  Solar flare hard X-rays (HXRs) are produced as bremsstrahlung when an
accelerated population of electrons interacts with the dense chromospheric
plasma. HXR observations presented by using the Ramaty High-Energy Solar
Spectroscopic Imager (RHESSI) have shown that HXR source sizes are 3-6 times
more extended in height than those predicted by the standard collisional thick
target model (CTTM). Several possible explanations have been put forward
including the multi-threaded nature of flare loops, pitch-angle scattering, and
magnetic mirroring. However, the nonuniform ionisation (NUI) structure along
the path of the electron beam has not been fully explored as a solution to this
problem. Ionised plasma is known to be less effective at producing nonthermal
bremsstrahlung HXRs when compared to neutral plasma. If the peak HXR emission
was produced in a locally ionised region within the chromosphere, the intensity
of emission will be preferentially reduced around this peak, resulting in a
more extended source. Due to this effect, along with the associated density
enhancement in the upper chromosphere, injection of a beam of electrons into a
partially ionised plasma should result in a HXR source which is substantially
more vertically extended relative to that for a neutral target. Here we present
the results of a modification to the CTTM which takes into account both a
localised form of chromospheric NUI and an increased target density. We find 50
keV HXR source widths, with and without the inclusion of a locally ionised
region, of ~3 Mm and ~0.7 Mm, respectively. This helps to provide a theoretical
solution to the currently open question of overly-extended HXR sources.
