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INSTITUTET FÖR RYMDFYSIK	UPPSALA

Swedish Institute of Space Physics	(59°50.272′N, 17°38.786′E)

Student project at IRF Uppsala

Project work / Examensarbete (30 hp)

Modelling of Rosetta Langmuir probe measurements

Student: Alexander Sjögren, Uppsala University
Supervisor: Anders Eriksson
Period: Spring 2009

[Rosetta ion wake]

Simulation of the plasma density around the Rosetta spacecraft in the solar wind. The wake forming behind the spacecraft body and behind the large (32 m wing span) solar panels is clearly visible.

Background

ESA's Rosetta mission is currently en route to comet Churyomov-Gerasimenko, where it will arrive in 2014. Among the instruments onboard, the Langmuir probe instrument LAP, built and operated by the Swedish Institute of Space Physics (IRF) in Uppsala, can be expected to be particularly sensitive to early activation of the comet, as it can detect small plasma density changes even at the very low densities typical of the solar wind plasma. However, interpreting the data in terms of plasma properties (density and temperature) requires an understanding of the spacecraft-plasma interaction and its impact on the LAP measurements.

Project

LAP data obtained in the solar wind (relevant conditions for the early comet phase) shows that the s/c attitude (pointing) influences the LAP measurement. The project modelled this influence by use of the SPIS code package for s/c-plasma interaction studies. The project included:

Gathering background information on Rosetta and the SPIS code
Setting up and running SPIS
Visiting SPIS experts at ESTEC in the Netherlands
Collaborating with a related project on Cassini
Creating a model of Rosetta and LAP in SPIS
Running simulations and anlyzing the results
Report writing and seminar presentation
Supporting two other project students starting up SPIS

Results

Project report (final version 091008): PDF

Abstract

The Rosetta spacecraft, which is on its way to the comet 67P/Churyumov-Gerasimenko, has two Langmuir probes onboard to measure plasma properties on its journey and in the region around the comet. The measurements depend on the potential around Rosetta, which is mostly disturbed by the spacecraft potential, the wake created behind the spacecraft, and photoelectrons emitted from the surface of the spacecraft. In order to make a correct analysis of the measurements made with the two probes, it is needed to understand what parts of the potential measured is due to the various effects presented above, and what part is the actual potential in space. To better understand this, simulations have been made with the software SPIS (Spacecraft Plasma Interaction System) for the cases of Rosetta in vacuum, in the flowing solar wind without photoelectrons emitted from the spacecraft, and in the solar wind with photoelectrons. The plasma pa- rameters and solar distance as well as spacecraft potential have been varied to understand the scaling of the effects. Two simple models of Rosetta have been used and compared, except for the case when photoelectrons are introduced where only one model could be used. The simulations show that of the various cases studied, the photoelectrons have the biggest effect on the potential measured. It is shown that the potential measured is lowered by about 10% when the probes are in the photoelectron cloud in front of the spacecraft with respect to the Sun. The wake created behind the spacecraft will lower the potential measured on the order of a couple percents. It is also shown that the potential variations due to the asymmetric shape of Rosetta is small compared to the effects of the photoelectrons and the wake.

https://space.irfu.se/exjobb/2009_alex_sjogren/index.html
last modified on Thursday, 08-Oct-2009 21:05:41 CEST