Impact of radial interplanetary magnetic fields on the inner coma of comet 67P/Churyumov-Gerasimenko: Hybrid simulations of the plasma environment

Context. The direction of the interplanetary magnetic field determines the nature of the interaction between a Solar System object and the solar wind. For comets, it affects the formation of both a bow shock and other plasma boundaries, as well as mass-loading. Around the nucleus of a comet, there is a diamagnetic cavity, where the magnetic field is negligible. Observations by the Rosetta spacecraft have shown that, most of the time, the diamagnetic cavity is located within a solar-wind ion cavity, which is devoid of solar wind ions. However, solar wind ions have been observed inside the diamagnetic cavity on several occasions. Understanding what determines whether or not the solar wind can reach the diamagnetic cavity also advances our understanding of cometsolar wind interaction in general. Aims. We aim to determine the influence of an interplanetary magnetic field directed radially out from the Sun that is, parallel to the solar wind velocity on the cometsolar wind interaction. In particular, we explore the possibility of solar wind protons entering the diamagnetic cavity under radial field conditions. Methods. We performed global hybrid simulations of comet 67P/Churyumov-Gerasimenko using the simulation code Amitis for two different interplanetary magnetic field configurations and compared the results to observations made by the Rosetta spacecraft. Results. We find that, when the magnetic field is parallel to the solar wind velocity, no bow shock forms and the solar wind ions are able to enter the diamagnetic cavity. A solar wind ion wake still forms further downstream in this case. Conclusions. The solar wind can enter the diamagnetic cavity if the interplanetary magnetic field is directed radially from the Sun, and this is in agreement with observations made by instruments on board the Rosetta spacecraft.