By Victor Steinwand (Imperial College London)
The ESA/Rosetta mission escorted comet 67P/Churyumov─Gerasimenko for 2 yr, exploring its plasma environment across diverse outgassing conditions. Plasma density observations from the Rosetta Plasma Consortium (RPC) are broadly categorized into two regimes for the ion dynamics, linked to the presence of a diamagnetic cavity at Rosetta's location. With a diamagnetic cavity present, ions detected by Rosetta are accelerated with respect to the neutral coma. Without a diamagnetic cavity present, at lower outgassing, and nearer the nucleus, ions co-move with the neutrals. We examine the transition between regimes following Rosetta's last detection of the cavity in 2016 February. During this transition, global 3D plasma models of the cometary ionosphere underestimate plasma densities. To investigate this underestimation, we assess the sensitivity of cometary ion densities to different parameters using a 3D collisional ion test particle model, driven by electromagnetic fields from hybrid modelling. We show that considering cometary electron cooling is necessary to model cometary ion dynamics within 100 km of the surface. Electron temperatures derived from collisional electron modelling affect ion dynamics via the ambipolar electric field, increasing ion number densities. We further show that the cometary electron cooling exobase organizes Rosetta plasma density observations; different ion dynamics regimes are linked to the position of Rosetta relative to the exobase. These findings demonstrate that Rosetta was below this exobase for much of the post-perihelion period. They justify the absence of ion acceleration in plasma density assessments and the use of uniform electron-impact ionization frequencies between Rosetta and the surface during post-perihelion.
See publication for more details:
https://academic.oup.com/mnras/article/550/2/stag1256/8723309
Two-dimensional histograms of post-perihelion number density data from 2016 February 17 to 2016 September 30, for a total ion production rate 19.85 ≤ log10(νQ[s−2]) ≤ 20.15, showing the organization of the data in the parameter space r/rexo against u_i r/rexo = (ν/100σn_i)(1 − rc/r). The boxes and points are values of (ν/100σni)(1 − rc/r) binned in r/rexo. The local gradient in this parameter space gives the local ion velocity u_i. The straight line represents ions moving at the speed of the neutral background u_n. Below the exobase, the data are clustered around the ui = un line, showing the utility of the acceleration-free model inside the flattened region of the ambipolar potential well. They depart from the line above r ∼ rexo, indicating acceleration.