Download Saturn`s icy satellites

MCANDREWS: RISHBETH PRIZES
Saturn’s icy satellites
I
n July 2004 the Cassini-Huygens spacecraft
entered Saturn orbit after a seven-year voyage.
This long-awaited event provides the first
opportunity since the brief Voyager flybys in 1980
and 1981 to collect data in situ at Saturn. The
mission will ultimately provide more than four
years of dedicated measurements of the planet
and its system of rings and moons.
One major area of research is the physics of
Saturn’s internally generated magnetic field and
the resultant magnetic cavity – the magnetosphere – around the planet. How this is affected
by the solar wind is also of great importance.
The solar wind is a constant stream of electrons
and ions that propagates supersonically from
the Sun and carries with it the “frozen-in”
heliospheric magnetic field. Planetary bodies or,
as is the case at Saturn, a planetary magnetic
field, provide obstacles to this flow. The magnetopause is the outer boundary of the magnetic
cavity; its nose sits at an average distance of
22 RS from Saturn (where RS = 60 268 km).
Saturn rotates with a period of 10 hours, as
does the planetary field close to Saturn. This
then “tows” the plasma inside the magnetosphere along with it because the field is “frozen
in”. The corotating plasma region dominates
the magnetosphere to a distance of about 10 RS.
Within about 4 RS the trapped high-energy radiation belts are present, as they are at Earth,
where they are called the Van Allen radiation
belts. However, within around 2 RS of the surface, Saturn’s rings are present and they soak up
the radiation-belt particles. Hence the radiation
environment at Saturn is much reduced in this
region. The effect that the orbiting icy satellites
can have on this local magnetospheric environment is the main topic of this brief report.
The Electron Spectrometer (ELS) is one of
three instruments of the Cassini Plasma Spectrometer (CAPS) (Young et al. 2003). CAPS
measures plasma populations in the magnetospheric regions Cassini encounters. It has collected comprehensive electron and ion data with
an unprecedented temporal and energy response
for the first 11 months of the Saturnian tour. The
ELS measures the electron component of the
plasma, with energies from 0.58 eV to 28 keV.
4.22
3.4
10000
log10 counts/2s
electron energy (eV/q)
Hazel J McAndrews – and
colleagues including the CAPS
Team – presents initial results
from the Cassini Plasma
Spectrometer, based on the talk
which won her a Rishbeth Prize
at the UK MIST meeting in April.
1000
100
10
2.3
1.2
0.17
1
03.20
03.30
time (UT)
03.40
1: A time–energy spectrogram for the period when
the electron drop-out due to Enceladus occurs.
The colour scale indicates the number of counts
per two second accumlation period, with red
indicating high intensities and blue the lowest. The
high count rates at low energy (<10eV) consist of
photoelectrons generated on the spacecraft
surface and also the low-energy corotation
plasma. Greater than 10eV the constant counts at
all energies are indicative of the penetrating
radiation. The dip in counts at ~03:27UT is striking.
average
effect
of moon
to Sun
higher energy
particles
move in a
westwards
sense
corotation
Saturn
3.95 RS
Enceladus
Cassini
trajectory
orbital wake of
thermal particles
2: A cartoon of the geometry of the encounter
from the north pole. Saturn is in the centre with
Enceladus orbiting at a distance of 3.95Rs. Here
the corotation direction is anticlockwise while the
high-energy electrons (> MeV) move clockwise.
Both the low-energy corotational wake and the
average wake over an orbit are indicated.
During February and March 2005, the ELS
data showed several significant drop-outs in the
number of radiation-belt electrons. Although
these have energies greater than the dynamic
range of the instrument (MeV and above), they
can penetrate the instrument housing directly and
register counts. The times of these drop-outs were
found to coincide with crossings by Cassini of the
orbital path of the moon Enceladus, which orbits
Saturn at a distance of 3.95 RS. One such drop
out was seen on 17 February at 03:27 UT and the
electron data observed during this crossing can
be seen in figure 1. An energy-versus-time spectrogram is shown with the analyser electron
energy (which does not correspond to the energy
of the penetrating particles) plotted on the verti-
cal axis and time along the horizontal.
The severe drop-out in the numbers of these
high-energy electrons is due to the sweeping
effect that Enceladus has on the radiation-belt
electrons. The moon is inert, so any electron
that hits it can be assumed to be absorbed
(Mead 1972). Therefore the moon leaves a trail
of flux tubes of depleted of particles as it orbits
the planet. This depleted region will continue to
move around the planet with the same velocity
and direction as the surrounding non-absorbed
particles (Van Allen et al. 1980). Of course,
when a discontinuity in the density of the
plasma is created, surrounding particles will diffuse into the region such that equilibrium is
regained over time. By analysis of the depth of
the depletion and knowledge of the time since
the “hole” was generated, a value for the highenergy electron diffusion rates at these distances
in the magnetosphere can be obtained.
Figure 2 shows the geometry, where the corotation direction, shown by the curved arrow, is
anticlockwise. The moon also travels anticlockwise, but more slowly. As the plasma drifts past
the moon it forms a wake ahead of the moon.
Over an orbit, the plasma may not fully recover
from the depletion before the moon passes by
again, giving a longitudinally averaged effect on
the plasma distribution at the moon’s orbit.
In this encounter the trajectory of the spacecraft passes behind the moon, “upstream” of
the moon with respect to corotation of the
plasma. Little to no absorption should be seen
in the ambient plasma, since the “hole” carved
out by the moon should have had sufficient time
(more than a corotation period) to fill in.
However, for the penetrating particles a significant drop-out is observed, as in figure 1. This
can be explained by considering the motion of
electrons at these energies (MeV). They have a
drift velocity greater than and in an opposite
sense to the corotation velocity. Therefore their
net motion is westwards around the planet,
clockwise in figure 2. Consequently Cassini lies
within the wake of these particles just downstream from the moon, so it can observe almost
immediately this newly created “hole”.
The physics behind the process of the absorption and consequent filling in of the moon’s
plasma wake is an interesting one and will be
investigated with further work. It will allow
increased understanding of plasma motions in the
magnetosphere, and, in particular, calculation of
diffusion rates which is of great interest and has
not been explored since the times of Voyager. ●
Hazel McAndrews, Mullard Space Science Laboratory, University College London, UK.
A&G • August 2005 • Vol. 46