodynamical (1D-HD) modelling. The analysis was performed for the flare on
20 September 2002. We estimated the geometry of the flaring loop, necessary for
1D-HD modelling, based on images from RHESSI and SOHO/EIT. The distribution of
non-thermal electrons (NTEs) was determined from RHESSI spectra. The 1D-HD
model assumes that non-thermal electrons with a power-law spectrum were
injected at the apex of the flaring loop. The NTEs then heat and evaporate the
chromosphere, filling the loop with hot and dense plasma radiating in soft
X-rays. The total energy of electrons was constrained by comparing observed and
calculated fluxes from GOES 1 - 8 A data. We determined the temperature and
density at every point of the flaring loop throughout the evolution of the
flare, calculating the resulting X-ray spectra. The synthetic spectra
calculated based on the results of hydrodynamic modelling for the 20 September
2002 flare are consistent within a factor of two with the observed RESIK
spectra during most of the duration of the flare. This discrepancy factor is
probably related to the uncertainty on the cross-calibration between RESIK and
RHESSI instruments.
