Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1989 Rosetta Observations of Plasma and Dust at Comet 67P FREDRIK LEFFE JOHANSSON ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 ISBN 978-91-513-1070-1 UPPSALA urn:nbn:se:uu:diva-425953 2020 Dissertation presented at Uppsala University to be publicly examined on Zoom, Friday, 15 January 2021 at 13:15 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Dr. Nicolas André (IRAP, Toulouse, France). Online defence: https://uu-se.zoom.us/j/67552597754 Contact person for questions about participation is Prof. Mats André 0707-792072 Abstract Johansson, F. L. 2020. Rosetta Observations of Plasma and Dust at Comet 67P. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1989. 35 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-1070-1. In-situ observations of cometary plasma are not made because they are easy. The historic ESA Rosetta mission was launched in 2004 and traversed space for ten years before arriving at comet 67P/Churyumov-Gerasimenko, which it studied in unprecedented detail for two years. For the Rosetta Dual Langmuir Probe Experiment (LAP), the challenge was increased by the sensors being situated on short booms near a significantly negatively charged spacecraft, which deflects low-energy charged particles away from our instrument. To disentangle the cometary plasma signature in our signal, we create a charging model for the particular design of the Rosetta spacecraft through 3D Particle-in-Cell/hybrid spacecraft-plasma interaction simulations, which also can be applicable to similarly designed spacecraft in cold plasma environments. By virtue of this model, we find a way to cross-calibrate (with the Mutual Impedance probe, MIP) the LAP spacecraft potential to a plasma density estimate with increased temporal resolution and dynamic range than any single plasma instrument alone. To characterise and disentangle the Sun-driven photoelectric current from the positive cometary ion current signal, using three different methods (where we believe one is novel), we find a signature of an attenuation of the Extreme Ultraviolet (EUV) radiation from the Sun that follows the cometary out-gassing activity. We discuss possible reasons for this, where the scattering and absorption of radiation by ~20 nm sized dust grains created by the disintegration of far larger cometary dust grains far from the nucleus appears most likely. By cross-calibrating also our current measurements to MIP, we find a cometary ion speed estimate, which, when applied to a simple comet ionosphere model using the LAP photoemission as a photoionisation proxy, predicts the measured comet plasma densities near perihelion, when comet activity was highest. This demonstrates that the LAP cross-calibration estimates are self-consistent, but also strongly suggests that the EUV attenuation we reported is apparent also in the comet ionosphere, as less plasma is ionised by EUV radiation. The ion speed estimates from LAP are consistent with recent results of cometary water ion velocities from the Ion Composition Analyser (ICA), and much elevated above the comet neutral speed, often by a factor of 5. This verifies that the cometary ions are not collisionally coupled to the neutrals, and instead rapidly accelerated by some electric field, such as an ambipolar electric field or from plasma wave activity. Keywords: Rosetta, Comets, Plasma, Langmuir Probes, Dust, Spacecraft-plasma interaction, Electrostatic Probes Fredrik Leffe Johansson, Swedish Institute of Space Physics, Uppsala Division, Box 537, Uppsala University, SE-75121 Uppsala, Sweden. Department of Physics and Astronomy, Box 516, Uppsala University, SE-751 20 Uppsala, Sweden. © Fredrik Leffe Johansson 2020 ISSN 1651-6214 ISBN 978-91-513-1070-1 urn:nbn:se:uu:diva-425953 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-425953) As a result of their long observations, the Egyptians have prior knowledge of earthquakes and floods, of the rising of comets, and ofall things which the ordinary man looks upon as beyond all finding out. - Diodorus of Sicily (ca. 60-21 B.C.) List of papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I Rosetta photoelectron emission and solar ultraviolet flux at comet 67P Johansson, F. L.; Odelstad, E; Paulsson, J. J. P.; Harang, S. S.; Eriksson, A. I., Mannel, T.; Vigren, E.; Edberg, N. J. T.; Miloch, W. J.; Simon Wedlund, C.; Thiemann, E.; Eparvier, F.; L. Andersson Monthly Notices of the Royal Astronomical Society , vol. 469, S626–S635 2017, doi:10.1093/mnras/stx2369 II A charging model for the Rosetta spacecraft Johansson, F. L.; Eriksson, A. I.; Gilet, N.; Henri, P.; Wattieaux, G.; Taylor, M. G. G. T.; Imhof, C.; Cipriani, F. Astronomy & Astrophysics, vol. 642, 2020. doi:10.1051/0004-6361/202038592. III Plasma densities, flow and solar EUV flux at comet 67P: A cross-calibration approach Johansson, F. L.; Eriksson, A. I.; Vigren, E.; Bucciantini, L.; Henri, P.; Nilsson, H.; Bergman, S.; Edberg, N.; Stenberg Wieser, G.; Odelstad,E. Submitted to Astronomy & Astrophysics, Nov 2020 IV Ionisation and EUV attenuation at comet 67P Johansson, F. L.; Eriksson, A. I.; Vigren, E.; Nilsson, H.; Edberg, N. J. T.; Stephenson, P. Manuscript in prep. Reprints were made with permission from the publishers. List of papers not included in this thesis 1. Gunell, H.; Götz, C.; Odelstad, E.; Beth, A.; Hamrin, M.; Henri, P.; Johansson, F. L.; Nilsson, H.; and Stenberg Wieser, G., Ion acoustic waves near a comet nucleus: Rosetta observations at comet 67P/Churyumov-Gerasimenko, Ann. Geophys. Discuss., Accepted, 2020. doi:10.5194/angeo-2020-59 2. Nilsson, H.; Williamson, H.; Bergman, S.; Stenberg Wieser, G.; Wieser, M.; Behar, E.; Eriksson, A. I.; Johansson, F. L.; Richter, I.; Götz, C., Average cometary ion flow pattern in the vicinity of comet 67P from moment data, MNRAS, vol. 498, no. 4, pp. 5263–5272, 2020. doi:10.1093/mnras/staa2613. 3. Gilet, N.; Henri, P.; Wattieaux, G.; Traoré, N.; Eriksson, A. I.; Vallières, X.; Moré, J.; Randriamboarison, O.; Odelstad, E.; Johansson, F. L.; Rubin, M Observations of a mix of cold and warm electrons by RPC-MIP at 67P/Churyumov- Gerasimenko, A&A, vol. 640, 2020. doi:10.1051/0004-6361/201937056. 4. Bergman, S., Stenberg Wieser, G., Wieser, M., Johansson, F. L., and Eriks- son, A., The influence of varying spacecraft potentials and Debye lengths on insitu low-energy ion measurements, JGR, vol. 125, no. 4, 2020. doi:10.1029/2020JA027870. 5. Bergman, S., Stenberg Wieser, G., Wieser, M., Johansson, F. L., and Eriks- son, A., The influence of spacecraft charging on low-energy ion measurements made by RPC-ICA on Rosetta, JGR, vol. 125, no. 1, 2020. doi:10.1029/2019JA027478. 6. Edberg, N. J. T.; Johansson, F. L.; Eriksson, A. I.; Andrews, D. J.; Hajra, R.; Henri, P.; Wedlund, C. S.; Alho, M.; Thiemann, E. Solar flares observed by Rosetta at comet 67P/Churyumov-Gerasimenko A&A A49 Volume 630, October 2019, doi: 10.1051/0004-6361/201834834 7. Myllys, M.; Henri, P.; Galand, M.; Heritier, K. L.; Gilet, N.; Goldstein, R.; Eriksson, A. I. ; Johansson, F. L.; Deca, J. Plasma properties of suprathermal electrons near comet 67P/Churyumov-Gerasimenko with Rosetta A&AA42 Vol- ume 630, October 2019, doi: 10.1051/0004-6361/201834964 8. Breuillard, H.; Henri, P.; Bucciantini, L.; Volwerk, M.; Karlsson, T.; Eriks- son, A.; Johansson, F.; Odelstad, E.; Richter, I.; Götz, C.; Vallières, X.; Ha- jra, R. Properties of the singing comet waves in the 67P/Churyumov-Gerasimenko plasma environment as observed by the Rosetta mission A&A A39 Volume 630, October 2019, doi: 10.1051/0004-6361/201834876 9. Vigren, E.; Edberg, N. J. T.; Eriksson, A. I.; Galand, M.; Henri, P.; Johansson, F. L.; Odelstad, E.; Rubin, M.; Vallières, X. The evolution of the electron number density in the coma of comet 67P at the location of rosetta from 2015 November through 2016 March ASJ, Volume 881, Number 1, August 2019, doi: 10.3847/1538-4357/ab29f7 10. Edberg, N J; T., Eriksson, A I.; Vigren, E; Johansson, F. L.; Götz, C; Nilsson, H; Gilet, N.; Henri, P. The convective electric field influence on the cold plasma and diamagnetic cavity of comet 67P ASJ, Volume 158, Number 2, July 2019, doi: 10.3847/1538-3881/ab2d28 11. Heritier, K. L.; Galand, M.; Henri, P.; Johansson, F. L.; Beth, A.; Eriksson, A. I.; Vallières, X.; Altwegg, K.; Burch, J. L.; Carr, C.; Ducrot, E.; Hajra, R.; Rubin, M. Plasma source and loss at comet 67P during the Rosetta mission A&A A77 Volume 618, October 2018 , doi: 10.1051/0004-6361/201832881 12. Odelstad, E.; Eriksson, A. I.; Johansson, F. L.; Vigren, E.; Henri, P.; Gilet, N.; Heritier, K. L.; Vallières, X.; Rubin, M.; André, M. Ion velocity and electron temperature inside and around the diamagnetic cavity of comet 67P JGR Volume 123, Issue 7 July 2018 Pages 5870-5893, June 2018, doi: 10.1029/2018JA025542 13. Heritier, K. L.; Altwegg, K.; Berthelier, J. -J.; Beth, A.; Carr, C. M.; De Keyser, J.; Eriksson, A. I.; Fuselier, S. A.; Galand, M.; Gombosi, T. I.; Henri, P.; Jo- hansson, F. L.; Nilsson, H.; Rubin, M.; Simon Wedlund, C.; Taylor, M. G. G. T.; Vigren, E. On the origin of molecular oxygen in cometary comae Nature Communications, Volume 9, id. 2580, July 2018, doi: 10.1038/s41467-018-04972-5 14. Eriksson, A. I.; Engelhardt, I. A. D.; André, M.; Boström, R.; Edberg, N. J. T.;Johansson, F. L.; Odelstad, E.; Vigren, E.; Wahlund, J. -E.; Henri, P.; Lebreton, J. -P.; Miloch, W. J.; Paulsson, J. J. P.; Simon Wedlund, C.; Yang, L.; Karlsson, T.; Jarvinen, R.; Broiles, T.; Mandt, K.; Carr, C.