  Solar flare ribbon fronts appear ahead of the bright structures that normally
characterise solar flares, and can persist for an extended period of time in
spatially localised patches before transitioning to `regular' bright ribbons.
They likely represent the initial onset of flare energy deposition into the
chromosphere. Chromospheric spectra (e.g. He I 10830A and the Mg II near-UV
lines) from ribbon fronts exhibit properties rather different to typical flare
behaviour. In prior numerical modelling efforts we were unable to reproduce the
long lifetime of ribbon fronts. Here we present a series of numerical
experiments that are rather simple but which have important implications. We
inject a very low flux of nonthermal electrons ($F = 5\times10^{8}$ erg
s$^{-1}$ cm$^{-2}$) into the chromosphere for 100 s before ramping up to
standard flare energy fluxes $(F = 10^{10-11}$ erg s$^{-1}$ cm$^{-2}$).
Synthetic spectra not only sustained their ribbon front-like properties for
significantly longer, in the case of harder nonthermal electron spectra the
ribbon front behaviour persisted for the entirety of this weak-heating phase.
Lengthening or shortening the duration of the weak-heating phase commensurately
lengthened or shortened the ribbon front lifetimes. Ribbon fronts transitioned
to regular bright ribbons when the upper chromosphere became sufficiently hot
and dense, which happened faster for softer nonthermal electron spectra. Thus,
the lifetime of flare ribbon fronts are a direct measure of the duration over
which a relatively low flux of high energy electrons precipitates to the
chromosphere prior to the bombardment of a much larger energy flux.
