ARTICLE IN PRESS
Planetary and Space Science ] (]]]]) ]]]–]]]
Contents lists available at ScienceDirect
Planetary and Space Science
journal homepage: www.elsevier.com/locate/pss
The distribution of atomic hydrogen and oxygen in the magnetosphere
of Saturn
Henrik Melin , Don E. Shemansky, Xianming Liu
Planetary and Space Science Division, Space Environment Technologies, 320 N. Halstead Street, Suite 110, Pasadena, CA 91107, USA
a r t i c l e in fo
abstract
Article history:
Received 27 November 2008
Received in revised form
17 April 2009
Accepted 21 April 2009
The intensity of H Lya 1216 Å (2P–1S) and OI 1304 Å (2p3 3s3 S22p4 3 P) is mapped in the magnetosphere
of Saturn using the ultraviolet imaging spectrograph (UVIS) [Esposito, L.W., Barth, C.A., Colwell, J.E.,
Lawrence, G.M., McClintock, W.E., Stewart, A.I.F., Keller, H.U., Korth, A., Lauche, H., Festou, M.C., Lane,
A.L., Hansen, C.J., Maki, J.N., West, R.A., Jahn, H., Reulke, R., Warlich, K., Shemansky, D.E., Yung, Y.L., 2004.
The Cassini ultraviolet imaging spectrograph investigation. Space Science Reviews 115, 299–361]
onboard Cassini. Spatial coverage is built up by stepping the slit sequentially across the system (system
scan). Data are obtained at a large range of space-craft–Saturn distances.
The observed atomic hydrogen distribution is very broad, extending beyond 40RS in the equatorial
plane, with the intensity increasing with decreasing distances to Saturn. The distribution displays
persistent local-time asymmetries, and is seen connecting continuously to the upper atmosphere of the
planet at sub-solar latitudes located well outside of the equatorial (ring) plane. This is consistent with
the source of the atomic hydrogen being located at the top of the atmosphere on the sun-lit side of the
planet on the southern hemisphere. In addition there are a number of temporally persistent features in
the intensity distribution, indicating a complex hydrogen energy distribution.
The emission from OI 1304 Å is generally distributed as a broad torus centered around 4RS
although the position of the peak intensity can vary by as much as 1RS . There is significant intensity
present out to 10RS . HST observations of hydroxyl (OH) are re-analyzed and display a distribution half
as broad as that of oxygen, also centered at 4RS .
The observed atomic oxygen distribution requires a sourcing of 1:3 1028 atoms s1 against loss due
to charge capture with the plasma. Using the ion partitioning of Schippers et al. [2008. Multi-instrument
analysis of electron populations in Saturn’s magnetosphere. Journal of Geophysical Research (Space
Physics) 113 (A12) 7208–+] then recombination of H2 Oþ and H3 Oþ will account for about a quarter of
the mass-loss in the inner magnetosphere, with charge capture of Oþ accounting for the rest. The
oxygen loss rate is seen to vary by 2 1027 atoms s1 over periods of weeks.
& 2009 Elsevier Ltd. All rights reserved.
Keywords:
Saturn
Magnetosphere
Cassini
UVIS
1. Introduction
Atomic hydrogen was discovered in the inner magnetosphere
of Saturn by Weiser et al. (1977), using a rocket-mounted UV
spectrograph, reporting a H Lya intensity of 200 Rayleigh. A few
years later, during the Saturn encounters of Voyager 1 and
Voyager 2, in 1980 and 1981, respectively, an extensive atomic
hydrogen distribution was observed (Shemansky and Hall, 1992)
extending from the center of the system to beyond 20RS , with
significant dusk intensity enhancements and rarefactions in the
pre-dawn region.
The arrival of Cassini at Saturn in 2004 provided, and continues
to provide, an unrivaled opportunity to study the temporal and
Corresponding author. Tel.: +1 323 319 6433.
E-mail address: hmelin@spacenvironment.net (H. Melin).
spatial distributions of the neutral species present in the
magnetosphere. As a testament to this, atomic oxygen was
discovered in the magnetosphere of Saturn by UVIS (Esposito
et al., 2005) as Cassini approached Saturn (pre-Saturn orbit
insertion, or pre-SOI), showing a variable neutral oxygen cloud
with a peak total oxygen population of 4 1034 oxygen atoms.
It has been speculated that the water/ice found in the rings and
on the icy satellites are the source of both the oxygen and
hydrogen found in the magnetosphere, via sputtering mechanisms driven by both photons and energetic particles (Cheng and
Lanzerotti, 1978; Ip, 1978; Carlson, 1980; Shemansky and Hall,
1992). Shemansky and Hall (1992) proposed that the source of the
hydrogen is Saturn itself, where hot hydrogen escapes the sun-lit
side of the planet, by collisional electron-dissociation of H2 .
The discovery of oxygen by Esposito et al. (2005), together with
the presence of hydroxyl (Shemansky et al., 1993), strongly
indicated the presence of water see Jurac et al. (2002). This was
0032-0633/$ - see front matter & 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.pss.2009.04.014
Please cite this article as: Melin, H., et al., The distribution of atomic hydrogen and oxygen in the magnetosphere of Saturn. Planet.
Space Sci. (2009), doi:10.1016/j.pss.2009.04.014
ARTICLE IN PRESS
2
H. Melin et al. / Planetary and Space Science ] (]]]]) ]]]–]]]
Table 1
The Cassini UVIS system scan observations from 2003 to 2005.
Group
Start time
End time
Resolution (R2S )
IðsÞ
N exp
N scans
1
2
3
4
5
6
7
2003-359 11:36
2004-037 13:23
2004-070 20:14
2004-096 06:14
2004-122 13:03
2004-315 16:37
2005-074 06:23
2004-008 01:21
2004-057 02:50
2004-094 02:10
2004-116 15:55
2004-142 15:15
2004-319 16:55
2005-086 14:43
3.14
1.78
1.04
0.62
0.27
0.003
0.002
900
452
509
396
194
33
44
986
1278
1209
1032
1775
12838
12026
124
86
88
60
54
11
45
Resolution is the average resolution of a UVIS pixel as the spacecraft distance changes. IðsÞ is the total integration time put into the central most 1R2S . N exp is the number of
exposures and N scans is the number of scans in a particular group.
confirmed when Hansen et al. (2006) reported observing water
vapor in the plume emerging from the southern pole of Enceladus.
This spectacular source injects water into the magnetosphere of
Saturn (Tokar et al., 2006), where it is subsequently dissociated,
producing the observed water products. The magnetospheric
oxygen is lost mainly via charge exchange (Johnson, 2004), where
the ion is accelerated up to co-rotation velocities and subsequent
recombination with electrons would lead to the atom escaping
the system. The charge-exchange process can also re-distribute
the water dissociation products within the magnetosphere
(Johnson et al., 2006a), such that the relatively narrow torus of
water produced by Enceladus produces a much broader distribution of oxygen and hydroxyl (Jurac and Richardson, 2005; Johnson
et al., 2006a). Water group ions were found to be the dominant
ionic species in the inner magnetosphere (Young et al., 2005;
Sittler et al., 2005, 2006, 2008). Martens et al. (2008) observed Oþ
2
and water group ions (Wþ ), consistent with dominant sourcing
inward of 4:5RS , in addition to a tentative source at the orbit of
Rhea.
This article presents a study of the temporal and spatial
variabilities of atomic hydrogen and atomic oxygen in the
magnetosphere of Saturn using Cassini UVIS system scan
observations. The probable sources are discussed.
2. The UVIS system scans
Between the day of year (DOY) 259 2003 and DOY-139 2005,
the UVIS instrument performed a long-term campaign to monitor
ultraviolet emissions inside the magnetosphere of Saturn. The
emission from hydrogen and oxygen were mapped both in the
outer magnetosphere (40RS from the center of Saturn) and
the inner magnetosphere (10RS ) by performing system scans—a
series of sequential UVIS exposures, each spatially separated by
1.5 mrad or more (the width of the slit).
During the period of the campaign, UVIS completed 663
system scans, contained within 50230 individual spectra. This
paper will present the analysis of a large subset of these
observations, containing consecutive days of observations. The
preliminary analysis of a smaller subset was presented by
Esposito et al. (2005). The groups of observations are listed in
Table 1. The Cassini–Saturn distance varies between 1500 and
24RS . The very distant scans provide a large scale view of the
system out to 40RS , with the ones close in providing a detailed
view of the inner magnetosphere. For all system scans, UVIS was
operated using the far-ultraviolet (FUV) channel. The slit has 64
spatial elements and the field of view is 1.5 mrad, using the lowresolution slit setting (Esposito et al., 2004).
The observations listed in Table 1 can be categorized into preSOI scans and post-SOI scans, as Cassini went into orbit at Saturn
on the 1st of July 2004. These are described below.
Fig. 1. A UVIS FUV spectrum of the inner magnetosphere of Saturn, located over
the rings.
2.1. Pre-SOI system scans
The first five groups of observations in Table 1 were obtained
as Cassini approached Saturn. The instrument returned the data
with a spectral compression of 2, producing a spectrum between
1115 and 1912 Å with 512 spectral resolution elements (pixels).
This is the full spectral range of the FUV channel.
2.2. Post-SOI system scans
For groups 6 and 7 a spectral windowing between 1180 and
1380 Å was used in conjunction with a spectral compression of 4,
producing 64 spectral resolution elements. These windowed
spectra exclude the solar reflection seen beyond 1600 Å in Fig. 1.
2.3. The FUV spectrum
Fig. 1 shows a binned UVIS FUV pre-SOI spectrum of the inner
magnetosphere of Saturn. Readily identified is the H Lya line
produced in solar fluorescence at 1216 Å. This line is present in all
spectra in every pointing direction. In addition to hydrogen, the
solar continuum beyond 1600 Å can be seen reflected from both
the sun-lit side of the planet and the rings. The icy moons of
Saturn also reflect sunlight (e.g. Hendrix and Hansen, 2005), but
the fractional area that a moon occupies in a UVIS pixel is too
small for the reflection component to be detected given these
relatively large distances from the system. The third identifiable
species in the FUV spectrum, given a long enough integration
time, is the 1304 Å atomic oxygen triplet. It is very weak and a
total exposure per mosaic element of 105 seconds is required to
get a signal to noise ratio of 5. The broad wings of the H Lya
Please cite this article as: Melin, H., et al., The distribution of atomic hydrogen and oxygen in the magnetosphere of Saturn. Planet.
Space Sci. (2009), doi:10.1016/j.pss.2009.04.014
ARTICLE IN PRESS
H. Melin et al. / Planetary and Space Science ] (]]]]) ]]]–]]]
point-spread function contaminate the oxygen feature. The background is removed by subtracting the averaged signal on either
side of the 1304 Å OI feature. The observed atomic oxygen must
predominantly be excited by solar photons as the electron-excited
transition at 1356 Å (2p3 3s 5 S22p4 3 P) is never observed.
2.4. System scan reduction process
Each group in Table 1 is comprises a number of system scans,
which in turn each are a series of individual spectra. In the
procedure that reduces these spectra to a mosaic, each spectrum
is mapped onto a pre-defined virtual spatial grid located at the
object (Saturn, in this case). The counts in a spectrum is fractioned
according to spatial overlap with the mosaic elements and
exposure time. The procedure takes the distance from the planet
into account and the resulting spectra for each mosaic element
has units of counts per mosaic element per column per second,
which can be converted directly into brightness.
The system scan procedure produces as 2D image for each of
the spectral pixels along the line of sight of the observations. A
more complete 3D view of the morphology can be built up by
combining system scans obtained at right angles from each other.
As of yet, only limited viewing geometries for Saturn exist within
the current set of Cassini UVIS system scans.
Due to variations in spatial coverage within each group of
observations listed in Table 1, the signal to noise ratio (S/N) can
vary across a particular mosaic. This is particularly evident for the
OI 1304 Å feature, which has a very weak signal. In contrast, the H
Lya feature is very bright, giving much higher S/N.
3
2.7. Mosaic plots
The figures showing the mosaics presented in this paper have
units of Rayleigh. The scale to the right specifies the range of
values that are displayed—any value greater than the maximum
on the scale is displayed as white, and any value smaller than the
smallest value is shown as black.
The plots all have the same basic layout; the x coordinate is in
the horizontal east–west direction, z is the in the vertical
north–south direction and y is in the direction of line-of-sight.
The sun is toward positive x and the shadow side is at negative x.
Plotted at the center of each mosaic is a circle representing
Saturn together with the A-ring (solid line closest to the planet),
G-ring (solid), the orbit of Enceladus (dashed) and the shadow of
Saturn (the tube coming off Saturn to the upper left). Mosaic
elements for which there is no coverage are filled with a ‘’.
The brightest H Lya emission emanates from the sun-lit
hemisphere and auroral regions, but this emission is not discussed
in this paper.
The mosaic plots show the line-of-sight integrated intensity of
hydrogen and oxygen, and are a 2-D representation of gas that is
distributed in three dimensions. Both the local hydrogen Lya and
OI 1304 Å are stimulated by sunlight, and thus will not emit, in the
absence of energetic electrons, when they are in shadow of the
sun, i.e. behind the planet or in the shadow of the rings.
3. Results
3.1. Atomic hydrogen in the magnetosphere of Saturn
2.5. Calibration
Each individual spectrum was flat-fielded before being processed by the system scan routine. The flat-field was derived from
observations of the local interstellar medium (LISM), correcting
for non-uniformity of response in the detector.
Converting counts per mosaic element per second to photometric units of Rayleighs is done by applying a flux calibration
curve, based on laboratory measurements of standard H2 and
N2 spectra, and modified with time using observations of Spica
(a Virgo).
The general morphology of the H Lya distribution can be described by looking at the mosaics for groups 2, 5– 7 (see Table 1).
These can be seen in Figs. 2–5.
During group 2, seen in Fig. 2, Cassini was 1100RS from
Saturn, providing spatial coverage of 35RS . The hydrogen
distribution is clearly very broad in x (east–west) direction, and
is still decreasing at the horizontal edge of the system scan. There
is also significant asymmetry in x, with the sun-facing side being
much more compacted than at negative x, such that the contour of
2.6. Geometry of observations
Table 2 details the geometric parameters for each group of
observations, with the range to Saturn listed in Table 1.
Groups 1–6 have similar sub-Cassini latitudes, about 15 .
For Group 7 the system is observed ring-edge-on, providing a
unique view of the system, unobstructed by the rings. Group 6 has
the largest phase angle, the difference between sub space-craft
longitude and sub-solar longitude, of 114 .
Table 2
The geometric parameters of the system scan observations listed in Table 1.
Group
Ring opening angle
1
2
3
4
5
6
7
16:3
16:3
16:3
16:3
16:3
13:4
0:00
a
Sub-solar latitude
25:6
25:4
25:2
25:1
24:9
23:5
22:3
Scan performed during outbound orbit.
Phase angle
Range (RS )
1500–1397
1172–1023
918–739
722–518
518–360
67–74a
39–24
62:5
64:1
65:3
66:1
66:9
114:1
77:3
Fig. 2. Lya map of Group 2 (2004-037), as defined in Tables 1 and 2, rendered at a
resolution of 0.78 0.78 RS . The sub-spacecraft latitude was 16:3 and the subsolar latitude was 25:6 .
Please cite this article as: Melin, H., et al., The distribution of atomic hydrogen and oxygen in the magnetosphere of Saturn. Planet.
Space Sci. (2009), doi:10.1016/j.pss.2009.04.014
ARTICLE IN PRESS
4
H. Melin et al. / Planetary and Space Science ] (]]]]) ]]]–]]]
Fig. 3. Lya map of Group 5 (2004-122), as defined in Tables 1 and 2, rendered at a
resolution of 0:23 0:23RS . The sub-spacecraft latitude was 16:3 and the subsolar latitude was 24:9 .
Fig. 5. Lya map of Group 7 (2005-074), as defined in Tables 1 and 2, rendered at a
resolution of 0:1 0:1RS . The sub-spacecraft latitude was 0 and the sub-solar
latitude was 22:3 .
Fig. 6. Average intensity of Lya as a function of x distance from Saturn for the two
most central mosaic in Fig. 3. The dotted line is the orbit of Enceladus and the
dashed line is the body of Saturn.
Fig. 4. Lya map of Group 6 (2004-315), as defined in Tables 1 and 2, rendered at a
resolution of 0:1 0:1RS . The sub-spacecraft latitude was 13:4 and the sub-solar
latitude was 23:5 .
250 Rayleigh lies 7RS closer to the planet at positive x. The
intensity increases toward the center of the planet (see Fig. 6).
Fig. 3 shows the H Lya distribution for group 5. The rings block
some of the H Lya emission that emanates from beyond them,
creating an apparent reduction in intensity toward the center of
the planet. Fig. 7 shows the x-profile at z ¼ 0 (east–west) of the
hydrogen distribution, showing a clear axial asymmetry with
positive x being brighter at equal distances from the planet. The
effects of the opaque rings is clearly seen in this figure—without
them the distribution would extend to the edge of the exobase.
During group 6 Cassini was 40RS from Saturn, resulting in a
very high spatial resolution mosaic of the inner magnetosphere.
The opacity of the rings is clearly seen against the distribution of
hydrogen. The solar-phase is 114 , and there is an asymmetry in
the distribution at negative x in z, with the distribution being
brighter at negative z (i.e. it is bottom heavy).
Group 7, plotted in Fig. 5, during which Cassini had an edge-on
view of the rings, thus removing the shadowing effect of the open
views of the rings, enabling a clear view of the hydrogen
distribution in the inner part of the magnetosphere. On the sunlit side, the hydrogen distribution continuously connects to the
top of Saturn’s atmosphere, below the rings at a latitude of
13 , in a plume like structure. The hydrogen intensity peaks
below the rings on both sides of the planet. A north–south profile
across the rings at x ¼ 1:9RS can be seen in Fig. 7. The Lya
distribution peaks 0:3RS , or 18,000 km, south of the ring plane.
There is significant intensity asymmetry across both x and z,
where the distribution beyond 3RS is inverted, although it is
noteworthy that the distribution is more asymmetric about z ¼ 0
at positive x than at negative x. Both the shadow of the rings on
the atmosphere and auroral emission can be seen on the northern
hemisphere in Fig. 5.
Please cite this article as: Melin, H., et al., The distribution of atomic hydrogen and oxygen in the magnetosphere of Saturn. Planet.
Space Sci. (2009), doi:10.1016/j.pss.2009.04.014
ARTICLE IN PRESS
H. Melin et al. / Planetary and Space Science ] (]]]]) ]]]–]]]
5
Table 3
Features of the Lya distribution.
Feature
Location (RS )
Fig.
Broad distribution
Still decreasing
Asymmetric in z
Ledge
Ledge
Ledge
40
40
All distances
23
20
33
2, 8
8
8
8
8
8
Fig. 7. The Lya intensity profile at constant x ¼ 1:9RS across the rings for Group 5
(2005-074).
Fig. 9. Lya intensity as a function of distance from Saturn for z ¼ 0, for groups 6
and 7 listed in Table 1.
Fig. 8. Lya intensity as a function of distance from Saturn for z ¼ 0, for all the five
groups of observations listed in Table 1.
In the mosaics shown in Figs. 2–5 there are persistent
large-scale asymmetries in the distribution of H Lya, both in x
and in z. Given the long integration time that each group of
system scans represent, any temporal variability on time-scales
shorter than a few weeks would have been averaged out. These
features, in addition to the presence of the ‘plume’ in Fig. 5, is
consistent with the hydrogen source being located in the atmosphere on the sun-lit side of Saturn, on the southern part of the
hemisphere.
3.2. Long-term temporal variability of H Lya
Fig. 8 shows the H Lya intensity profiles for the pre-SOI system
scans (groups 1–5), rendered at a resolution of 1 1RS . They are
plotted across the center of the system at z ¼ 0 in the x direction
(east–west). During this period Saturn presents almost an
identical geometry to Cassini (see Table 2). The figure shows
variability on the order of 10%, with intensities at positive x
showing less variability than at negative x. This is of similar order
to the variation in H Lya observed during solar minimum
(e.g. Lean and Skumanich, 1983).
The intensity profiles in Fig. 8 share persistent features fixed in
local time—there is a clear intensity ledge at 23RS after which
the intensity decreases more rapidly. Between 23 and 10RS the
intensity distribution is more or less flat, i.e. no significant change
in the column integrated abundances. At positive x there are two
ledges—one at 20RS and one at 33RS . This suggests an intricate
shape of the energy distribution, with some orbits more probable
than others, perhaps due to tidal forces, radiation pressure or
inherent biases introduced by the source process.
The H Lya distribution still has a non-zero gradient at 40RS ,
indicating that the source process, or subsequent re-distribution
mechanisms, can provide velocities at and above the system
escape velocity. In addition, there is a 25R difference in intensity
between the two edges, with negative x being the brighter. 210
Rayleigh is reached at 20RS at positive x, whereas 210 Rayleigh is
reached at x ¼ 25RS on the opposite side. The large scale features
of the Lya distribution are summarized in Table 3.
The H Lya x profiles at z ¼ 0 (east–west) for groups 6 and 7 can
be seen in Fig. 9 together with the average profile of groups 1–5.
These two scans present two different Saturn–space-craft
geometries, with a difference in solar phase of 40 . For this
reason, the intensity distributions are not directly comparable, but
they both clearly show asymmetries fixed in local-time. There is
an intensity difference of 10–30% between the averaged pre-SOI
hydrogen (x ¼ 0) distribution and those of groups 6 and 7. The
intensity distribution of group 6 is brighter at positive x whereas
group 7 is brighter at negative x.
Fig. 10 shows the intensity profiles for groups 1–5, cutting
along x ¼ 0 in the z direction. The distribution is much narrower
than in the east–west direction (Fig. 8) and the distribution falls
off faster south of the planet with an average rate is 10:5RR1
S
(Rayleighs per Saturn radii) between 3 and 10RS and only
1
6:8RRS between 3 and 10RS above the north pole of the planet.
The profile evens out toward 20RS , at a level of 170R. There is
still a large column of planetary H Lya being observed, but the
intensity is much smaller than that of 1300R of LISM background
reported by Shemansky and Hall (1992). This is mainly because of
Please cite this article as: Melin, H., et al., The distribution of atomic hydrogen and oxygen in the magnetosphere of Saturn. Planet.
Space Sci. (2009), doi:10.1016/j.pss.2009.04.014
ARTICLE IN PRESS
6
H. Melin et al. / Planetary and Space Science ] (]]]]) ]]]–]]]
3.3. Atomic oxygen in the magnetosphere of Saturn
Figs. 12–18 show the mosaics for the integrated OI 1304 Å
triplet for each of the groups listed in Table 1. The background is
subtracted from the oxygen feature by averaging the intensity of
the surrounding pixels. However, for a mosaic element on the
body of Saturn, H2 emission features makes a reliable background
subtraction impossible, and as a consequence, intensities on the
disk are not a reliable measure of oxygen.
Through Figs. 12–16, where the geometry of observation is
approximately the same, there is a general intensity morphology
consistent with a broad torus centered at 4RS . However, there is
variability between the different groups—Fig. 14 clearly shows a
shadow side enhancement, whereas Figs. 15 and 16 show a more
even distribution.
Fig. 10. Lya intensity as a function of distance from Saturn for x ¼ 0, for groups
1–5, listed in Table 1.
Fig. 11. The variation of the total number of hydrogen atoms between x ¼ 10RS
and z ¼ 5RS between 2003-359 and 2004-142.
the different viewing geometry with respect to the LISM between
Voyager 1980/1981 and Cassini 2004/2005.
Shemansky and Hall (1992) reported a very wide distribution
of H Lya, based on Voyager UVS observations. Based on the results
presented in this paper, geometry of observation is an important
factor when comparing distributions. However, the levels of
planetary Lya reported here are comparable to those reported
by Shemansky and Hall (1992).
Carlson (1980) suggested that hydrogen was sourced via
photo-sputtering from the rings. If the rings were the main
source of the hydrogen, the neutral density in the inner magnetosphere would peak in the ring plane (Johnson et al., 2006b). Fig. 7
shows the Lya intensity profile across the rings at constant
x ¼ 1:9RS for group 7, and it is not consistent with a dominant
atomic hydrogen ring source since it peaks 18,000 km below the
ring plane. In addition, the observed hydrogen distribution does
not indicate a hydrogen Titan torus (e.g. Hilton and Hunten, 1988),
to any measurable extent. The atomic hydrogen abundance
increases toward the sun-lit upper thermosphere of Saturn
(see Broadfoot et al., 1981; Shemansky and Hall, 1992; Shemansky
et al., 2009).
Fig. 11 shows the number of hydrogen atoms present between
x ¼ 10RS and z ¼ 5RS , for the pre-SOI system scans divided up
into 13 temporal segments. It shows variability of the order of 10%
on the order of weeks.
Fig. 12. OI 1304 Å map of group 1 (2003-359), as defined in Tables 1 and 2,
rendered at a resolution of 1:03 1:03RS . The sub-spacecraft latitude was 16:3
and the sub-solar latitude was 25:6 .
Fig. 13. OI 1304 Å map of group 2 (2004-037), as defined in Tables 1 and 2,
rendered at a resolution of 0:78 0:78RS . The sub-spacecraft latitude was 16:3
and the sub-solar latitude was 25:6 .
Please cite this article as: Melin, H., et al., The distribution of atomic hydrogen and oxygen in the magnetosphere of Saturn. Planet.
Space Sci. (2009), doi:10.1016/j.pss.2009.04.014
ARTICLE IN PRESS
H. Melin et al. / Planetary and Space Science ] (]]]]) ]]]–]]]
7
Fig. 14. OI 1304 Å map of group 3 (2004-070), as defined in Tables 1 and 2,
rendered at a resolution of 0:45 0:45RS . The sub-spacecraft latitude was 16:3
and the sub-solar latitude was 25:6 .
Fig. 16. OI 1304 Å map of group 5 (2004-122), as defined in Tables 1 and 2,
rendered at a resolution of 0:23 0:23RS . The bright spot at (2,6) is a star. The subspacecraft latitude was 16:3 and the sub-solar latitude was 25:6 .
Fig. 15. OI 1304 Å map of group 4 (2004-096), as defined in Tables 1 and 2,
rendered at a resolution of 0:37 0:37RS . The bright spot at (1,8) is a star. The subspacecraft latitude was 16:3 and the sub-solar latitude was 25:6 .
Fig. 17. OI 1304 Å map of group 6 (2004-315), as defined in Tables 1 and 2,
rendered at a resolution of 0:1 0:1RS .
Group 6, seen in Fig. 17, provides higher spatial resolution and
also shows a broad toroidal distribution with the intensity
peaking around 4RS . Note the low S=N outside z ¼ 3RS .
The OI mosaic for the edge-on view of the rings of group 7 is
seen in Fig. 18. It shows a distribution with a half-width of about
1RS in the z direction and is also asymmetric in z, especially so at
positive x. It is unclear why the OI distribution is tilted in a similar
way to the H Lya distribution in Fig. 5.
3.4. Temporal variability of atomic oxygen
Fig. 19 shows the z ¼ 0 (east–west) profiles for atomic oxygen
for the first five groups listed in Table 1, rendered at a resolution of
1RS square. The peak of the intensity distribution on either side of
the planet peaks at around 4RS , but the location of the peak
emission can be found 1RS of 4RS on both sides of the planet.
There is significant emission, 30% of peak intensity, out to
10RS .
The abundance of the atomic oxygen can be calculated
given the input of the solar 1304 Å feature. The variations in the
1304 Å emission intensity due to solar cycle are of the order
of a few percent, so generally, for the data presented in this
paper, 1 Rayleigh of emission of atomic oxygen is equivalent
to 6:3 1012 atoms cm2 . For hydrogen, 1 Rayleigh equals
5 1010 atoms s1 cm2 .
Fig. 20 shows the total number of oxygen atoms present
between x ¼ 10RS and z ¼ 5RS in z for the period between
DOY-359 2003 and DOY-142 2004, sub-divided into 13 temporal
segments. Assuming a constant source of oxygen, the short-term
Please cite this article as: Melin, H., et al., The distribution of atomic hydrogen and oxygen in the magnetosphere of Saturn. Planet.
Space Sci. (2009), doi:10.1016/j.pss.2009.04.014
ARTICLE IN PRESS
8
H. Melin et al. / Planetary and Space Science ] (]]]]) ]]]–]]]
Fig. 20. The variation of the total number of oxygen atoms between x ¼ 10RS and
z ¼ 5RS between 2003-359 and 2004-142.
Fig. 18. OI 1304 Å map of Group 7 (2005-074), as defined in Tables 1 and 2,
rendered at a resolution of 0:1 0:1RS . Note that the intensity scale has a
maximum value of 4 Rayleighs per mosaic element.
Table 4
Summary of the life-times of the neutral species in the magnetosphere of Saturn.
Species
Loss mechanism
Lifetime
H
OI
OI
OI
OI
OI
OH
OH
O2
H2 O
H2 Oþ
Photo-ionization
Photo-ionization
Charge exchange at 3:6RS
Charge exchange at 8RS
Charge exchange at 10RS
Electron-ionization
Photo-dissociation
Electron dissociation
Photo-dissocation
Photo-dissociation
Electron-recombination at 4RS
35 years
13 years
4 days
30 days
8 years
1.4 years
78 days
12 days
234 days
4 days
12 h
the upper atmosphere of Saturn, from below the ring plane,
whereas the oxygen is concentrated in a broad torus centered at
x ¼ 4RS . These distinctly different distributions indicate that the
bulk of the hydrogen and oxygen originate from different sourcing
mechanism.
4.1. The magnetospheric hydrogen
Fig. 19. Intensity of OI 1304 Å as a function of distance from Saturn for z ¼ 0, for
groups 1–5 listed in Table 1.
variability seen in Fig. 20 is the result of variations in the loss rate.
Between DOY-96 2004 and DOY-122 2004 the average change in
atomic oxygen loss rate is 2 1027 atoms s1 in order to produce
the observed change in total abundance. Long-term, however, the
total abundance does not vary significantly and in order to
maintain a near-constant number of oxygen atoms against charge
exchange and other loss mechanisms (see Section 4.5) there must
be a near-constant injection of OI into the system.
4. Discussion and conclusions
H Lya and OI have very different distributions within the
magnetosphere of Saturn. Hydrogen is seen being sourced from
Fig. 5 shows a column integrated H Lya distribution that is
inconsistent with a dominant ring source and displays a ‘plume’
feature that connects to the upper atmosphere of Saturn.
Shemansky et al. (2009) details the mechanism by which hot
hydrogen can be produced in the upper atmosphere of Saturn.
The H Lya distribution along z ¼ 0 in Fig. 8 shows substantial
emission of H Lya at large distance from the planet. This means
that there is a significant density of orbiting hydrogen at those
distances, distributed from the inner magnetosphere either
directly by the source or by secondary mechanisms such as
charge exchange (Johnson et al., 2006a).
The lifetimes of the neutral species expected to be found in the
magnetosphere of Saturn are given in Table 4. Left undisturbed,
orbiting hydrogen has a very long lifetime in the outer
magnetosphere. Its life-time will be shortened by collisions and
its orbit will be altered by gravitational tides, radiation pressure
and orbital precession produced by the oblateness of Saturn
(Hamilton and Krivov, 1996). In general, the source rate for
orbiting hydrogen is very small compared to the source rate of the
ballistic hydrogen.
Please cite this article as: Melin, H., et al., The distribution of atomic hydrogen and oxygen in the magnetosphere of Saturn. Planet.
Space Sci. (2009), doi:10.1016/j.pss.2009.04.014
ARTICLE IN PRESS
H. Melin et al. / Planetary and Space Science ] (]]]]) ]]]–]]]
9
4.2. The source of the atomic hydrogen
This paper supports the idea that the atomic hydrogen in the
magnetosphere of Saturn originates from the sun-lit side of
the planet’s atmosphere at a location south of the ring-plane. The
exact production mechanism of the hot hydrogen in the atmosphere of Saturn is not known Shemansky et al. (2009), but it is
likely to be the product of dissociative excitation of H2 X to b 3 Sþ
u
by low energy electrons. The cross section of this transition peaks
at low electron energies (Stibbe and Tennyson, 1998; Khakoo and
Segura, 1994). This mechanism is explored in detail by Shemansky
et al. (2009).
The plume feature at 13 latitude cannot be produced by a
ring source of hydrogen by virtue of the fact that the density peaks
18,000 km away from the ring plane. In addition, the force due to
radiation pressure northward of the rings, and thus incapable of
producing the observed feature. The fact that the distribution
physically connects to the upper atmosphere, together with the
local-time asymmetries, is consistent with the source being
located on the southern part of the sun-lit hemisphere of Saturn.
Although the rings are likely both a sink and a source of hydrogen,
this contribution is drowned out by the dominant atmospheric
hydrogen.
4.3. The sources of atomic oxygen
Shemansky et al. (1993) observed emission from the OH
(A2 Sþ –X2 P)(0, 0) band in the magnetosphere of Saturn using the
Faint Object Spectrograph on the Hubble Space Telescope. HST
observations of OH in the magnetosphere of Saturn from 1992
(cycle 4) are re-analyzed here, due to issues with flux-calibration,
using g values obtained from from ab-initio OH structure
calculations. During these observations the sub-Earth latitude
was 16 , so they are directly comparable to the pre-SOI UVIS
system scans that had a sub-space-craft latitude of 16 ,
assuming that the oxygen distribution is symmetric about the
x2y plane and does not vary with time. Fig. 21, shows the
normalized and averaged z ¼ 0, x ¼ x, profiles of groups 1–5 in
Table 1, rendered at a resolution of 1 1 RS (dashed). The solid line
is an empirical smooth fit to the data that is used to derive
volumetric densities (shown in Fig. 22). The dotted line is the
re-reduced HST observation and the dot-dashed is the neutral OH
distribution modeled by Johnson et al. (2006). In the Johnson et al.
(1996) model the neutrals are re-distributed by means of chargeexchange and reactive collisions from a source of H2 O located at
Fig. 22. The radial volumetric density profile of oxygen (solid) and hydroxyl
(dotted), derived from Cassini UVIS and HST observations.
the orbit of Enceladus. This model fits the Jurac et al. (2002) OH
observations well but does not fit the OH HST observations
presented here — this is likely due to the application of different
g-values in the reduction process.
Using the line-of-sight O and OH profiles the radial density
distribution can be modeled, seen in Fig. 22). The intensities
inward of 2 RS are ignored in the modeling process, since the rings
are likely to shadow at least a part of the distribution. The peak
density of oxygen is 680 cm3 and the peak density of hydroxyl is
700 cm3 . Jurac et al. (2002) calculated the maximum OH density
to be 103 cm3 at the orbit of Enceladus.
The radial profiles of OI and OH are different in shape, with the
former being twice as broad as the latter. The hydroxyl distribution is centered around 3:9RS and falls symmetrically from there,
with a scale-height of 2:5RS . The oxygen distribution, on the
other hand, is centered at 4:5RS and falls off much slower with
increasing distance from the planet, with a scale-height of 4RS .
Almost 40% of the total oxygen population is found at radial
distances greater than 10RS . The difference in the two profiles hint
that there is a difference in the way the two are sourced. Although
Enceladus provides large amounts of H2 O, additional sources
of oxygen in the magnetosphere of Saturn may be present (Jurac
et al., 2002; Martens et al., 2008).
4.4. Asymmetries in distribution
When many system scans are combined to produce a mosaic,
each mosaic element represents a temporally averaged intensity
over the period of observations. This means that variability on
time-scales shorter than the period of observation is averaged out.
As a consequence, it is difficult to explain the asymmetries seen in
the oxygen mosaics, as the orbital period of Enceladus is 1.4 days
and the period of observations is on the order of weeks.
4.5. Loss rates
The main mechanism by which OI is lost in the inner
magnetosphere of Saturn is charge exchange:
O þ Oþ ! Oþ þ O
(1)
þ
Fig. 21. The line-of-sight intensity profiles for oxygen (dashed) and hydroxyl
(dotted) derived from Cassini UVIS and HST observations. Also plotted are the
modeled oxygen distribution (solid) and the modeled neutral line of sight
abundance due to charge exchange (dash-dot).
In this process, the O product is accelerated to Saturn co-rotation
velocities by the magnetic field—at 3:95RS the co-rotation velocity
1
required
is 39 km s1 which is more than double the 18 kms
þ
to escape the system. Once the O charge exchanges once more,
the oxygen has a large enough velocity to escape the system.
Reaction 1 is a net loss of OI, while leaving the plasma population
unchanged.
Please cite this article as: Melin, H., et al., The distribution of atomic hydrogen and oxygen in the magnetosphere of Saturn. Planet.
Space Sci. (2009), doi:10.1016/j.pss.2009.04.014
ARTICLE IN PRESS
10
H. Melin et al. / Planetary and Space Science ] (]]]]) ]]]–]]]
The distribution of the OI density, dOI ðx; y; zÞ, can be seen in
Fig. 22. The total number of oxygen atoms lost in the charge
exchange process, nloss , can be calculated by
Z 1Z 1Z 1
nloss ¼
sde ðx; y; zÞdOI ðx; y; zÞjDvin ðx; yÞj dx dy dz
(2)
Table 5
Upper limits of kinetic energy injection to OI via neutral photo-dissociative
excitation.
Reaction
Dissociation
energy (eV)
Ionization
energy (eV)
OI
KEmax (eV)
where de ðx; y; zÞ is the volumetric plasma density (Persoon et al.,
2006), s is the cross section of the O+ Oþ charge exchange process
(Lindsay et al., 2001), Dvin ðx; yÞ is the velocity difference between
the plasma velocity (the Saturn co-rotation velocity) and the
orbital velocity and x, y and z are spatial coordinates (as per
Section 2.7).
The plasma density model of Persoon et al. (2006) is valid
between 5 and 8:6RS . In this calculation it is assumed that the
plasma density distribution inward of 5RS is constant and equal to
the distribution at 5RS . Since the plasma density of water group
ions, Wþ , is about 10 times larger than that of Hþ (Sittler et al.,
2008) only charge exchange with Oþ is considered. This assumes
that Oþ is the dominant Wþ ion, given that it is intrinsically much
longer lived.
Solving Eq. (1) gives a total loss of oxygen to charge exchange
of 1:3 1028 atoms s1 , with a peak volumetric loss rate of
8:7 104 atoms cm3 s1 . Therefore, in order to sustain a constant number of oxygen atoms in the magnetosphere 1:3 1028
atoms need to be added every second into the magnetosphere,
putting a lower limit on the sourcing process of water. The total
loss rate gives an average lifetime of the oxygen of 37 days.
The H2 O source rate has previously been estimated to be
1 1028 molecules s1 (Burger et al., 2007; Hansen et al., 2006).
OH ! H(1s) + O(3 P)
H2 O ! H2 + O(1 D)
H2 O ! H2 + O(3 P)
O2 ! O(3 P) + O(3 P)
O2 ! O(1 D) + O(3 P)
4.412
7.001
5.034
5.116
7.083
13.017
12.621
12.621
12.070
12.070
0.506
0.624
0.843
3.477
2.493
0
0
0
4.6. Implication of OI distribution
If the gravitational effect of satellites is neglected, the sum of
potential and kinetic energy of an OI atom orbiting around Saturn
at distance R from the center, by virtue of the virial theorem, is
GMm=2R, where M and m are masses of Saturn and OI atom,
respectively. Consequently, it requires 4 eV of kinetic energy per
atom to move OI from 3:95RS to 10RS . Likewise, it requires a
minimum of 1.43 eV to move OI at 3:95RS a distance of 2RS in the z
direction.
The upper limits of kinetic energy that an oxygen atom can
acquire via neutral dissociative photo-excitation of an oxygen
containing molecule can be estimated from its first ionization
potential and first dissociation limit. In the magnetosphere of
Saturn, H2 O, OH and O2 are probably the most abundant oxygen
containing molecules. While O2 cannot be directly produced from
H2 O in gas phase, it has been experimentally shown that it is
formed on particle impacted water ice surface (Sieger et al., 1998;
Orlando, 2003). In a two-body breakup of the molecule, the
conservation of momentum dictates that the fraction of kinetic
energy of a fragment is inversely proportional to the mass of the
fragment. In case of OH þ hn ! Oð3 PÞ þ Hð1 sÞ dissociation the O
and H atoms carry 1/17 and 16/17 of the total released kinetic
energy. Since the dissociation and ionization energies of OH is
4.412 and 13.017 eV, respectively, the maximum kinetic energy
that the O(3P) atom can acquire via the neutral dissociation
process is 0.506 eV. Table 5 lists the estimated maximum kinetic
energy pickup by an oxygen atom for several neutral photodissociation process. It should be mentioned that the tabulated
value represents an upper limit for the given process. For various
photochemical reasons, the actual maximum energy pickup can
be significantly lower than the tabulated value. For instance, the
3 strong O2 transition from X 3 S
g to valence and Rydberg E Su
states between 9.75 and 10.62 eV results in the production
Oð1D Þ þ Oð3 PÞ (Lambert et al., 2004), which yields an upper limit
of 1.78 eV per oxygen atom.
In addition to the energy given to the oxygen atoms via their
formation process, they can be re-distributed in the magnetosphere via charge capture, giving them a wide distribution
throughout the magnetosphere (Johnson, 2004). The plasma corotation velocity at 4RS is larger than the escape velocity at that
distance. Given that there is a distribution of ion velocities that
the neutrals charge exchange with (Tokar et al., 2006), charge
exchange can re-distribute oxygen out to a significant distance.
This range of ion velocities is only observed in a volume extending
0:1RS from the location of Enceladus, such that anywhere else
along the moons orbit, charge exchange is purely loss mechanism.
The model of Johnson et al. (2006a) predicts that charge exchange
re-distribution process results in about 2% of the peak line-ofsight intensity out at 10RS (see Fig. 21). In this paper a line-ofsight intensity of 30% of the maximum oxygen intensity is
observed at 10RS , an order of magnitude greater than produced by
charge exchange in Johnson et al. (2006a). This indicates either
additional mechanism by which oxygen is re-distributed, or
additional sources of oxygen in the inner magnetosphere of
Saturn.
Table 4 lists the lifetimes of the neutral species likely to be
present in the magnetosphere of Saturn. The lifetime of oxygen is
on the order of days within the plasma sheet, but since the OI
distribution has a FWHM of 1RS and the plasma only has a
FWHM of 0:1RS Persoon et al. (2006), the OI will orbit in and out
of the plasma sheet. The lifetime of OI outside the plasma sheet is
very long, so the actual lifetime of the OI will depend on how
much of its orbit is spent inside the plasma sheet. If the loss rate is
1:3 1028 then the implied lifetime is 38 days.
At large distances from the planet, 10RS , both electron
ionization and photo-ionization will compete with charge capture
for the dominant role as a loss process for the oxygen. Using the
electron population at 10RS described by Schippers et al. (2008),
the volumetric electron ionization rate is 2:8 107 cm3 s1 ,
whereas the photoionization rate is 8:4 108 cm3 s1 , using the
cross section of Huebner et al. (1992). Charge exchange still has a
important role to play at these distances with a rate of
1:2 107 cm3 . Whilst electron impact ionization just barely
dominates, the oxygen still has a very long lifetime (see Table 4)
and the source rate required to keep the density at 40% of the total
is slow compared to the source rate required at 4RS .
Given the ion partitioning of Schippers et al. (2008) and the
rate coefficient of Mitchell and McGowan (1983), the mass loss
rate due to recombination of H2 Oþ and H3 Oþ at the core of the
neutral torus at the orbit of Enceladus is about a fourth of that due
to Oþ charge exchange.
Acknowledgment
This research was supported by the University of Colorado
Cassini UVIS Program contract 1531660 to Space Environment
Technologies.
Please cite this article as: Melin, H., et al., The distribution of atomic hydrogen and oxygen in the magnetosphere of Saturn. Planet.
Space Sci. (2009), doi:10.1016/j.pss.2009.04.014
ARTICLE IN PRESS
H. Melin et al. / Planetary and Space Science ] (]]]]) ]]]–]]]
References
Broadfoot, A.L., Sandel, B.R., Shemansky, D.E., Holberg, J.B., Smith, G.R., Strobel, D.F.,
McConnell, J.C., Kumar, S., Hunten, D.M., Atreya, S.K., Donahue, T.M., Moos,
H.W., Bertaux, J.L., Blamont, J.E., Pomphrey, R.B., Linick, S., 1981. Extreme
ultraviolet observations from Voyager 1 encounter with Saturn. Science 212,
206–211.
Burger, M.H., Sittler, E.C., Johnson, R.E., Smith, H.T., Tucker, O.J., Shematovich, V.I.,
2007. Understanding the escape of water from Enceladus. Journal of
Geophysical Research (Space Physics) 112 (11) 6219–+.
Carlson, R.W., 1980. Photo-sputtering of ice and hydrogen around Saturn’s rings.
Nature 283 461–+.
Cheng, A.F., Lanzerotti, L.J., 1978. Ice sputtering by radiation belt protons and the
rings of Saturn and Uranus. Journal of Geophysical Research 83, 2597–2602.
Esposito, L.W., Barth, C.A., Colwell, J.E., Lawrence, G.M., McClintock, W.E., Stewart,
A.I.F., Keller, H.U., Korth, A., Lauche, H., Festou, M.C., Lane, A.L., Hansen, C.J.,
Maki, J.N., West, R.A., Jahn, H., Reulke, R., Warlich, K., Shemansky, D.E., Yung,
Y.L., 2004. The Cassini ultraviolet imaging spectrograph investigation. Space
Science Reviews 115, 299–361.
Esposito, L.W., Colwell, J.E., Larsen, K., McClintock, W.E., Stewart, A.I.F., Hallett, J.T.,
Shemansky, D.E., Ajello, J.M., Hansen, C.J., Hendrix, A.R., West, R.A., Keller, H.U.,
Korth, A., Pryor, W.R., Reulke, R., Yung, Y.L., 2005. Ultraviolet imaging
spectroscopy shows an active Saturnian system. Science 307, 1251–1255.
Hamilton, D.P., Krivov, A.V., 1996. Circumplanetary dust dynamics: effects of solar
gravity radiation, pressure, planetary oblateness, and electromagnetism. Icarus
123, 503–523.
Hansen, C.J., Esposito, L., Stewart, A.I.F., Colwell, J., Hendrix, A., Pryor, W.,
Shemansky, D., West, R., 2006. Enceladus’ water vapor plume. Science 311,
1422–1425.
Hendrix, A.R., Hansen, C.J., 2005. An overview of cassini UVIS icy satellite results so
far. In: Mackwell, S., Stansbery, E. (Eds.), 36th Annual Lunar and Planetary
Science Conference. Lunar and Planetary Institute Conference Abstracts, vol. 36
2384–+.
Hilton, D.A., Hunten, D.M., 1988. A partially collisional model of the Titan hydrogen
torus. Icarus 73, 248–268.
Huebner, W.F., Keady, J.J., Lyon, S.P., 1992. Solar photo rates for planetary
atmospheres and atmospheric pollutants. Astrophysics and Space Science
195, 1–289.
Ip, W.-H., 1978. On the Lyman-alpha emission from the vicinity of Saturn’s rings.
Astronomy and Astrophysics 70, 435–437.
Johnson, R.E., 2004. The magnetospheric plasma-driven evolution of satellite
atmospheres. The Astrophysical Journal 609, L99–L102.
Johnson, R.E., Luhmann, J.G., Tokar, R.L., Bouhram, M., Berthelier, J.J., Sittler, E.C.,
Cooper, J.F., Hill, T.W., Smith, H.T., Michael, M., Liu, M., Crary, F.J., Young, D.T.,
2006a. Production, ionization and redistribution of O2 in Saturn’s ring
atmosphere. Icarus 180, 393–402.
Johnson, R.E., Smith, H.T., Tucker, O.J., Liu, M., Burger, M.H., Sittler, E.C., Tokar, R.L.,
2006b. The Enceladus and OH Tori at Saturn. The Astrophysical Journal 644,
L137–L139.
Jurac, S., McGrath, M.A., Johnson, R.E., Richardson, J.D., Vasyliunas, V.M., Eviatar, A.,
2002. Saturn: Search for a missing water source. Geophysical Research Letters
29 (24), 240000-1.
Jurac, S., Richardson, J.D., 2005. A self-consistent model of plasma and neutrals
at Saturn: neutral cloud morphology. Journal of Geophysical Research
(Space Physics) 110 9220–+.
Khakoo, M.A., Segura, J., 1994. Differential cross sections for the electron impact
3
excitation of the b Sþ
u continuum of molecular hydrogen. Journal of Physics B
Atomic Molecular Physics 27, 2355–2368.
Lambert, H.M., Dixit, A.A., Davis, E.W., Houston, P.L., 2004. Quantum yields for
product formation in the 120–133 nm photodissociation of O2 . Journal of
Chemical Physics 121, 10437–10446.
11
Lean, J.L., Skumanich, A., 1983. Variability of the Lyman alpha flux with solar
activity. Journal of Geophysical Research 88, 5751–5759.
Lindsay, B.G., Sieglaff, D.R., Smith, K.A., Stebbings, R.F., 2001. Charge transfer of keV
Oþ ions with atomic oxygen. Journal of Geophysical Research 106, 8197–8204.
Martens, H.R., Reisenfeld, D.B., Williams, J.D., Johnson, R.E., Smith, H.T., 2008.
Observations of molecular oxygen ions in Saturn’s inner magnetosphere.
Geophysical Research Letters 35 20103–+.
Mitchell, J.B.A., McGowan, J.W., 1983. Experimental studies of electron–ion
recombination. In: Brouillard, F., McGowan, J.W. (Eds.), NATO ASIB Proceedings
83: Physics of Ion–Ion and Electron–Ion Collisions 279–+.
Orlando, T., 2003. The role of electron-stimulated production of O2 from water ice
in the radiation processing of outer solar system surfaces. Surface Science 528,
1–3.
Persoon, A.M., Gurnett, D.A., Kurth, W.S., Groene, J.B., 2006. A simple scale height
model of the electron density in Saturn’s plasma disk. Geophysical Research
Letters 33 18106–+.
Schippers, P., Blanc, M., André, N., Dandouras, I., Lewis, G.R., Gilbert, L.K., Persoon,
A.M., Krupp, N., Gurnett, D.A., Coates, A.J., Krimigis, S.M., Young, D.T.,
Dougherty, M.K., 2008. Multi-instrument analysis of electron populations in
Saturn’s magnetosphere. Journal of Geophysical Research (Space Physics) 113
(A12) 7208–+.
Shemansky, D., Liu, X., Melin, H., 2009. The saturn hydrogen plume. Planetary and
Space Science, this issue, doi:10.1016/j.pss.2009.05.002.
Shemansky, D.E., Hall, D.T., 1992. The distribution of atomic hydrogen in the
magnetosphere of Saturn. Journal of Geophysical Research 97, 4143–4161.
Shemansky, D.E., Matheson, P., Hall, D.T., Hu, H.-Y., Tripp, T.M., 1993. Detection of
the hydroxyl radical in the Saturn magnetosphere. Nature 363, 329–331.
Sieger, M.T., Simpson, W.C., Orlando, T.M., 1998. Production of O2 on icy satellites
by electronic excitation of low-temperature water ice. Nature 394, 554–556.
Sittler, E.C., Andre, N., Blanc, M., Burger, M., Johnson, R.E., Coates, A., Rymer, A.,
Reisenfeld, D., Thomsen, M.F., Persoon, A., Dougherty, M., Smith, H.T., Baragiola,
R.A., Hartle, R.E., Chornay, D., Shappirio, M.D., Simpson, D., McComas, D.J.,
Young, D.T., 2008. Ion and neutral sources and sinks within Saturn’s inner
magnetosphere: Cassini results. Planetary and Space Science 56, 3–18.
Sittler, E.C., Thomsen, M., Chornay, D., Shappirio, M.D., Simpson, D., Johnson, R.E.,
Smith, H.T., Coates, A.J., Rymer, A.M., Crary, F., McComas, D.J., Young, D.T.,
Reisenfeld, D., Dougherty, M., Andre, N., 2005. Preliminary results on Saturn’s
inner plasmasphere as observed by Cassini: comparison with Voyager.
Geophysical Research Letters 32, 14 14–+.
Sittler, E.C., Thomsen, M., Johnson, R.E., Hartle, R.E., Burger, M., Chornay, D.,
Shappirio, M.D., Simpson, D., Smith, H.T., Coates, A.J., Rymer, A.M., McComas,
D.J., Young, D.T., Reisenfeld, D., Dougherty, M., Andre, N., 2006. Cassini
observations of Saturn’s inner plasmasphere: Saturn orbit insertion results.
Planetary and Space Science 54, 1197–1210.
Stibbe, D.T., Tennyson, J., 1998. Near-threshold electron impact dissociation of H2
within the adiabatic nuclei approximation. New Journal of Physics 1 2–+.
Tokar, R.L., Johnson, R.E., Hill, T.W., Pontius, D.H., Kurth, W.S., Crary, F.J., Young, D.T.,
Thomsen, M.F., Reisenfeld, D.B., Coates, A.J., Lewis, G.R., Sittler, E.C., Gurnett,
D.A., 2006. The Interaction of the Atmosphere of Enceladus with Saturn’s
Plasma. Science 311, 1409–1412.
Weiser, H., Vitz, R.C., Moos, H.W., 1977. Detection of Lyman-alpha emission from
the Saturnian disk and from the ring system. Science 197, 755–757.
Young, D.T., Berthelier, J.-J., Blanc, M., Burch, J.L., Bolton, S., Coates, A.J., Crary, F.J.,
Goldstein, R., Grande, M., Hill, T.W., Johnson, R.E., Baragiola, R.A., Kelha, V.,
McComas, D.J., Mursula, K., Sittler, E.C., Svenes, K.R., Szegö, K., Tanskanen, P.,
Thomsen, M.F., Bakshi, S., Barraclough, B.L., Bebesi, Z., Delapp, D., Dunlop,
M.W., Gosling, J.T., Furman, J.D., Gilbert, L.K., Glenn, D., Holmlund, C., Illiano,
J.-M., Lewis, G.R., Linder, D.R., Maurice, S., McAndrews, H.J., Narheim, B.T.,
Pallier, E., Reisenfeld, D., Rymer, A.M., Smith, H.T., Tokar, R.L., Vilppola, J.,
Zinsmeyer, C., 2005. Composition and dynamics of plasma in Saturn’s
magnetosphere. Science 307, 1262–1266.
Please cite this article as: Melin, H., et al., The distribution of atomic hydrogen and oxygen in the magnetosphere of Saturn. Planet.
Space Sci. (2009), doi:10.1016/j.pss.2009.04.014