Diamagnetic region(s): structure of the unmagnetized plasma around Comet 67P/CG

Ion Velocity and Electron Temperature Inside and Around the Diamagnetic Cavity of Comet 67P

“…Also, the red and dark blue points, obtained at the very beginning and end of the 300‐day period of cavity crossings, respectively, do not follow the general trend. We note here that the plasma density is distributed similarly to the neutral density in Figure c, a result of the close relationship between n e and n n inside the cavity observed by Henri et al (), though the plasma density seems to have less variance within each subinterval. Thus, the plasma density does not really provide any new information by which to organize the data.…”

“…Together with the intermittent nature of the signatures in LAP data, this was taken as evidence for strong filamentation of the cold plasma close to the nucleus, with individual filaments extending far outside the collisionally dominated region, perhaps even detaching from it entirely. Similar structures were observed to develop in connection to the diamagnetic cavity in global 3‐D hybrid simulations by Koenders et al () and have been proposed to result from the instability of the cavity boundary (e.g., Henri et al, ). However, observations of cold electrons are not limited to the diamagnetic cavity, so the relationship (if any) between the structure and dynamics of cold and unmagnetized plasma, respectively, is not clear.…”

“…Figure d shows the radial distance of the spacecraft in terms of the nominal electron exobase (Henri et al, ), denoted R ∗ and calculated as where σ en is the electron‐neutral cross section, which we have set to 5·10 −16 cm −3 in accordance with Itikawa and Mason (), n n is the neutral density observed by COPS on board the spacecraft, and r is the radial distance to the nucleus. The most prominent clusters of cavity crossings occur at small R ∗ , while many of the minor ones actually at larger R ∗ , as previously noted also by Henri et al (). We point out here that R ∗ primarily follows the (reciprocal of the) neutral density, which in turn largely varies in response to changes in the spacecraft latitude, shown in Figure e (black line), as a consequence of the seasonal variations in outgassing over the comet nucleus.…”

“…The plasma density inside the cavity was found to be entirely determined by the ionization of the cometary neutral atmosphere and the cavity boundary generally located close to the electron exobase. Hence, Henri et al, suggested that the cavity formation and extent was the result of electron‐neutral collisionality rather than the ion‐neutral collisionality previously invoked. They also proposed a Rayleigh‐Taylor‐type instability of the cavity boundary, driven by the electron‐neutral drag force acting as an effective gravity , instead of the Kelvin‐Helmholtz type suggested by Goetz, Koenders, Richter, et al ().…”

3D View of a Comet’s Neighborhood

“…When the gas production rate is high enough, plasma-neutral collisions eventually carve out a nonmagnetized region near the cometary nucleus [56]. This region is shaped by electron-neutral collisions [57]. Taking into account collisions will be necessary to extend this study for more active comets.…”