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

Project work (15 c)/Projektarbete (15 hp)

Degree project (30 c)/Examensarbete (30 hp)

Plasma structures in the Enceladus plume

Period: Autumn 2012

Background

Cassini is a NASA spacecraft in orbit around Saturn since 2004. Among the scientific instruments onboard is a Langmuir probe (LP) provided by the Swedish Institute of Space Physics in Uppsala, for studies of the plasma environment of Saturn, its moons, and its rings. One of the interesting Cassini findings is that the moon Enceladus spews out a plume, mainly consisting of water, from its south pole regions into space. In space, gaseous plume material can get ionized by solar EUV radiation and high energy particles in the Saturn magnetosphere, while grains can get charged by picking up free charges. Therefore, the plume has a clearly detectable plasma signature.

Project

Investigate the plasma density data from the Enceladus flybys, looking for fine structure in the plasma associated with the plume. This will mean combining and comparing densities obtained from the upper hybrid line in spectra from the RPWS wave measurements with LP data obtained at high time resolution (20 samples/s) and LP bias sweeps. Observed fine structure should be compared to known topography of the Enceladus plume.

The project is actually two: first a 15 credit student project, which was then continued as a 30 credid master project.

Abstract

Cassini-RPWS high resolution (20 Hz) Langmuir probe data was analyzed to find the source of fast variations in the electron density especially in the Enceladus plume region. The spatial scale on the variations is between 1 and 10 km in size. The approaches were to check for correlations between the plasma density and its variations on one hand, and boundary conditions such as the cracks on Enceladus surface as well as dust and single jets on the other hand. None of these mechanisms could be identified as the only or dominating source of observed fine structure, though partial correlation can sometimes be found and the comparison to dust presence is qualitative more than quantitative. Along the way the charging mechanism in the plume was found to be most likely due to solar UV ionization since the maximum electron density was found to be around 200km altitude. Also the deformation of the plume in the corotation direction is visible in the 20 Hz data.

Results

• Presentation at the European Planetary Science Contress, 8-13 Sep, London, UK
• Initial project report
• Final project report

Artist's impression of Cassini at Enceladus, showing the plume in the background [Image credit: NASA/JPL]