O2 AND O2+ DENSITY FROM THE RINGS THROUGH INNER
MAGNETOSPHERE
M.K. Elrod1, R.E. Johnson 1, T. A. Cassidy1, R. J. Wilson2, R. L. Tokar2, W. L. Tseng3, W.H. Ip3
1
University of Virginia, Charlottesville, VA 22904
2
Space Science and Applications, Los Alamos National Laboratory, MS D466,
Los Alamos, NM, 87545
3
Institute of Astronomy, National Central University, Chung Li 320, Taiwan
The main rings and the ice grains in the tenuous F and G rings are a source of O2+ ions for the inner
magnetosphere (Tokar et. al. 2005). These ions are formed from neutral O2 produced by the
decomposition of ice by incident radiation (Johnson et. al. 2006). Since the principal source of O2+ ions is
from the ionization of the neutral O2 molecules through photo-ionization and electron interactions, O2+
becomes a marker for the radiation-induced decomposition of ice and the presence of O2 neutrals.
Recently, Martens et al (2008) described O2+ beyond the orbit of Enceladus, noting the possibility that
Rhea is a source. Here we focus on O2+ inside the orbit of Enceladus. Through simulations of the neutral
cloud created by photo-induced decomposition of the ice in the main rings and the tenuous F and G rings
(Johnson et. al. 2006, Tseng et. al. 2008), it is possible to calculate the column density of the neutrals and
the O2+ source rate in the inner magnetosphere. Using the Cassini Plasma Spectrometer (CAPS) data, we
describe the density of the O2+ ions from the rings out to the orbit of Enceladus. The largest source of O2
neutrals is expected to be the main rings. However, here we examine whether or not the energetic ion
irradiation of grains in the F and G rings are significant sources of O2 and if ion-neutral reactions in the
Enceladus plume are a possible source.
INTRODUCTION:
The main rings of Saturn are primarily
composed of icy grains ranging in size from
microns to meters. When UV sunlight
interacts with these ice grains, it causes the
water in the ice to photo-disassociate. The
disassociated species can react to produce
H2 and O2 molecules which diffuse out of the
ice. H2 molecules, being much lighter in
mass, tend to escape from the system much
easier than the heavier O2 molecules. As a
result the O2 molecules collect over the rings
forming a tenuous O2 atmosphere that
extends into the inner magnetosphere. The
plasma from the inner magnetosphere of
Saturn and UV photons interacts with this
neutral cloud of O2 molecules, creating O2+
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ions. Photo-chemistry of oxygen has shown
the following reactions for molecular
oxygen:
O2 + hν -> O2+ + e
(1)
O2 + hν -> O + O+ + e
(2)
O2 + hν -> O + O
(3)
And for atomic oxygen:
O + hν -> O+ + e
(4)
O + O+ -> O+ O (charge exchange) (5)
Thus the most effective method for the
formation of a molecular oxygen ion over
the main rings is through ionization of
neutral molecular oxygen. When the
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Cassini Space craft entered Saturn’s orbit in
2004 in order to brake properly and enter a
stable orbit for the mission, it passed very
close to Saturn directly over the rings. This
was the only pass over the rings that has
occurred thus far in the mission. The
spacecraft passed over the B-ring and
crossed the ring plane just outside the F ring
between the F and G rings. It was during
this pass that the Cassini Plasma
Spectrometer (CAPS) instrument detected
ionized molecular oxygen (Tokar et. al.
2005, Johnson et. al., 2006). During
subsequent orbits of Saturn the CAPS
instrument detected O2+ ions beyond the
rings but in significantly lower
concentrations (Sittler et al, 2006). The fact
that there is no other point where O2+ ions
have such a high concentration indicates that
the main rings are the primary source for O2+
ions throughout the inner magnetosphere.
Our model of the neutral cloud from the
rings show that the highest column density
of molecular oxygen is over the A and B
ring and trails off outward as seen in figure
1. The broad distribution of neutrals outside
the rings is due to low energy, ion-neutral
collision and charge exchange. This model
also indicates that the inclination of the sun
with the respect to the rings will affect the
column density on the north side versus the
south side of the rings (or lit and unlit sides)
(Tseng et.al. 2009).
The model used to obtain the result in figure
1 is a particle tracking simulation describing
the neutral cloud where loss occurs through
ionization processes and ion-neutral
interactions. The model results shown in
figure 1 demonstrate both the trailing edge
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over the ring, and the impact of the angle of
inclination of the sun on the rings.
Figure1. The neutral O2 column density
(molecules/cm2) in four situations: red: 24° north
of the ring plane; green: 24° north; blue: 14°
north; pink: 4° north. (Tseng et.al. 2009)
Having modeled the CAPS data over the
rings, the subsequent goals of this study is to
analyze the CAPS data of O2+ ions from the
rings to just inside the orbit of Enceladus at
around 4 Rs where Rs is the Saturn Radius
(Rs = 60330 km).
CAPS DATA ANALYSIS
Early morning of July 1, 2004 the Cassini
craft entered orbit around Saturn. On this
initial pass over the rings, Cassini passed
approximately 1.79 Rs over the main part of
the B ring. On this initial pass starting at
around 2.2 Rs until the ring crossing
between the F and G ring at around 2.6 Rs,
the CAPS instrument detected an increase in
the ion density. Figure 2 shows a diagram
of the trajectory of the Cassini spacecraft’s
trajectory as it entered the Saturn system.
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singles mode for the instrument is designed
to simply count the number of strikes made
in a single sweep. Depending on the
telemetry mode there can be 1-16 sweeps/
Acycle. Each Acycle lasts 4 seconds long,
so in the highest telemetry mode, like that
used during the 2004 entry pass, the highest
amount of data will be collected. In all other
telemetry modes, the counts will be summed
up.
Figure 2. Schematic of the Cassini trajectory near the
Rings. Red regions indicate when CAPS rotated into
the direction of plasma flow and ion densities were
enhanced.
As this trajectory shows, the spacecraft
began at an altitude of approximately 0.25Rs
north of the ring to about 0.15 Rs when the
spacecraft rotated in the first area of interest,
and then crossed the ring plane in the second
area of interest.
To determine the column density of the O2+
ions, it is necessary to determine the ion
temperature, velocity relative to the space
craft, and density at its position. It is also
necessary to separate the different ion types
entering the detector into the different
species to correctly determine the
temperature density and velocities. Since
O2+ is twice the mass of the water group
(also a dominate ion in the inner
magnetosphere) and the O+ ion, it is much
easier to determine the peak from the O+
peak by mass. Using Maxwell distribution
to fit to the curve, it is possible to determine
the moments of the phase space i.e., the
temperature, density and velocity of the
individual ions. The singles detector, used
for this study, is part of the CAPS
instrument on Cassini. It has 63 different
energy ‘bins’ or settings. As the ion enters
the instrument, the charge/mass ratio will
cause the trajectory of the ion to change and
it will strike one of the different bins. The
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Since the ions have a temperature, the
counts need to be converted to phase space
and then fitted to a Maxwellian to determine
Ti. The counts are plotted vs energy. This
flux F(E) is calculated from the Maxwellian
fit to the counts vs energy bin:
(6)
Here eff(E) is the efficiency of the
instrument as a function of ion energy E,
G(E) is the geometrical factor of the
instrument, v is the velocity of the particle
. The key piece of this equation
comes from the f(E) which is the
Maxwellian fitting curve:
(7)
Here n is the number density, m is the ion
mass, T is the ion temperature in eV, and u
is the velocity of the ion relative to the
spacecraft.
This simple analysis gives a one
dimensional approximation to the ion phase
space distribution. When the spacecraft
passed over the rings, the CAPS instrument
was not actuating, meaning that the detector
was pointing in one direction during the
entire pass. To get a complete three
dimensional analysis, the detector needs to
be actuating or scanning across the field
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both in and out of the line of view of the
plasma, the peak of ion density.
CAPS DATA
Figure 3 shows a sample of the flux and
fitting process for when the detector was
over the B ring and pointed in the mean
plasma flow direction. The higher peak in
this graph is the O2+ ions while the lower
peak is the O+. The heavier ions will have
the higher energy while the lighter mass ions
will be at the lower energy.
Figure 4. Density versus Rs of O2+ and O+ ions. Red
line is O2+ and blue is O+. Each anode that has a
measurable peak, is individually analyzed, at each
point. Then the anodes are averaged together to
create one density per radial point. This curve was
smoothed using a three point averaging to remove
sharp jump in the density.
Figure 3. Time 03:46:17 The blue line is the actual
data, the red line is the O2+ flux fit and the green line
is the O+ flux fit line with the black line the sum of
the two. This is a snap shot measurement made near
1.92 Rs over the main rings specifically the B-ring.
There are eight anodes on the detector. To
accumulate the single densities from all
anodes per Acycle, the densities are summed
up to the Acycle resolution, then the anodes
are averaged together for each point of
measurement. Figure 4 is a graph of the
densities at the space craft location. Figure
5 is the ion temperatures at the spacecraft
location. Near the rings the plasma is
moving close to the rotational velocity of
Saturn’s Magnetosphere, ~ 13 – 16 km/s
depending on position over the rings. These
higher velocities of ions, as compared to
further out in the magnetosphere make for
the steep narrow curves seen in figure 3.
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Figure 5. Ion Temperature eV vs Rs.Red line is O2+
and blue is O+. Similar to the density the anodes with
measurable peaks are averaged together at each point
to get the temperature of the O2+ and O+. O2+ has
lower temperature due to the fact that more energy is
released in the O2+ reaction. This curve was also
smoothed using a three point averaging.
In order to determine the column density of
the ions, we calculate the scale height H, a
function of the temperature and ion mass.
The projected density at the magnetic
equator no, depends on the altitude and the
scale height.
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(8)
(9)
indicates that there is also a slight increase
in O2+ ions near Rhea indicating that Rhea
might be a second source, though much less
so than the rings (Martens et. al. 2007).
(10)
Here n = local density, z = altitude above the
magnetic equator, q = ion charge, and mi =
mass of the ion. Figure 6 shows the
projected equatorial density for the region
near the rings. Figure 6 shows the column
density over the rings.
Figure8. Column density of O2+ from the rings to
Rhea. Diamonds indicate the density over the rings
and squares out to Rhea (Tokar et al 2005, Martens
et.al., 2007)
DISCUSSION:
Figure 6. Projected density of O2+ to the magnetic
equator.
The relatively high column density of O2+
over the rings indicates that the rings are by
far the largest source of O2 and, thus, O2+
ions in the inner magnetosphere of Saturn.
While the Cassini spacecraft has passed
several times near the plume of Enceladus,
the source of the E-ring and the source of
water ice grains, there has been little to no
evidence for a strong O2+ source found in the
plume or near Enceladus as yet. At present
we are analyzing the CAPS data between
Enceladus and the main rings. In this region
very energetic ions interact with the tenuous
F, G and E ring and might be and additional
source of O2 and consequently O2+.
REFERENCES
Figure 7. Column density of the region over the ring
of O2+.
Figure 8 compares the column density of O2+
from the ring out to 10Rs. This figure
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