  Observations of the Sun with the Atacama Large Millimeter Array have now
started, and the thermal infrared will regularly be accessible from the NSF's
Daniel K. Inouye Solar Telescope. Motivated by the prospect of these new data,
and by recent flare observations in the mid infrared, we set out here to model
and understand the source of the infrared continuum in flares, and to explore
its diagnostic capability for the physical conditions in the flare atmosphere.
We use the 1D radiation hydrodynamics code RADYN to calculate mid-infrared
continuum emission from model atmospheres undergoing sudden deposition of
energy by non-thermal electrons. We identify and characterise the main
continuum thermal emission processes relevant to flare intensity enhancement in
the mid- to far-infrared (2-200 $\mu$m) spectral range as free-free emission on
neutrals and ions. We find that the infrared intensity evolution tracks the
energy input to within a second, albeit with a lingering intensity enhancement,
and provides a very direct indication of the evolution of the atmospheric
ionization. The prediction of highly impulsive emission means that, on these
timescales, the atmospheric hydrodynamics need not be considered in analysing
the mid-IR signatures.
