Student project at IRF Uppsala
| INSTITUTET FÖR RYMDFYSIK
| Swedish Institute of Space Physics
|| (59°50.272′N, 17°38.786′E)
Project work / Examensarbete (20 p)
Dust-plasma interactions in the Saturnian E-ring
Student: Mikael Lundberg
Supervisors: Jan-Erik Wahlund, Ronan Modolo
Preliminary measurements with the RPWS Langmuir probe instrument
onboard Cassini near the equatorial plane of Saturn within 8 RS
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.
- 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
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.
Artist's impression of Cassini and the rings. (c) NASA.