Nonuniform Target Ionisation and Fitting Thick Target Electron Injection Spectra to RHESSI Data 

Eduard Kontar Submitted: 20030326 15:02
Past analysis of flare Hard XRay (HXR) spectra have largely
ignored the effect of nonuniform ionisation along the electron
paths in the thick target model, though it is very significant for
wellresolved spectra. The inverse problem (photon spectrum to
electron injection spectrum {cal F}_0(E_0)) is disturbingly
nonunique. However, we show that it is relatively simple to allow
for the effect in forward fitting of parametric models of {cal
F}_0(E_0) and provide an expression to evaluate it for the usual single
power law form of {cal F}_0(E_0). The expression involves
the column depth N_* of the transition region in the flare loop as
one of the parameters so data fitting can enable derivation of
N_* (and its evaporative evolution) as part of the fitting
procedure. The fit to RHESSI data on four flares for a single
power law {cal F}_0(E_0) is much improved when ionisation structure
is included compared to when the usual fully ionised approximation is used.
This removes the need, in these events at least, to invoke broken
powerlaws, or other forms, of the acceleration spectrum {cal F}_0(E_0)
to explain the observed photon spectrum.
Authors: E.P. Kontar, J.C. Brown, and G. K. McArthur
Projects: RHESSI

Publication Status: Solar Physics, (in press)
Last Modified: 20051207 05:20


