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Rosetta spacecraft potential and activity evolution of comet 67P
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Space Plasma Physics. (Rymd- och plasmafysik, Space Plasma Physics)ORCID iD: 0000-0001-7854-6001
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The plasma environment of an active comet provides a unique setting for plasma physics research. The complex interaction of newly created cometary ions with the flowing plasma of the solar wind gives rise to a plethora of plasma physics phenomena, that can be studied over a large range of activity levels as the distance to the sun, and hence the influx of solar energy, varies. In this thesis, we have used measurements of the spacecraft potential by the Rosetta Langmuir probe instrument (LAP) to study the evolution of activity of comet 67P/Churyumov-Gerasimenko as it approached the sun from 3.6 AU in August 2014 to 2.1 AU in March 2015. The measurements are validated by cross-calibration to a fully independent measurement by an electrostatic analyzer, the Ion Composition Analyzer (ICA), also on board Rosetta.

The spacecraft was found to be predominantly negatively charged during the time covered by our investigation, driven so by a rather high electron temperature of ~5 eV resulting from the low collision rate between electrons and the tenuous neutral gas. The spacecraft potential exhibited a clear covariation with the neutral density as measured by the ROSINA Comet Pressure Sensor (COPS) on board Rosetta. As the spacecraft potential depends on plasma density and electron temperature, this shows that the neutral gas and the plasma are closely coupled. The neutral density and negative spacecraft potential were higher in the northern hemisphere, which experienced summer conditions during the investigated period due to the nucleus spin axis being tilted toward the sun. In this hemisphere, we found a clear variation of spacecraft potential with comet longitude, exactly as seen for the neutral gas, with coincident peaks in neutral density and spacecraft potential magnitude roughly every 6 h, when sunlit parts of the neck region of the bi- lobed nucleus were in view of the spacecraft. The plasma density was estimated to have increased during the investigated time period by a factor of 8-12 in the northern hemisphere and possibly as much as a factor of 20-44 in the southern hemisphere, due to the combined effects of seasonal changes and decreasing heliocentric distance.

The spacecraft potential measurements obtained by LAP generally exhibited good correlation with the estimates from ICA, confirming the accuracy of both of these instruments for measurements of the spacecraft potential. 

Place, publisher, year, edition, pages
Uppsala: Uppsala universitet, 2016. , 80 p.
Keyword [en]
Rosetta, comet, 67P, RPC-LAP, RPC-MIP, Langmuir probe, spacecraft potential, plasma
National Category
Natural Sciences
Research subject
Physics with specialization in Space and Plasma Physics
Identifiers
URN: urn:nbn:se:uu:diva-294395OAI: oai:DiVA.org:uu-294395DiVA: diva2:929762
Presentation
2016-06-02, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:00 (English)
Opponent
Supervisors
Funder
Swedish National Space Board, 109/12, 135/13, 166/14Swedish Research Council, 621-2013-4191
Available from: 2016-05-20 Created: 2016-05-19 Last updated: 2016-05-20Bibliographically approved
List of papers
1. Evolution of the plasma environment of comet 67P from spacecraft potential measurements by the Rosetta Langmuir probe instrument
Open this publication in new window or tab >>Evolution of the plasma environment of comet 67P from spacecraft potential measurements by the Rosetta Langmuir probe instrument
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2015 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 42, no 23Article in journal (Refereed) Published
Abstract [en]

We study the evolution of the plasma environment of comet 67P using measurements of the spacecraft potential from early September 2014 (heliocentric distance 3.5 AU) to late March 2015 (2.1 AU) obtained by the Langmuir probe instrument. The low collision rate keeps the electron temperature high (similar to 5 eV), resulting in a negative spacecraft potential whose magnitude depends on the electron density. This potential is more negative in the northern (summer) hemisphere, particularly over sunlit parts of the neck region on the nucleus, consistent with neutral gas measurements by the Cometary Pressure Sensor of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis. Assuming constant electron temperature, the spacecraft potential traces the electron density. This increases as the comet approaches the Sun, most clearly in the southern hemisphere by a factor possibly as high as 20-44 between September 2014 and January 2015. The northern hemisphere plasma density increase stays around or below a factor of 8-12, consistent with seasonal insolation change.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-277807 (URN)10.1002/2015GL066599 (DOI)000368343900005 ()
External cooperation:
Funder
Swedish National Space Board, 109/12Swedish National Space Board, 135/13Swedish National Space Board, 166/14Swedish Research Council, 621-2013-4191
Available from: 2016-02-23 Created: 2016-02-23 Last updated: 2017-11-30Bibliographically approved
2. Measurements of the electrostatic potential of Rosetta at comet 67P
Open this publication in new window or tab >>Measurements of the electrostatic potential of Rosetta at comet 67P
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2016 (English)In: Proceedings of the 14th Spacecraft Charging Technology Conference, Noordwijk, The Netherlands: ESA Publications Division, European Space Agency , 2016, Abstract 123- p.Conference paper, Published paper (Other academic)
Abstract [en]

We present and compare measurements of the spacecraft potential (Vs/c) of ESA:s Rosetta spacecraft, currently in orbit around comet 67P/Churyumov-Gerasimenko, by the Langmuir probe (RPC-LAP) and Ion Composition Analyzer (RPC-ICA) instruments. Vs/c has mainly been negative, driven so by the high (∼5 eV) temperature of the coma photoelectrons. LAP only picks up a portion of the full Vs/c since the two probes, mounted on booms of 2.2 and 1.6 m length, respectively, are generally in- side the potential field of the spacecraft. Comparison to the minimum energy of collected positive ions by ICA shows that this portion varies between 0.7 and 0.9Vs/c, with generally good correspondence between the two in- struments except when local ion production is weak and accelerated ions dominate the flux. 

Place, publisher, year, edition, pages
Noordwijk, The Netherlands: ESA Publications Division, European Space Agency, 2016
Keyword
Rosetta; comets; ESA; spacecraft potential; Langmuir probe; RPC-LAP; RPC-ICA; electrostatic an- alyzer; plasma; coma; ionization.
National Category
Natural Sciences
Research subject
Physics with specialization in Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-294393 (URN)
Conference
14th Spacecraft Charging Technology Conference, ESA/ESTEC, Noordwijk, NL, 04-08 APRIL 2016
Funder
Swedish National Space Board, 109/12, 135/13, 166/14Swedish Research Council, 621-2013-4191
Available from: 2016-05-19 Created: 2016-05-19 Last updated: 2016-07-27Bibliographically approved

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