The solar wind is mostly made up of high-velocity electrons and protons ejected from the upper atmosphere of the Sun (several hundreds of kilometres per second). This plasma interacts with the atmosphere and magnetic field of Solar System bodies.
In the case of comets, the nucleus has no intrinsic magnetic field, and its atmosphere — generated by cometary activity — is not gravitationally bound and escapes in the outer space. This continuous outflow of neutral elements, that are gradually photo-ionised, results in a very wide interaction region between the solar wind and the cometary atmosphere.
The interaction between the solar wind and the coma has significant effects at large distances from the cometary nucleus; therefore, monitoring the solar wind dynamics provides an indirect measurement of the cometary activity, removing the need for close in-situ measurements. Yet, this requires to have a good understanding of the mechanisms at play, which is the subject of the project initiated by Etienne Behar in which I am involved.
The RPC-ICA instrument onboard ESA's Rosetta mission has measured the speed and direction of solar and cometary ions in the neighbourhood of comet 67P/Churyumov-Gerasimenko. Observations reveal that the flux of solar protons is almost not decelerated (about 400 km/s), but it is more and more deflected as cometary activity increases. After a maximal deflection close to 180°, the instrument did not detect any solar particle any more for several months before and after the comet's perihelion passage. These observations have been a surprise for the scientific community, because people were used to studying very active comets near perihelion, for which the solar wind is strongly decelerated and produces a bow shock.
The regime of weak cometary activity, which is essentially non-collisional, requires a specific modelling said "kinetic", as opposed to the "fluid" description previously used. A simplified analytical model allows one to link the properties of the solar protons measured by RPC-ICA with the time-varying properties of the cometary nucleus [ix]. The continuous flux of solar protons naturally creates an asymmetric density structure, as well as a central cavity devoid of any particle [vii].
We retrieve the solar wind deflection without deceleration, as well as the central cavity whose radius goes beyond the Rosetta spacecraft when the comet is close to perihelion, which explains why no signal has been detected. Using our semi-analytical model, the RPC-ICA data provides a continuous indirect monitoring of the cometary outgassing rate through a unique scaling parameter [vi]. These results can be applied to other non-magnetised Solar System bodies having a tenuous atmosphere that is weakly gravitationally bound, such as Pluto or centaurs.
Last update: 2024-11.