Licentiate seminar: Observations of plasma and dust around comet 67P by Rosetta

  • Date: –12:00
  • Location: Ångströmlaboratoriet, Lägerhyddsvägen 1 Häggsalen (10132)
  • Lecturer: Fredrik Leffe Johansson
  • Organiser: Department of Physics and Astronomy, Division of Astronomy and Space Physics
  • Contact person: Anders Eriksson
  • Phone: 5945
  • Licentiatseminarium

Licentiate seminar in Space and Plasma Physics

The instruments of the Rosetta Plasma Consortium (RPC) on the ESA mission Rosetta studied the comet 67P/Churyumov-Gerasimenko surroundings from August 2014 to September 2016, past perihelion and peak activity in August 2015. In this thesis, we discuss some aspects of the measurements and data from two of these instruments, the Dual Langmuir Probe instrument (RPC-LAP) and the Mutual Impedance Probe Experiment (RPC-MIP). Although a cost-effective and highly flexible plasma instrument, the simple concept of measuring current to a spherical Langmuir probe does not mean that the analysis is straight-forward. We first quantify the effect of spacecraft charging and the associated plasma sheath around the spacecraft on the in-situ measurements of RPC-LAP and RPC-MIP through numerical simulations of spacecraft-plasma interaction by use of the SPIS software package. Secondly, we study the EUV radiation intensity inside the coma by use of the photoelectron emission current obtained from LAP.We report the results of several methods (one believed to be novel) to obtain the photoemission current of RPC-LAP and use this to characterise the solar EUV flux to the comet environment. We find the measured EUV flux to be signicantly diminished during perihelion, and have not been able to find a credible instrumental explanation of this. Instead we present a model of attenuation of EUV by small (tens of nm) dust grains at large ( > 2000 km) distances from the comet, consistent with ground observations. As we do not see any local variations of EUV intensity, these small dust grains must be rare close to the nucleus, consistent with reports by in situ dust observations. This suggests that some kind of erosion process must be active as the dust grains travel outward from the cometary nucleus.