We present Rosetta observations from comet 67P/Churyumov-Gerasimenko during the impact
of a coronal mass ejection (CME). The CME impacted on 5-6 Oct 2015, when Rosetta was
about 800 km from the comet nucleus, and 1.4 AU from the Sun. Upon impact, the plasma
environment is compressed to the level that solar wind ions, not seen a few days earlier when
at 1500 km, now reach Rosetta. In response to the compression, the flux of suprathermal
electrons increases by a factor of 5-10 and the background magnetic field strength increases
by a factor of ∼2.5. The plasma density increases by a factor of 10 and reaches 600 cm−3
,
due to increased particle impact ionisation, charge exchange and the adiabatic compression
of the plasma environment. We also observe unprecedentedly large magnetic field spikes at
800 km, reaching above 200 nT, which are interpreted as magnetic flux ropes. We suggest
that these could possibly be formed by magnetic reconnection processes in the coma as the
magnetic field across the CME changes polarity, or as a consequence of strong shears causing
Kelvin-Helmholtz instabilities in the plasma flow. Due to the limited orbit of Rosetta, we are
not able to observe if a tail disconnection occurs during the CME impact, which could be
expected based on previous remote observations of other CME-comet interactions.