  Magnetohydrodynamics simulations have been carried out in studying the solar
wind and cometary plasma interactions for decades. Various plasma boundaries
have been simulated and compared well with observations for comet 1P/Halley.
The Rosetta mission, which studies comet 67P/Churyumov-Gerasimenko, challenges
our understanding of the solar wind and comet interactions. The Rosetta Plasma
Consortium observed regions of very weak magnetic field outside the predicted
diamagnetic cavity. In this paper, we simulate the inner coma with the Hall
magnetohydrodynamics equations and show that the Hall effect is important in
the inner coma environment. The magnetic field topology becomes complex and
magnetic reconnection occurs on the dayside when the Hall effect is taken into
account. The magnetic reconnection on the dayside can generate weak magnetic
filed regions outside the global diamagnetic cavity, which may explain the
Rosetta Plasma Consortium observations. We conclude that the substantial change
in the inner coma environment is due to the fact that the ion inertial length
(or gyro radius) is not much smaller than the size of the diamagnetic cavity.
