  The Langmuir Probe instrument on Rosetta monitored the photoelectron emission
cur- rent of the probes during the Rosetta mission at comet
67P/Churyumov-Gerasimenko, in essence acting as a photodiode monitoring the
solar ultraviolet radiation at wave- lengths below 250 nm. We have used three
methods of extracting the photoelectron saturation current from the Langmuir
probe measurements. The resulting dataset can be used as an index of the solar
far and extreme ultraviolet at the Rosetta spacecraft position, including
flares, in wavelengths that are important for photoionisation of the cometary
neutral gas. Comparing the photoemission current to data measurements by
MAVEN/EUVM and TIMED/SEE, we find good correlation when 67P was at large
heliocentric distances early and late in the mission, but up to 50 percent
decrease of the expected photoelectron current at perihelion. We discuss
possible reasons for the photoemission decrease, including scattering and
absorption by nanograins created by disintegration of cometary dust far away
from the nucleus.
