No spacecraft visiting a comet has been equipped with instruments to
directly measure the static electric field. However, the electric field
can occasionally be estimated indirectly by observing its effects on the
ion velocity distribution. We present such observations made by the
Rosetta spacecraft on 19th of April 2016 when comet 67P was at a low
outgassing rate and the plasma environment was relatively homogeneous.
The ion velocity distributions show the cometary ions on the first half
of their gyration. We estimate the bulk drift velocity and the gyration
speed from the distributions. By using the local measured magnetic field
and assuming an E x B drift of the gyrocentre, we get an estimate for
the average electric field driving this ion motion. We analyse a period
of 13h, during which the plasma environment does not change drastically.
We find that the average strength of the electric field is 0.21mV/m. The
direction of the electric field is mostly anti-sunward. This is in
agreement with previous results based on different methods