  In a standard method of determining electron density for soft X-ray (SXR)
flare kernels it is necessary to assume what is the extension of a kernel along
the line of sight. This is a source of significant uncertainty of the obtained
densities. In our previous paper (Bak-Steslicka and Jakimiec, 2005) we have
worked out another method of deriving electron density, in which it is not
necessary to assume what is the extension of a kernel along the line of sight.
The point is that many flares, during their decay phase, evolve along the
sequence of steady-state models [quasi-steady-state (QSS) evolution] and then
the scaling law, derived for steady-state models, can be used to determine the
electron density. The aim of the present paper is: (1) to improve the two
methods of density determination, (2) to compare the densities obtained with
the two methods. We have selected a number of flares which showed QSS evolution
during the decay phase. For these flares the electron density, N, has been
derived by means of standard method and with our QSS method. Comparison of the
N values obtained with the two different methods allowed us: (1) to test the
obtained densities, (2) to evaluate the volume filling factor of the SXR
emitting plasma. Generally, we have found good agreement (no large systematic
difference) between the values of electron density obtained with the two
methods, but for some cases the values can differ by a factor up to 2. For most
flare kernels estimated filling factor turned out to be about 1, near the flare
maximum.
