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INSTITUTET FÖR RYMDFYSIK UPPSALA
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Swedish Institute of Space Physics (59o50.272'N, 17o38.786'E)
Student project at IRF Uppsala

Project work / Examensarbete (20 p)

Dust-plasma interactions in the Saturnian E-ring

Student: Mikael Lundberg
Supervisors: Jan-Erik Wahlund, Ronan Modolo
Period: 2006-2007

Background

Preliminary measurements with the RPWS Langmuir probe instrument onboard Cassini near the equatorial plane of Saturn within 8 RS suggested that negatively charged dustconstitute a sizeable part of the charge balance of the plasma there. The plasma is the cold and dense plasma disk (torus) that also surrounds some of the inner icy moons to Saturn (e.g., Enceladus, Dione and Rhea). The interaction between the plasma and the charged dust (water ice) grains seemed to result in two ion populations that rotates around Saturn with different speeds, one hotter population that follows the magnetic field and one colder that is stuck near the negatively charged grains. This was the subject of investigation in this project.

Results

  • Project report (PDF, 25 MB)
  • J.-E. Wahlund, M. André, A. I. Eriksson, M. Lundberg, M.W. Morooka, M. Shafiq, T.F. Averkamp, D.A. Gurnett, G.B. Hospodarsky, W.S. Kurth, K.S. Jacobsen, A. Pedersen, W. Farrell, S. Ratynskaia, N. Piskunov. Detection of dusty plasma near the E-ring of Saturn. Planetary and Space Science, doi:10.1016/j.pss.2009.03.011

Report abstract

This report gives further substance to the claim for the existence of an interaction between the negatively charged dust grains in the E-ring and positively charged ions of mainly hydrogen and water in the surrounding plasma. It also estimates how large a part of the total ion population is being coupled with the dust and in effect is reduced in azimuthal speed around the planet.

Furthermore, a look at certain orbits of interest -- among them including flybys of icy moons in the region -- revealed significant increases in dust-plasma interaction during the flyby of a moon. The same effect could also be seen in the trail left behind along the orbit of the icy moons, suggesting narrow regions of high dust content formed by particles from passing moons.

This study was conducted by analysing data given by the Langmuir probe on board Cassini, and focusing on the ion side part of the current from the voltage sweeps. From the total ion current, Ii , a model for photoelectrons as a function of spacecraft attitude was obtained, in order to deduce an accurate estimation of the ion speed, vi . Using this, the fraction of the total number of ions which had been coupled to dust, ndust , was calculated within the E-ring.

In correlation with vi , ndust was plotted, showing a clear increase at distances ≤ 5 RS , with additional increases during flybys of icy moons, as well as in the trail of the moons around their orbital distances. This effect was clearly seen near Enceladus, a moon well known to spew out massive amounts of ice particles into the E-ring, but, most surprisingly, could also be found near Tethys and Dione!

The results of this thesis confirm that two ion populations -- one hotter and one colder -- are consistent with the Langmuir probe data in the inner magnetosphere of Saturn. The results also show that dust-plasma interaction is indeed taking place in the E-ring. A ratio, ndust , of 20 % or more is commonly found in the inner magnetosphere, and around the orbits of the icy moons it can be as high as 50 % to 100 %!

It is very likely that several of the icy moons are feeding the E-ring with icy dust particles, which then become charged and interact with the surrounding plasma. While Enceladus most certainly is a major source, it is probably not the only source of particles for the E-ring. From the results presented here it
is shown that the contribution from the moons leads to an increase in dust-plasma coupling, which is unrelated to the coupling already present in the inner part of the ring and magnetosphere.

[Cassini and the rings]
Artist's impression of Cassini and the rings. (c) NASA.

http://space.irfu.se/exjobb/2006_mikael_lundberg/index.html
last modified on Wednesday, 30-Sep-2009 15:20:00 CEST