  The interaction between a Coronal Mass Ejection (CME) and a comet has been
observed several times by in-situ observations from the Rosetta Plasma
Consortium (RPC), which is designed to investigate the cometary magnetosphere
of comet 67P/Churyumov-Gerasimenko (CG). Goetz et al. (2019) reported a
magnetic field of up to 300 nT measured in the inner coma, which is among the
largest interplanetary magnetic fields observed in the solar system. They
suggested the large magnetic field observations in the inner coma come from
magnetic field pile-up regions, which are generated by the interaction between
a CME and/or corotating interaction region and the cometary magnetosphere.
However, the detailed interaction between a CME and the cometary magnetosphere
of comet CG in the inner coma has not been investigated by numerical
simulations yet. In this manuscript, we will use a numerical model to simulate
the interaction between comet CG and a Halloween class CME and investigate its
magnetospheric response to the CME. We find that the plasma structures change
significantly during the CME event, and the maximum value of the magnetic field
strength is more than 500nT close to the nucleus. Virtual satellites at similar
distances as Rosetta show that the magnetic field strength can be as large as
250nT, which is slightly less than what Goetz et al. (2019) reported.
