What is the density of solar flares usually?
There are some numbers here, but the article doesn't say anything about specific location, i.e. how far from the surface of the sun the data are reported.
Solar flares would ostensibly have particle densities on the order of the chromosphere and corona.
See - RELEASE OF HELIUM FROM CLOSED-FIELD REGIONS OF THE SUN
The Structure of the Sun
What about modeling them as having the density of the sun as in the beginning a lot of the stuff are blown out from the sun.
In the article in last link (pdf) -
So modeling the initial conditions of the flare based on the density of the chromosphere near sun spots. One would need a good plasma MHD model with knowledge of the plasma density and currents, magnetic field strengths and rate of change of magetic fields (for induced EMF).
See if one can obtain access to the journal Solar Physics
Solar Flare Statistics with a One-Dimensional Mhd Model
Turbulent plasma model of the narrowband dm-spikes
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