  We present observations from the Rosetta Plasma Consortium of the effects of
stormy solar wind on comet 67P/Churyumov-Gerasimenko. Four corotating
interaction regions (CIRs), where the first event has possibly merged with a
CME, are traced from Earth via Mars (using Mars Express and MAVEN) and to comet
67P from October to December 2014. When the comet is 3.1-2.7 AU from the Sun
and the neutral outgassing rate $\sim10^{25}-10^{26}$ s$^{-1}$ the CIRs
significantly influence the cometary plasma environment at altitudes down to
10-30 km. The ionospheric low-energy \textcolor{black}{($\sim$5 eV) plasma
density increases significantly in all events, by a factor $>2$ in events 1-2
but less in events 3-4. The spacecraft potential drops below -20V upon impact
when the flux of electrons increases}. The increased density is
\textcolor{black}{likely} caused by compression of the plasma environment,
increased particle impact ionisation, and possibly charge exchange processes
and acceleration of mass loaded plasma back to the comet ionosphere. During all
events, the fluxes of suprathermal ($\sim$10-100 eV) electrons increase
significantly, suggesting that the heating mechanism of these electrons is
coupled to the solar wind energy input. At impact the magnetic field strength
in the coma increases by a factor of ~2-5 as more interplanetary magnetic field
piles up around of the comet. During two CIR impact events, we observe possible
plasma boundaries forming, or moving past Rosetta, as the strong solar wind
compresses the cometary plasma environment. \textcolor{black}{We also discuss
the possibility of seeing some signatures of the ionospheric response to tail
disconnection events
