Hall effect in the coma of 67P/Churyumov–Gerasimenko

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 field 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.