  Numerical models of solar flares typically focus on the behaviour of
directly-heated flare models, adopting magnetic field- aligned, plane-parallel
methodologies. With high spatial- and spectral-resolution ground-based optical
observations of flares, it is essential also to understand the response of the
plasma surrounding these strongly heated volumes. We investigate the effects of
the extreme radiation field produced by a heated column of flare plasma on an
adjacent slab of chromospheric plasma, using a two-dimensional radiative
transfer model and considering the time-dependent solution to the atomic level
populations and electron density throughout this model. The outgoing spectra of
H$\alpha$ and Ca II 854.2 nm synthesised from our slab show significant
spatial-, time-, and wavelength-dependent variations (both enhancements and
reductions) in the line cores, extending on order 1 Mm into the non-flaring
slab due to the incident transverse radiation field from the flaring boundary.
This may lead to significant overestimates of the sizes of directly-heated
flare kernels, if line-core observations are used. However, the radiation field
alone is insufficient to drive any significant changes in continuum intensity,
due to the typical photospheric depths at which they forms, so continuum
sources will not have an apparent increase in size. We show that the line
formation regions near the flaring boundary can be driven upwards in altitude
by over 1 Mm despite the primary thermodynamic parameters (other than electron
density) being held horizontally uniform. This work shows that in simple models
these effects are significant and should be considered further in future flare
modelling and interpretation.
