  The electron temperature of the plasma is one important aspect of the
environment. Electrons created by photoionization or impact ionization of
atmospheric gas have energies $\sim$10 eV. In an active comet coma, the gas
density is high enough for rapid cooling of the electron gas to the neutral gas
temperature (a few hundred kelvin). How cooling evolves in less active comets
has not been studied before. Aims. We aim to investigate how electron cooling
varied as comet 67P/Churyumov-Gerasimenko changed its activity by three orders
of magnitude during the Rosetta mission. We used in situ data from the Rosetta
plasma and neutral gas sensors. By combining Langmuir probe bias voltage sweeps
and mutual impedance probe measurements, we determined at which time cold
electrons formed at least 25\% of the total electron density. We compared the
results to what is expected from simple models of electron cooling, using the
observed neutral gas density as input. We demonstrate that the slope of the
Langmuir probe sweep can be used as a proxy for the presence of cold electrons.
We show statistics of cold electron observations over the two-year mission
period. We find cold electrons at lower activity than expected by a simple
model based on free radial expansion and continuous loss of electron energy.
Cold electrons are seen mainly when the gas density indicates that an exobase
may have formed. Collisional cooling of electrons following a radial outward
path is not sufficient to explain the observations. We suggest that the
ambipolar electric field keeps electrons in the inner coma for a much longer
time, giving them time to dissipate energy by collisions with the neutrals. We
conclude that better models are required to describe the plasma environment of
comets. They need to include at least two populations of electrons and the
ambipolar field.
