  Aims. We aim to determine whether dissociative excitation of cometary
neutrals by electron impact is the major source of far-ultraviolet (FUV)
emissions at comet 67P/Churyumov-Gerasimenko in the southern hemisphere at
large heliocentric distances, both during quiet conditions and impacts of
corotating interaction regions observed in the summer of 2016.
  Methods. We combined multiple datasets from the Rosetta mission through a
multi-instrument analysis to complete the first forward modelling of FUV
emissions in the southern hemisphere of comet 67P and compared modelled
brightnesses to observations with the Alice FUV imaging spectrograph. We
modelled the brightness of OI1356, OI1304, Lyman-$\beta$, CI1657, and CII1335
emissions, which are associated with the dissociation products of the four
major neutral species in the coma: CO$_2$, H$_2$O, CO, and O$_2$. The
suprathermal electron population was probed by RPC/IES and the neutral column
density was constrained by several instruments: ROSINA, MIRO and VIRTIS.
  Results. The modelled and observed brightnesses of the FUV emission lines
agree closely when viewing nadir and dissociative excitation by electron impact
is shown to be the dominant source of emissions away from perihelion. The
CII1335 emissions are shown to be consistent with the volume mixing ratio of CO
derived from ROSINA. When viewing the limb during the impacts of corotating
interaction regions, the model reproduces brightnesses of OI1356 and CI1657
well, but resonance scattering in the extended coma may contribute
significantly to the observed Lyman-$\beta$ and OI1304 emissions. The
correlation between variations in the suprathermal electron flux and the
observed FUV line brightnesses when viewing the comet's limb suggests electrons
are accelerated on large scales and that they originate in the solar wind. This
means that the FUV emissions are auroral in nature.
