Balance at the edge of the diamagnetic cavity: first results from a Particle-In-Cell simulation

A common plasma feature which has been observed at comets during a relatively high outgassing activity is the presence of a magnetic field-free region, the so-called diamagnetic cavity. Observed for the first time at 1P/Halley, such structures have also been crossed many times at low relative speed at 67P/Churyumov-Gerasimenko by the ESA/Rosetta spacecraft.

Many questions have been raised about the origin of this boundary. It is quite clear that, as one goes from an unmagnetised to a magnetised medium, one of the forces playing a role is the magnetic (pressure and tension) force. But what other force counter-balances the latter and helps form the boundary? For 1P/Halley, one formerly accepted explanation was the ion-neutral friction which has been investigated many times with magneto-hydrodynamic and hybrid simulations. However, the ion-neutral friction does not explain the observations at 67P/C-G as the outgassing rate was much lower than that of 1P/Halley.

In this work, we investigate the balance between the electromagnetic forces at the boundary with a collisionless Particle-In-Cell 1D3V simulation and the open source code Smilei. It allows us to go down to scales which are not modelled by the more common MHD or hybrid simulations. In addition, this fully kinetic simulation give us access to the different moments (e.g., number density, mean velocity, pressure tensor) of the distribution function without extra assumptions (e.g., Ohm's law and adiabatic electrons). In particular, we investigate at the balance between the different forces at play on the electrons, i.e., the electron pressure gradient and the Lorentz force.

For example, first results show that there is a sharp increase in the electric field at the boundary which decelerates ions coming from the diamagnetic cavity before reaching the magnetised part and being backstreamed towards the comet.