Abstract
We present partial ring distributions of solar wind protons observed by
the Rosetta spacecraft at comet 67P/Churyumov-Gerasimenko. The formation
of ring distributions is usually associated with high activity comets,
where the spatial scales are larger than multiple ion gyroradii. Our
observations are made at a low-activity comet at a heliocentric distance
of 2.8 AU on April 19th, 2016, and the partial rings occur at a spatial
scale comparable to the ion gyroradius. We use a new visualisation
method to simultaneously show the angular distribution of median energy
and differential flux. A fitting procedure extracts the bulk speed of
the solar wind protons, separated into components parallel and
perpendicular to the gyration plane, as well as the gyration velocity.
The results are compared with models and put into context of the global
comet environment. We find that the formation mechanism of these partial
rings of solar wind protons is entirely different from the well-known
partial rings of cometary pickup ions at high-activity comets. A density
enhancement layer of solar wind protons around the comet is a focal
point for proton trajectories originating from different regions of the
upstream solar wind. If the spacecraft location coincides with this
density enhancement layer, the different trajectories are observed as an
energy-angle dispersion and manifest as partial rings in velocity space.