  We present and compare measurements of the spacecraft potential (Vs/c) of the
Rosetta space- craft throughout its stay in the inner coma of comet
67P/Churyumov-Gerasimenko, by the Langmuir probe (RPC-LAP) and Ion Composition
Analyzer (RPC-ICA) instruments. Vs/c has mainly been negative, driven by the
high temperature (~5-10 eV) of the coma photoelectrons. The magnitude of the
negative Vs/c traces heliocentric, cometocentric, seasonal and diurnal
variations in cometary outgassing, consistent with production at or inside the
cometocentric distance of the spacecraft being the dominant source of the
observed plasma. LAP only picks up a portion of the full Vs/c since the two
probes, mounted on booms of 2.2 and 1.6 m length, respectively, are generally
inside the potential field of the spacecraft. Comparing with the minimum energy
of positive ions collected by ICA, we find numerous cases with strong cor-
relation between the two instruments, from which the fraction of Vs/c picked up
by LAP is found to vary between about 0.7 and 1. We also find an ICA energy
offset of 13.7 eV (95% CI: [12.5, 15.0]). Many cases of poor correlation
between the instruments are also observed, predominantly when local ion
production is weak and accelerated ions dominate the flux, or during quiet
periods with low dynamic range in Vs/c and consequently low signal-to-noise
ratios.
