2.A Material sources of gas and plasma
The magnetosphere, extending from the top of the Saturn magnetosphere to beyond the
magnetopause is dominated by neutral gas. The main components are atomic hydrogen,
H I, atomic oxygen, O I, and the hydroxyl radical, OH. O I and OH are evidently the
dissociation products of H2O. A major source of H2O has been identified in observations
of what is now described as the Enceladus plume ( Hansen et al., 2006; ---------------)
with an estimated ejection rate of a few x 1028 molecules s-1. Although the Enceladus
plume is a major source it is not clear that Enceladus alone is a strong contributor of the
observed water products, based on the spatial distribution and temporal properties of the
O I observed in UVIS system scans ( Shemansky et al., 2004; Melin et al., 2009).
Identification of other significant specific sources of H2O is unresolved at this time. OH
was first observed in HST exposures by Shemansky et al.(1993) as predicted by
Shemansky and Hall(1992). HST was the last experimental facility with the capability of
measuring OH in the Saturn magnetosphere, and this component has not been observed
since the epoch of HST capability. The spatial morphology of OH is poorly known, and
there is no information on temporal variability in this component. Temporal variability in
the source at Enceladus is also essentially unknown. Temporal variability and spatial
distribution of O I has been obtained in the Cassini UVIS system scans (Shemansky et
al., 2004; Melin et al., 2009), and to date these results raise more questions than answers,
pointing to a more complex system. Asymmetry in O I spatial distribution, and temporal
effects that are substantially shorter than known rate processes seriously complicate
identifying the controlling factors in this system. An example of the observed image of O
I is shown in Figure 1_uvis. The distribution of H I in the magnetosphere was first
obtained by Shemansky and Hall (1992) using Voyager 1 (V1) post encounter UVS
system scans. The V1 observations identified a strong local time asymmetry in noisy
images of the H I distribution with a general peak in density in the dusk region , and a
minimum in the predawn region of the magnetosphere. Typical densities in the 3 -- 4 RS
region of the sub-solar magnetosphere are O I:OH:H I = 500:700:450 cm-3 (Shemansky
et al., 2009; Melin et al., 2009). Shemansky & Hall (1992) argued that the primary source
of H I is escaping dissociation products of physical chemistry at the top of the Saturn
sunlit atmosphere. Definitive images obtained with Cassini UVIS system scans show
distinctive structure in H I escaping from the top of the sunlit atmosphere, with a strong
feature creating a propeller shaped distribution originating at -8o to the ring plane. The
density of H I at the rings is of the order 104 cm-3 . There is no observational evidence
for a component of the H I sourced from the rings. At the orbit of Titan the only
detectable neutral component is H I at densities ranging from 50 -- 150 cm-3, with no
measurable torus at 20 RS. The image of H I in the close vicinity of Saturn is shown in
Figure 2_uvis as a contour map of H Lyα brightness at a resolution of 0.1 X 0.1 RS. The
rings in this observation are edge-on so that no scattered ring particle emission is
detectable. Figure 3_uvis shows an image of the system in H Lyα extending ± 30 RS
from planet center. H I is measurable to beyond ± 45 RS in the orbital plane and ± 30 RS
perpendicular to the orbital plane. The temporal variation of total O I population is shown
in Figure 4_uvis, indicating a minimum near 2004 DOY 240, with an implied relaxation
time scale of ~10 days. Table 1_uvis shows estimated populations and loss rates
(Shemansky et al., 2004; Shemansky et al., 2009).
The dominant neutral gas population throughout the magnetosphere is the limiting factor
for development of the plasma environment. The loss of atomic oxygen from the system
is determined mainly by the reaction
O+ + H > O + H+
This reaction does not affect the ion population, but is a net loss mechanism for the
neutral product. Similarly the reaction
H+ + H > H + H+
is a net loss for atomic hydrogen. The primary limiting factor for the development of the
plasma is the collisional cooling of the electron population by the neutral gas through
excitation of electronic states and momentum transfer (Shemansky and Hall, 1992). If the
rates quoted by Shemansky and Hall(1992) (Table 6 of S & H) are used with the current
knowledge of neutral densities the energy loss by the ambient electron population is ~3
X 10-15 ergs cm-3 s-1 in the 3 -- 4 RS region. Energy deposition from pickup ions is
negligible, and most of the energy loss would need to come from heterogenous deposition
by inward plasma diffusion from the outer magnetosphere.
Table 1
Species
OI
OH
HI
H2O
NI
* Theoretical
Density (cm-3)
3 -4 RS
500
700
450
~200*
minor
Total system
population
3. x 1034
~ 4 x 1034
2. x 1035
Loss rate (s-1)
~ 1029
~ 1029
3. x 1030
RS (N - S)
RS (E - W)
Figure 1. Image of O I 130.4 nm emission from the Saturn magnetosphere obtained 2004
DOY 51 -- 72. Sub-spaceraft latitude -13.5o. Sub-solar latitude -23o. Solar flux impacts
the system from the right. Brightness is indicated on the contour lines in Rayleighs. The
O I emission is entirely forced by fluorescence of the solar O I flux.
RS (N - S)
1
0
-1
-4
-3
-2
-1
0
1
2
3
4
RS (E - W)
Figure 2. Image of Saturn and inner magnetosphere in H Lya emission showing
structured outflow of atomic hydrogen from the top of the atmosphere. The emission is
forced by solar flux from the right side of the image, at latitude -17o. The rings are
viewed edge-on from the spacecraft. A ridge of emission at -8o latitude forms a propeller
shaped feature from both sides of the planet, measurable to beyond 4 RS as indicated in
the shape of the image contours. Outflow from the sub-solar atmosphere is evident over a
broad range of latitudes, apparently affected by ring shadow. Auroral emission is evident
at the poles.
RS (N - S)
30
0
-30
-29
0
29
RS (E - W)
Figure 3. Image of the Saturn magnetosphere in H Lyα emission showing the distinct
local time asymmetry in distribution. The sub-spacecraft latitude is -13.7o. The sun is on
the right side of the image at sub-solar latitude -23o. The separate small bright feature
north of the planet is a star.
O I total (1034 atoms)
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0
100
200
300
400
500
600
700
DOY from 356 2003
Figure 4. Temporal variation in measured total O I population in the Saturn inner
magnetosphere. See Figure 1 for spatial distribution (also temporally variable), and Table
1 for populations of other neutral species.