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

Flare Kernels May be Smaller than You Think: Modelling the Radiative Response of Chromospheric Plasma Adjacent to a Solar Flare