  Strong electron cooling on the neutral gas in cometary comae has been
predicted for a long time, but actual measurements of low electron temperature
are scarce. We present in situ measurements of plasma density, electron
temperature and spacecraft potential by the Rosetta Langmuir probe instrument,
LAP. Data acquired within a few hundred km from the nucleus are dominated by a
warm component with electron temperature typically 5--10 eV at all heliocentric
distances covered (1.25 to 3.83 AU). A cold component, with temperature no
higher than about 0.1 eV, appears in the data as short (few to few tens of
seconds) pulses of high probe current, indicating local enhancement of plasma
density as well as a decrease in electron temperature. These pulses first
appeared around 3 AU and were seen for longer periods close to perihelion. The
general pattern of pulse appearance follows that of neutral gas and plasma
density. We have not identified any periods with only cold electrons present.
The electron flux to Rosetta was always dominated by higher energies, driving
the spacecraft potential to order -10 V. The warm (5--10 eV) electron
population is interpreted as electrons retaining the energy they obtained when
released in the ionisation process. The sometimes observed cold populations
with electron temperatures below 0.1 eV verify collisional cooling in the coma.
The cold electrons were only observed together with the warm population. The
general appearance of the cold population appears to be consistent with a
Haser-like model, implicitly supporting also the coupling of ions to the
neutral gas. The expanding cold plasma is unstable, forming filaments that we
observe as pulses.
