Icarus 200 (2009) 176–187
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Icarus
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Characteristics of Saturn’s polar atmosphere and auroral electrons derived from
HST/STIS, FUSE and Cassini/UVIS spectra
J. Gustin a,∗ , J.-C. Gérard a , W. Pryor b , P.D. Feldman c , D. Grodent a , G. Holsclaw d
a
Laboratoire de Physique Atmosphérique et Planétaire, Université de Liège, Allée du 6 Août, 17, 4000 Liège, Belgium
Central Arizona College, Coolidge, AZ 85228, USA
c
Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA
d
Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA
b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Received 24 July 2008
Revised 21 October 2008
Accepted 10 November 2008
Available online 6 December 2008
Ultraviolet (UV) spectra of Saturn’s aurora obtained with the Hubble Space Telescope Imaging Spectrograph (STIS), the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Far Ultraviolet Spectroscopic
Explorer (FUSE) have been analyzed. Comparisons between the observed spectra and synthetic models
of electron-excited H2 have been used to determine various auroral characteristics. Far ultraviolet (FUV:
1200–1700 Å) STIS and UVIS spectra exhibit, below 1400 Å, weak absorption due to methane, with a
vertical column ranging between 1.4 × 1015 and 1.2 × 1016 cm−2 . Using the low-latitude Moses et al.
[Moses, J.I., Bézard, B., Lellouch, E., Feuchtgruber, H., Gladstone, G.R., Allen, M., 2000. Icarus, 143, 244–298]
atmospheric model of Saturn and an electron energy–H2 column relationship, these methane columns are
converted into the mean energy of the primary precipitating electrons, estimated to lie in the range 10–
18 keV. This result is confirmed by the study of self-absorption with UVIS and FUSE extreme ultraviolet
(EUV: 900–1200 Å) spectra. Below 1200 Å, it is seen that transitions connecting to the v < 2 vibrational
levels of the H2 electronic ground state are partially self-absorbed by H2 molecules overlying the auroral
emission. Because of its low spectral resolution (∼5.5 Å), the UVIS EUV spectrum we analyzed does
not allow us to unequivocally determine reasonable ranges of temperatures and H2 columns. On the
other hand, the high spectral resolution (∼0.2 Å) of the FUSE LiF1a and LiF2a EUV spectra we examined
resolve the H2 rotational lines and makes it possible to determine the H2 temperature. The modeled
spectrum best fitting the FUSE LiF1a observation reveals a temperature of 500 K and self-absorption by a
H2 vertical column of 3 × 1019 cm−2 . When converted to energy of precipitating electrons, this H2 column
corresponds to primary electrons of ∼10 keV. The model that best fits the LiF2a spectrum is characterized
by a temperature of 400 K and is not self-absorbed, making this segment ideal to determine the H2
temperature at the altitude of the auroral emission. The latter value is in agreement with temperatures
obtained from H+
3 infrared polar spectra. Self-absorption is detectable in the LiF2a segment for H2
columns exceeding 6 × 1019 cm−2 , which sets the maximum mean energy determined from the FUSE
observations to ∼15 keV. The total electron energy range of 10–18 keV deduced from FUV and EUV
observations places the auroral emission peak between the 0.1 and 0.3 μbar pressure levels. These values
should be seen as an upper limit, since most of the Voyager UVS spectra of Saturn’s aurora examined by
Sandel et al. [Sandel, B.R., Shemansky, D.E., Broadfoot, A.L., Holberg, J.B., Smith, G.R., 1982. Science 215,
548] do not exhibit methane absorption. The auroral H2 emission is thus likely located above but close
to the methane homopause. The H2 auroral brightness in the 800–1700 Å bandwidth varies from 2.9 kR
to 139 kR, comparable to values derived from FUV Faint Object Camera (FOC) and STIS images.
© 2008 Elsevier Inc. All rights reserved.
Keywords:
Aurorae
Saturn
Spectroscopy
Ultraviolet observations
1. Introduction
Observations of Saturn’s aurora in the UV were first carried out
by the Pioneer 11 and Voyager 1 and 2 spacecraft in 1979 and
*
Corresponding author.
E-mail address: gustin@astro.ulg.ac.be (J. Gustin).
0019-1035/$ – see front matter
doi:10.1016/j.icarus.2008.11.013
©
2008 Elsevier Inc. All rights reserved.
1980. The data collected by the Voyager UV spectrometer (UVS) revealed auroral emissions from both polar regions. Saturn’s aurora
appeared as a narrow circumpolar region near 80◦ latitude with no
apparent emission present in the polar cap, with typical intensities
in the range 10 to 15 kilo-Rayleighs (kR) (Broadfoot et al., 1981;
Sandel and Broadfoot, 1981). Intensity variations up to a factor
of 5 were observed (Sandel et al., 1982), suggesting a solar wind
controlled aurora. As for Jupiter, comparisons between observed
UV spectroscopy of Saturn’s aurora with STIS, FUSE and UVIS
spectra and models showed that the auroral emission is produced
by the interaction between the H2 atmosphere and precipitating
electrons (Shemansky and Ajello, 1983). Between 800 and 1800 Å,
the auroral emission is dominated by atomic H lines from the
Lyman series and H2 vibronic lines from the B 1 Σu+ → X 1 Σ g+ ,
C 1 Πu → X 1 Σ g+ , B 1 Σu+ → X 1 Σ g+ , D 1 Πu → X 1 Σ g+ , B 1 Σu+ →
X 1 Σ g+ , D 1 Πu → X 1 Σ g+ system bands. While the Lyman (B → X )
and Werner (C → X ) bands and the Lyman continuum prevail in
the FUV spectral region, the EUV bandwidth includes transitions
from the B and higher Rydberg electronic states.
Analysis of images obtained with STIS onboard the Hubble
Space Telescope (HST) revealed a highly dynamic aurora, in the
form of a ∼2◦ wide ring, located between 70◦ and 80◦ latitude
with brightness ranging from below the STIS threshold of ∼1 up
to ∼100 kR (Gérard et al., 2004; Clarke et al., 2005). Significant
changes in morphology have also been observed during short intervals, where the auroral region forms a spiral followed by intervals of reinflation of the auroral region (Grodent et al., 2005).
Recent STIS images of Saturn combined with simultaneous Cassini
measurement of the solar wind (Crary et al., 2005) and Saturn
kilometric radio emission (Kurth et al., 2005) confirmed that the
morphology of Saturn’s aurora differs from those of both Earth
and Jupiter. In particular, brightening of the aurora with a movement of the emissions toward higher latitudes was observed and
correlated with Cassini’s solar wind dynamic pressure measurements, suggesting that the aurora is controlled by the solar wind
conditions at Saturn. On the other hand, considerable longitudinal structure and time variations over interval of a few hours were
also observed, despite the absence of observable external triggers
and solar wind activations (Gérard et al., 2006). This suggests that
the aurora is also influenced by an intrinsically dynamical magnetosphere.
From a theoretical point of view, auroral rings can either be
formed by field-aligned currents, associated with the breakdown
of corotation of the plasma rotating in the middle magnetosphere
(as in the case of Jupiter), or by interactions between the solar wind and the planet’s magnetosphere at the open-closed field
line boundary (as for the Earth). Theoretical work (Cowley et al.,
2004a), statistical studies of the location of the auroral emission
(Badman et al., 2006) and the recent observation of a field-aligned
current at the open-closed field line boundary coincident with HST
images of the aurora (Bunce et al., 2008) indicate that the latter is more likely in Saturn’s case. In particular, using a simple
quantitative model, Cowley et al. (2004b) showed that field-aligned
voltages of ∼10 kV are required to accelerate outer magnetospheric
electrons in order to produce an auroral oval of a few tens of kR.
The FUV and EUV auroral emissions are known to interact
with the atmosphere through absorption by hydrocarbons and selfabsorption by H2 . These two processes are used to estimate the
altitude of the aurora. First, the altitude of the aurora relative to
the hydrocarbon homopause may be derived from the absorption
by methane, which attenuates the H2 emission below 1400 Å, leaving the H2 emission above 1400 Å unattenuated. This absorption is
measured by the color ratio CR = I (1550–1620 Å)/ I (1230–1300 Å),
which relates the attenuation of the H2 auroral emission to the
amount of methane overlying the peak of the emitting layer. CR is
1.1 for an unattenuated spectrum expressed in Rayleighs. From six
STIS Saturn spectra of southern aurora in the 1150–1750 Å spectral
range, Gérard et al. (2004) found that the auroral emission was
little absorbed by methane, with a nearly constant vertical CH4
column of 6 × 1015 cm−2 . Using the Saturn model atmosphere of
Moses et al. (2000) at 30◦ North relating the H2 and CH4 column
density profiles and the Grodent et al. (2001) atmospheric model
that links the H2 column density to the mean energy of the precipitated electrons, Gérard et al. (2004) showed that the energy of the
177
precipitated primary electrons was in the range 12 ± 3 keV. Second, a direct measurement of the H2 column overlying the auroral
emission can be obtained from EUV spectra. Below 1200 Å, the
photons connecting to the v = 0, 1, 2 levels may be partially or
totally absorbed by the overlying column of H2 and redistributed
to the FUV portion of the spectrum. This self-absorption process
provides a measure of the H2 column overlying the UV emission
peak and provides information on the population of the groundstate vibrational levels (Gustin et al., 2004a). Self-absorption on
Saturn was earlier pointed out by Broadfoot et al. (1981), Sandel
et al. (1982) and Shemansky and Ajello (1983), who analyzed one
of the auroral spectra at ∼30 Å resolution obtained with Voyager
UVS. Sandel et al. (1982) and Shemansky and Ajello (1983) used a
modeled spectrum with foreground H2 abundances of 1 × 1020 and
1 × 1016 cm−2 , respectively, to best fit the UVS data. On the other
hand, the low resolution of the UVS observations and the high uncertainty of the vibronic branching ratios available at that time did
not allow an accurate measure of self-absorption. Also, the modeled absorption by H2 used to fit the UVS spectra was based on
single scattering losses only and did not take into account a line
by line treatment.
We present in this paper comparisons between synthetic H2
spectra, generated by a spectral generator previously described by
Dols et al. (2000) and Gustin et al. (2002, 2004a), and UV spectra of Saturn’s aurora obtained with HST/STIS, FUSE, and Cassini
UVIS instruments. Spectra obtained with STIS and the FUV channel of UVIS, with spectral resolution ∼12 and ∼5.5 Å, respectively,
are potentially attenuated by the hydrocarbons overlying the auroral emission. Although methane (CH4 ), ethane (C2 H6 ) and acetylene (C2 H2 ) are known to be present within the atmosphere, only
CH4 has been previously detected from auroral UV spectra in the
1150–1700 Å range (Gérard et al., 2004). FUSE and UVIS spectra in
the EUV have spectral resolutions ∼0.20 and ∼5.5 Å, respectively.
As mentioned above, lines connecting to ν 2 may be attenuated by the H2 molecules overlying the emission. Several series
of synthetic spectra are generated, parameterized by the methane
column density in the FUV and by the H2 temperature and H2
column density in the EUV. Each resulting spectrum is compared
to the data, and the synthetic spectrum that minimizes the chisquare (χ 2 ) is considered as the best fit, providing an accurate
estimate of the auroral characteristics. The pressure level of the
energy deposition of the aurora is then derived from the best parameters and yields the energy of the precipitating electrons, using
a relationship between the energy of the incident electrons and
the H2 column described hereafter. Results obtained with all three
instruments are compared and implications for the auroral characteristics are considered.
2. Observations and data reduction
2.1. STIS
The STIS observations presented here were already processed
and analyzed by Gérard et al. (2004). They consist of six spatially
resolved spectra obtained on 7 and 8 December 2000 with the
G140L grating, combined with the 24.7 × 0.5 arcsec2 slit, providing spectra at ∼12 Å resolution in the 1150–1750 Å spectral range.
They were imaged on the FUV-MAMA photon counting detector
providing 1024 pixels in the spatial direction and 1024 pixels in the
spectral direction. The methodology used to process these spectra
was described in detail by Gérard et al. (2004) and will not be
repeated here. Table 2 of Gérard et al. (2004) summarizes the observation characteristics and their Fig. 6 shows the position of the
slit, superimposed on a 24.7 × 24.7 arcsec2 STIS image obtained on
7 December 2000.
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J. Gustin et al. / Icarus 200 (2009) 176–187
Fig. 2. Viewing geometry of the UVIS observation of Saturn’s aurora from 17 February 2005. The long side of the slit represents the 64 spatial pixels of the UVIS
aperture, while each small rectangle inside the slit can be seen as a 1024 pixels
wide spectrum. The slit was parallel to the planet’s equator, moving from the east
side to the west side of the pole. The slit labeled “A” corresponds to the first record
of 240 s, starting at 14:59:23 UT. The “B” slit corresponds to the middle exposure
(start time: 18:56:23), while the “C” slit represents the last record of the observation (start time: 22:44:13).
Fig. 1. The white square illustrates the field of view of the FUSE LWRS aperture,
using an unrelated STIS image taken on 7 December 2000. Assuming an auroral
oval situated at 75◦ latitude, the viewing angle of the STIS image (which was taken
at the same period as the STIS spectra described in this paper) is 53.1◦ , while the
FUSE viewing angle is 53.4◦ .
2.2. FUSE
Spectra of Saturn’s aurora in the wavelength range 905 to
1180 Å were obtained by FUSE on 15 January 2001, starting at
03:20:06 UT for a total exposure time of 9966 seconds. The low
resolution aperture LWRS (30 × 30 ) was used, providing spectra
at ∼0.20 Å spectral resolution, determined by the small but finite
extent of the aurora in the aperture. The data presented here were
processed using the FUSE pipeline, calfuse, version 2.2.1. Fig. 1
shows the field of view of the LWRS aperture, using an HST/STIS
image obtained on 7 December 2000. It is seen that FUSE collected
a large portion of photons from Saturn’s disc, as well as its rings.
Background emissions from Saturn’s airglow and sunlight reflected
by the rings are identified in the observed spectra. This aspect will
be discussed in Section 4.4.
A detailed description of the FUSE spectrometer has been given
by Moos et al. (2000) and Sahnow et al. (2000). FUSE consists
of four separate spectrographs, two using lithium fluoride coatings (LiF) and two employing silicon carbide (SiC). Each of four
separate detectors, denoted 1a, 1b, 2a, 2b simultaneously records
a spectrum from LiF and a spectrum from SiC, giving a total of
eight separate raw spectra. In this study, we used the LiF1a and
LiF 2a channels. They provide the best signal to noise ratio, while
spanning a major portion of the total wavelength range available.
Unlike STIS or UVIS spectra presented here, FUSE observations do
not provide spatial information of the intensity distribution within
the aperture.
2.3. UVIS
Saturn’s aurora was observed with UVIS on 17 February 2005,
from 14:59:23 UT to 22:48:13 UT. The data consist of a set of 48
consecutive records of 240 s each. The geometry of the observation
is illustrated in Fig. 2. The characteristics of the UVIS instrument
have been discussed in detail by Esposito et al. (2004). In brief,
UVIS includes a two-channel imaging spectrograph, from 1115 to
1912 Å in the FUV, and from 563 to 1182 Å in the EUV. Three
slits are available: a high resolution slit (75 and 100 μm slit width
for the FUV and EUV channel, respectively), a low resolution slit
(150 and 200 μm slit width for the FUV and EUV respectively), and
an occultation slit of 800 μm width, identical for both channels.
The data discussed here were obtained with the low resolution slit
for both the EUV and FUV channels, providing spectra at ∼5.5 Å
spectral resolution. The detector is a Codacon (CODed Anode array
CONverter), consisting of 1024 pixels in the spectral direction and
64 pixels in the spatial direction.
Calibration of the EUV and FUV data follows the pre-flight
measurements as described in Esposito et al. (2004). Additionally,
a background noise level due to the radioisotope thermoelectric
generator (RTG) onboard Cassini is removed and a flat-field correction derived from observations of Spica (Steffl et al., 2004) is
applied. Examination of the raw data shows that the RTG background is ∼4.5 × 10−4 count per spatial and per spectral pixel,
which corresponds to the value determined by other datasets. In
the EUV, two wavelength-dependent contaminating signals affect
the recorded spectra. The first is due to internal instrument scattering of Ly-α , which is focused just beyond the long-wavelength
end of the EUV detector. This contribution to the EUV signal is
small below 1170 Å, and is estimated to be less than 7% of the total
signal, based on comparisons with a modeled H2 spectrum (Ajello
et al., 2005). Our analysis of UVIS in the EUV focuses on the 900–
1170 Å region, where this effect is negligible. The second contamination source is due to a small light leak that allows undispersed
interplanetary Ly-α to reach the EUV detector into one-third of the
detector, corresponding to wavelengths between 920 and 1020 Å.
A recent calibration observation has been performed on 16 April
2007, when the telescope was pointed on the sky. It allowed us to
record this background alone, which was scaled and removed from
the EUV data presented here. For the FUV channel, in flight calibrations from September 1999 to November 2007 have revealed
that above 1550 Å, the FUV detector absolute sensitivity slowly
increases with time. This effect has been considered in a timedependent sensitivity curve (the UVIS team, personal communication) and has been included in our pipeline procedure in order to
convert the raw counts into brightness units of Rayleigh Å−1 .
Each of the 48 raw records has been carefully examined to
determine which spatial pixels were measuring auroral emission.
For each EUV record, the spectral pixels in the 1045–1167 Å
range (which is the brightest portion of the H2 emission in the
EUV bandwidth) are summed, and the result is displayed as a
function of the spatial pixels. We then arbitrarily set an auroral
emission threshold to 6.3 × 10−3 count pixel−2 s−1 and all spatial pixels above this value were assumed to be auroral. The same
procedure is used in the FUV, were all pixels above 7 × 10−3
counts pixel−2 s−1 in the 1270–1740 Å range were assumed to be
auroral. For both channels, the selected pixels are then summed to
create a single EUV and FUV raw spectrum. These resulting spectra
are then processed and used in the present analysis. It should be
UV spectroscopy of Saturn’s aurora with STIS, FUSE and UVIS
179
Fig. 3. UVIS FUV spectrum (black line) and best synthetic fit, absorbed with a CH4 vertical column of 1.4 × 1015 cm−2 (dash–dot green line). When compared to an unabsorbed
synthetic spectrum (dashed blue line), the UVIS spectrum is very slightly absorbed. For comparison, the red curve shows one of the six STIS spectra of Saturn, which is best
fitted with a model absorbed by a vertical methane column of 5.4 × 1015 cm−2 .
noted that UVIS observed Saturn’s nightside northern pole, while
the STIS and FUSE data were acquired from Saturn’s southern dayside aurora.
3. Spectral generator and absorption process
3.1. Spectral generator
The generator of H2 spectra simulates the effects of the impact
of auroral electrons on the atmospheric H2 molecules. It has been
described and used for jovian spectroscopy analysis by Dols et al.
(2000), Gustin et al. (2002, 2004a, 2004b) and Gérard et al. (2002),
and for Saturn’s spectroscopy by Gérard et al. (2004). The generator includes the X 1 Σ g+ , B 1 Σu+ , E , F 1 Σ g+ , C 1 Πu , B 1 Σu+ , D 1 Πu ,
B 1 Σu+ , and D 1 Πu electronic states, whose population is controlled by the direct primary and secondary electron impact on
ground state H2 ( X 1 Σ g+ ), assumed to be in local thermodynamic
equilibrium (LTE). As seen in Fig. 3 of Gustin et al. (2004a), spectra
longward of the atomic H Ly-α line can be described by synthetic
spectra including the Lyman (B– X transitions) and Werner (C – X
transitions) bands only, taking into account the enhanced rate of
excitation of the B 1 Σu+ state due to cascading from the E , F 1 Σ g+
state. Shortward of Ly-α , the modeled spectra require the addition
of the B – X , D– X , B – X and D – X bands to accurately fit the data.
3.2. Hydrocarbon absorption
Methane is the fourth most abundant compound in the atmosphere of giant planets, after H, He and H2 . It is produced
in the troposphere and transported to the stratosphere by convection and turbulent diffusion. Through photolysis, methane is
the source of other hydrocarbons and produces ethane and acetylene, in that order of importance on Saturn (Moses et al., 2000;
Ollivier et al., 2000; Moses and Greathouse, 2005). Methane
has a large wavelength-dependent absorption cross-section in
the 900–1400 Å domain. Accordingly, it partially absorbs auroral emissions in this wavelength range and leaves the emission
above 1400 Å unattenuated (see Fig. 3 of Gérard et al. (2003)
for an illustration of this differential absorption). The color ratio provides a measure of the attenuation of the H2 emission
by methane overlying the emitting layer. From comparisons between the observed spectra and several series of synthetic spectra
attenuated by CH4 , the methane column overlying the auroral
emission is first determined. The model atmosphere of Moses
et al. (2000) is then used to relate the CH4 column obtained
from the best fit to the pressure level and the H2 column overlying the auroral emission peak. The energy deposition of the
precipitated electrons can then be estimated from a stopping
power table, giving the average path length traveled by monoenergetic electrons into H2 , as they slow down to rest. We have
derived a relationship between the primary electron energy and
the H2 column from two sources. The first is the ESTAR database
(http://physics.nist.gov/PhysRefData/Star/Text/ESTAR.html), which
spans the 10 to 106 keV electron energy range. The second is
the two-stream electron transport model of Grodent et al. (2001),
which relates the mean electron energy of the primary electrons to
their penetration depth in Jupiter’s auroral atmosphere. This degradation model is used to determine the electron energy–H2 column
relationship for electrons from 200 eV to 10 keV (since H2 is the
dominant constituent for Jupiter and Saturn, the penetration depth
of electrons is identical for both planets). Results obtained from
the Grodent et al. (2001) model have also been compared to values provided by the stopping power table. It has been found that
the mean energy of the Maxwellian electronic distributions used
in the Grodent et al. (2001) model and the mono-energetic electrons found in the stopping power table provide similar H2 column
values, yielding the energy deposition at the same pressure level
(D. Grodent, private communication). This method has been used
in a slightly different form for jovian auroral studies by Dols et
al. (2000), Gustin et al. (2002, 2004a, 2004b, 2006) and Gérard et
al. (2002, 2003) in order to determine electron energies from hydrocarbon absorption. It was also used by Gérard et al. (2004) to
study the STIS spectra of Saturn also presented here.
3.3. Self-absorption
When an auroral photon is emitted from a given transition
in the direction of the observer, the photon can undergo an absorption by an H2 molecule, following the same transition. This
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J. Gustin et al. / Icarus 200 (2009) 176–187
Table 1
Main results.
a
Spectral range (Å)
FWHM (Å)
CH4 column (cm−2 )b
H2 column (cm−2 )c
Pressure (μbar)d
Temperature (K)e
Mean energy (keV)
H2 brightness (kR)
STISf
UVIS FUV
UVIS EUVg
FUSE LiF1a
FUSE LiF2ah
1230–1620
12
4.2 × 1015 –1.2 × 1016
–
0.17–0.28
–
13.4–17.6
3.3–14.0
1230–1660
5.5
1.4 × 1015
–
0.10
–
10.2
5.8i
910–1170
5.5
–
2.4 × 1020
0.82
700
21.6
2.9i
1030–1080
0.2
–
3.0 × 1019
0.10
500
10.1
139j
1090–1180
0.2
–
6.0 × 1019
0.21
400
14.8
129j
a
Spectral range used for the fitting procedure, not the full spectral range of the observed spectra.
Vertical values, deduced from the FUV spectra, taking into account the angle between the local vertical and the line of sight in planetographic coordinates. The H2
columns deduced from UVIS EUV and FUSE best fits may be used to derive the corresponding vertical methane column, using the Moses et al. (2000) atmospheric model.
We find methane columns of 7.6 × 1016 , 1.3 × 1015 , and 6.3 × 1015 cm−2 for UVIS EUV, FUSE LiF1a and FUSE LIF 2a, respectively.
c
Vertical values, measured from self-absorption for UVIS EUV and FUSE spectra. The methane columns deduced from STIS and UVIS FUV spectra are used determine the
corresponding H2 column, using the Moses et al. (2000) model. We find H2 columns from 5.0 × 1019 to 8.2 × 1019 cm−2 for STIS and 3.1 × 1019 cm−2 for UVIS FUV.
b
Pressure level of the auroral emission, calculated from the hydrostatic law (weight of the H2 column, with g at 75◦ ).
Measured from self-absorption for UVIS EUV and FUSE spectra.
f
Minimum and maximum values obtained from the lesser and most absorbed of the six spectra.
g
The extreme values deduced from the χ 2 limits are 9.7 × 1018 to 1.3 × 1022 cm−2 for the vertical H2 column, which correspond to pressure from 0.033 to 45.2 μbar and
mean electron energies from 4.4 to 342 keV, respectively (see point 4.3 for details).
d
e
h
The H2 column deduced from LiF2a spectrum should be considered as an upper limit value (see Section 4.4 for details).
Although the EUV and FUV observations are simultaneous, the two slits are not perfectly aligned, which explains the different auroral brightness derived for both
channels.
j
As the aurora does not completely fill the LWRS aperture, the FUSE values must take into account a filling factor, estimated to be ∼330. Since the auroral morphology
was unknown during the FUSE observation, this factor is very uncertain and could differ by a factor of ∼5.
i
absorption is followed by a radiative de-excitation, producing a
photon with an energy equal to or lower than the photon initially absorbed. Self-absorption is therefore a fluorescence process
changing the intensity distribution of the H2 lines in the auroral
spectra. A measure of the attenuation of lines affected by selfabsorption gives information on the rovibrational temperature of
the absorbing molecules through the population of the absorbing
rovibrational level, controlled by temperature. It also indicates the
pressure level of the auroral emission through the total number of
absorbing H2 molecules, i.e. the H2 column density. Self-absorption
is included in our spectral model in the form of an optical depth
τs , determined for each vibronic transition:
τs = P X ( v , j) × C H2 × σλ ,
where P X ( v , j ) is the population of the ground rovibrational absorbing level, controlled by the rovibrational temperature of the absorbing molecules, assumed to equal the rovibrational temperature
of the emitting molecules; C H2 is the total number of molecules
crossed by the emitted photons (i.e. the column density), directly
related to the depth of the emission; and σλ is the absorption
cross-section of the transition considered, described by Wolven
and Feldman (1998).
This model has been validated by an H2 laboratory spectrum
obtained at high pressure, simulating self-absorption, and used
previously in the analysis of FUSE spectra of jovian aurora by
Gustin et al. (2004a). The redistribution of spectral intensity is illustrated in Fig. 6 of Gustin et al. (2004a). Comparisons between
synthetic spectra (parameterized by the H2 temperature and H2
column) and observed spectra then allow one to derive the parameters that best fit the observations. The energy of the precipitating
electrons is then derived from the stopping power table mentioned
above. As for the emitting H2 molecules, it is assumed that the
absorbing H2 molecules are in the X 1 Σ g+ electronic ground state
and that their vibrational and rotational populations are in LTE
and follow a Maxwell–Boltzmann distribution. According to models
(Moses et al., 2000) or observations (Smith et al., 1983), rovibrational temperatures in Saturn’s stratosphere are expected to lie in
the range 100–450 K. At a temperature of 400 K, the fraction of
molecules in the v = 1 and v = 2 states compared to v = 0 is
3 × 10−7 and 2 × 10−13 , respectively. This means that in the case of
Saturn, an attenuation of transitions connecting to the v = 0 and
1 levels is expected. Examination of a synthetic spectrum shows
that lines connecting to v = 0, 1 are emitted at wavelengths
shorter than Ly-α , which means that only lines in the EUV part
of the UV spectrum are expected potentially to be absorbed.
4. Analysis
Determination of the characteristics of Saturn’s auroral precipitation is based on a comparison between the observed spectra
and several series of synthetic spectra. It is assumed that the auroral emission is produced in a single emitting layer, characterized
by an average rovibrational H2 temperature. This layer is overlaid
by an absorbing layer of CH4 and H2 . In the FUV bandwidth, CH4
is taken as the only parameter, since it is the major absorbing
species affecting the auroral emission. In the EUV, as the auroral emission is influenced by self-absorption through the overlying
H2 column and the H2 temperature, these variables are used as
parameters in the model. Since they are inter-dependent, nested
loops with H2 and T as variable parameters are employed. It is
supposed here that the primary electrons do not directly participate into the excitation of the H2 molecules producing the aurora.
Through an energy degradation process, primary electrons produce
secondary electrons that effectively excite H2 and create the aurora (see Grodent et al., 2001, for a thorough study of the electron
energy degradation process). Besides, the excitation cross-section
curves of the H2 electronic states are very similar for exciting electrons above 30 eV, meaning that the shape of the H2 spectrum is
moderately influenced by the electron energy variations above this
value. Consequently, we used 100 eV mono-energetic electrons to
simulate secondary electrons and generate the modeled H2 spectra.
This reasoning has been validated by the work of Liu and Dalgarno
(1996), who demonstrated that auroral UV spectra are not a sensitive indicator of the primary electron energy distribution. Each
resulting modeled spectrum is compared with the data and the
model that minimizes χ 2 is considered as the best fit. All the results are summarized in Table 1.
4.1. STIS
As mentioned previously, Saturn’s spectra obtained with STIS
have been analyzed by Gérard et al. (2004). Six spectra of the
UV spectroscopy of Saturn’s aurora with STIS, FUSE and UVIS
southern aurora were compared to synthetic spectra, and columns
of methane have been derived. An example of best fit is displayed
in Fig. 7 of Gérard et al. (2004), and one of the STIS spectra is
reproduced by the red line in Fig. 3. The CR values derived from
these observations are nearly constant, with values from 1.31 to
1.48, compared with 1.1 for an unabsorbed spectrum. The attenuation by CH4 is thus clear in this case, with best fits obtained for
vertical columns of methane from 4.2 × 1015 to 1.2 × 1016 cm−2 .
Using the atmospheric model of Moses et al. (2000), it is seen that
these values correspond to vertical H2 columns from 5.0 × 1019 to
8.2 × 1019 cm−2 which places the auroral emission peak at pressure levels between 0.19 and 0.31 μbar. The mean energy of the
precipitating electrons then lies in the range 13.4 to 17.6 keV. We
also tested models including C2 H2 and C2 H6 , likely important on
Saturn. The fits were not improved, confirming the small amount
of these hydrocarbons compared to CH4 . Using a synthetic spectrum, the total brightness of the STIS spectra, converted to an
unattenuated H2 emission in the 800–1700 Å bandwidth, ranges
from 3.3 to 14 kR.
4.2. FUV channel of UVIS
The UVIS FUV spectrum we analyzed is shown by the black line
in Fig. 3. The observed spectrum is little absorbed, with a best fit
obtained for a vertical CH4 column of 1.4 × 1015 cm−2 , shown in
green in Fig. 3. Comparison with a model unaffected by methane
(dashed blue line) clearly shows that the addition of weak absorption improves the fit. The vertical H2 column density derived from
the hydrocarbon absorption is 3.1 × 1019 cm−2 , which corresponds
to a pressure level of 0.11 μbar and precipitating electrons with
a mean energy of ∼10 keV. The unattenuated H2 emission in the
800–1700 Å range provides a total brightness of 5.8 kR, in accord
with those obtained with STIS.
4.3. EUV channel of UVIS
The UVIS EUV spectrum has been compared to several series of
synthetic spectra, with the H2 column and temperature as free parameters. The vertical methane column was set to 1.4 × 1015 cm−2 ,
i.e. the value determined from the UVIS FUV channel. Comparison
between the observation (black curve in Fig. 4a) and a model without self-absorption (dashed green curve in Fig. 4a) shows that Saturn’s spectrum undergoes strong attenuations below 1060 Å, while
the intensity of some observed lines above 1110 Å are increased.
Lines with clear attenuation or intensification have been identified
with the help of modeled spectra and are listed in Table 2. It is
seen that all the attenuated lines (900–1060 Å) connect to v = 0
and v = 1 vibrational levels, with stronger absorption for lines
connecting to v = 0. Above 1110 Å, all the lines amplified by fluorescence connect to v = 3 and v = 4. These considerations well
illustrate the intensity redistribution caused by self-absorption. The
peak around 1100 Å is more complex to interpret. Comparison with
the unabsorbed synthetic spectrum shows that this peak, mainly
due to W (0–2) transitions, is slightly intensified. While most lines
connected to v < 2 are attenuated and v = 3 and 4 connected
transitions are intensified, lines connecting to v = 2 can be seen
as an intermediate case, since they do not exhibit clear absorption
or intensification.
An extensive study of self-absorption has been made by Gustin
et al. (2004a), who analyzed FUSE spectra of the jovian aurora.
Their Fig. 15 shows the effect of self-absorption on lines connecting
to v 3, for temperatures of 400 and 900 K, in the 1090–1180 Å
window. Panels a, b and d of their Fig. 15 show that for both temperatures, an increase in the total H2 column implies an attenuation of lines connecting to v = 0, 1 levels, while lines connecting
to v = 3 are brightened. Panel c of Fig. 15 shows that at 400 K,
181
lines connecting to v = 2 slightly brighten as the H2 column increases. This means that the v = 2 vibrational level is virtually
unpopulated at 400 K and thus cannot absorb photons, whatever
the amount of H2 attainable. When the rovibrational temperature
is in the 400 K range, transitions connecting to v = 2 can only
be enhanced through fluorescence. At 900 K, H2 columns below
5 × 1019 cm−2 tend to brighten lines connecting to v = 2, while
higher H2 columns start to absorb these lines. The best fit is obtained for T = 700 K and a vertical H2 column of 2.4 × 1020 cm−2
(red line in Fig. 4a) with a χ 2 value of 0.0307. Features near 1027
and 1053 Å, strongly self-absorbed, are well reproduced, as well
as brightened lines near 1145 and 1161 Å. Apart from substantial
discrepancies below 950 Å, the overall best fit shows major improvements compared to the unabsorbed model. In particular, it
exhibits significant improvements near 1118, 1146 and 1161 Å, implying that the brightening due to fluorescence is well modeled.
Previous studies showed that a minimum spectral resolution of
∼1 Å is required to derive a rotational temperature (Gustin et al.,
2002). At ∼5.5 Å spectral resolution, the UVIS spectrum does not
resolve the rotational lines and does not allow one to unequivocally determine a best T –H2 column pair. In order to determine
a range of satisfactory values of these parameters, the χ 2 variations with temperature and H2 column have been examined and
are displayed in Fig. 5. The best fit is indicated by a star symbol in
Fig. 5, as shown in Fig. 4a. Two other models (square symbols in
Fig. 5) at 500 K—1.2 × 1020 cm−2 (dashed orange line) and 900 K—
1.2 × 1019 cm−2 (dashed blue line) are compared to the UVIS EUV
data in Fig. 4b. A detailed examination shows that both models in
Fig. 4b, although still acceptable with χ 2 < 0.034, are less satisfactory than the best model (χ 2 = 0.0307) in red in Fig. 4a. We found
that models with χ 2 > 0.034 started to show some major discrepancies. We thus arbitrarily consider T –H2 column pairs with
χ 2 0.034 as satisfactory models to the UVIS EUV data. In the
lower left corner in Fig. 5 we also show the T –H2 relationship obtained from the Moses et al. (2000) low-latitude atmosphere. It is
seen that for a given H2 column, temperatures associated with the
Moses et al. (2000) model are significantly lower than the values
derived from our regression analysis. This point will be discussed
in Section 5 of the paper. The pressure level corresponding to our
best fit, derived from the weight of the observed H2 column, is
0.82 μbar. Using our H2 column–electron energy relationship, this
corresponds to an electron mean energy of 21.6 keV. If we take
into account the wide range of H2 column values limited by models with χ 2 < 0.034, we obtain a range of H2 vertical columns
from 9.7 × 1018 to 1.3 × 1022 cm−2 , corresponding to pressure levels from 0.033 to 45.2 μbar. These values correspond to primary
auroral electrons with mean energies ranging from 4.4 to 342 keV.
4.4. FUSE
This section describes the analysis of two FUSE LiF segments,
which provide the highest signal to noise ratio. The first is the
LiF1a segment in the 1030–1080 Å spectral window. The spectrum below 1030 Å is ignored to avoid the bright H-Ly β line at
1025.73 Å. The second spectrum is the entire LiF2a segment in the
1090–1180 Å window. The SiC spectra below 1000 Å were found to
be too noisy to provide valuable information on the auroral characteristics and won’t be discussed here. For both analyzed spectra, a
CH4 column was not determined, because of the small variation of
the methane cross-section with wavelength in these spectral windows.
The LiF1a spectrum is presented by the black line in Fig. 6. The
spectrum is contaminated by solar pumped fluorescence lines, as
well as solar lines reflected from Saturn’s disc and rings, identified
by vertical green lines. The best fit, shown by the red line in Fig. 6,
is characterized by a rovibrational H2 temperature of 500 K, as-
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J. Gustin et al. / Icarus 200 (2009) 176–187
Fig. 4. (a) UVIS EUV spectrum (black line) and models. When compared to a synthetic spectrum without self-absorption (dashed green line), it is seen that below 1060 Å,
the observed lines connecting to v 1 are attenuated, while lines near 1118, 1146 and 1161 Å, connecting to v 2, are brightened. The best fit, in red, is obtained for
a temperature of 700 K and a vertical H2 column of 2.4 × 1020 cm−2 . (b) The spectral resolution of UVIS (∼5.5 Å) does not allow us to resolve the rotational lines. As a
consequence, many (T , H2 column) combinations can offer a satisfactory fit to the data as shown here. See the text for a more detailed discussion on the fit quality.
sociated with a vertical H2 column of 3.0 × 1019 cm−2 . Compared
to a model without self-absorption (dashed curve in Fig. 6), the
best fit exhibits major improvements, especially near 1052.5 and
1054 Å. The main lines influenced by self-absorption, which correspond to those identified in the UVIS EUV spectrum, are listed in
Table 2. The effect of self-absorption at this resolution and for this
spectral window is illustrated in Fig. 11 of Gustin et al. (2004a),
who analyzed FUSE spectra of the jovian aurora. This figure shows
that the lines affected by self-absorption in the LiF1a window connect to v = 0 and v = 1. The modeled spectrum fits the data
very well, especially where self-absorption has a major effect, i.e.
near 1052.5 and 1054 Å. The major discrepancies are explained by
the contamination by the fluorescence and solar lines. Unlike UVIS,
FUSE provides spectra at high resolution (∼0.20 Å) that allow us to
resolve the rotational lines, which means that spectra at this res-
olution are sensitive to temperature variations. The W (0–1) P (3)
line at 1058.82 Å is particularly responsive to temperature and is
a good indicator of the overall rovibrational H2 temperature. The
T –H2 column pair determined by the best fit can be considered as
the unique solution of the fitting regression, since small parameter variations could clearly affect the fit and be visually identified.
Although not necessary, we tested models where the temperature
controlling the absorbing and emitting molecules is different. This
did not improve the fit, which suggests that the emission and
absorption of auroral photons occur approximately at the same altitude, as expected from the rapid drop of H2 density with altitude.
As seen in Fig. 1, the low resolution aperture of FUSE integrates a
large portion of the H2 airglow from Saturn’s disc and limb. In order to see if the airglow has a significant influence on the best
parameters we obtained, we used a modeled H2 spectrum of Sat-
UV spectroscopy of Saturn’s aurora with STIS, FUSE and UVIS
183
Fig. 5. Map of the χ 2 variations with the H2 rovibrational temperature and H2 column obtained from the comparison between the UVIS EUV spectrum and several series of
synthetic spectra. The star corresponds to the best fit displayed in Fig. 4a, the two squares correspond to cases shown in Fig. 4b and discussed within the text. Temperatures
and H2 column pairs inside the bold line (χ 2 0.034) are considered to provide a good fit to the data. The dashed line shows temperatures and H2 column from the Moses
et al. (2000) atmosphere.
Fig. 6. FUSE LiF1a spectrum and best fit model at T = 500 K and H2 vertical column of 3 × 1019 cm−2 . Comparisons between the best fit and a synthetic spectrum without
self-absorption (dashed curve) clearly demonstrate the effects of self-absorption, principally near 1052.5 and 1054 Å. The main discrepancies at 1031.86 Å (W (1, 1) Q (3)),
1037.15 Å (L (5, 0) R (1) + P (3)) and 1071.62 Å (L (6, 1) P (1)) correspond to H2 lines pumped by the solar emission (Liu and Dalgarno, 1996). It should be noted that the H2
lines pumped by the solar NIII line at 989.79 Å are mislabeled as solar pumped NII in Liu and Dalgarno (1996).
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J. Gustin et al. / Icarus 200 (2009) 176–187
H2 molecules is different. They did not improve the best fit. The
upper limit of the H2 column gives an upper limit for the precipitating electron mean energy of 14.8 keV.
Table 2
Main spectral lines affected by self-absorption at Saturn.
Attenuated lines
Intensified lines
Wavelength (Å)
Identification
Wavelength (Å)
Identification
960.449
965.062
967.673
969.088
970.560
976.548
978.217
985.642
986.519
987.767
991.378
996.124
997.638
997.824
1006.340
1008.383
1014.504
1017.001
1019.500
1028.246
1040.058
L (13–0) P (3)
W (2–0) R (1)
L (12–0) R (3)
L (12–0) P (3)
W (2–0) P (3)
W (2–0) P (5)
L (11–0) P (3)
W (1–0) R (1)
L (11–0) R (5)
L (10–0) P (3)
W (1–0) P (3)
L (10–0) P (5)
W (1–0) P (5)
L (9–0) P (3)
L (9–0) P (5)
L (8–0) P (3)
W (0–0) P (3)
L (8–0) P (5)
L (7–0) P (3)
L (7–0) P (5)
L (6–0) P (5)
1097.911
1098.115
1099.420
1100.521
1102.162
1104.328
1116.407
1145.904
1161.295
1193.246
1206.639
W (0–2)
W (0–2)
W (0–2)
W (0–2)
W (0–2)
W (0–2)
W (1–3)
W (0–3)
W (1–4)
W (0–4)
W (1–5)
R (1)
R (0)
Q (1)
Q (2)
Q (3)
Q (4)
Q (1)
Q (1)
Q (1)
Q (1)
Q (1)
At temperatures near 500 K and absorbing H2 columns of 3 × 1019 cm−2 , most
of the lines connecting to the v = 0 vibrational level of ground H2 are saturated
and drop to intensities 1% of the unabsorbed values. Very strong lines connecting
to v = 1 are reduced buy a factor 2 to 4 while lines connecting to v > 1 may
be slightly intensified by the fluorescence process of self-absorption. L stands for
Lyman and W for Werner H2 lines.
urn’s airglow (Hallett et al., 2005), convolved at FUSE resolution.
The intensity of the airglow spectrum was scaled to the total intensity of the FUSE auroral spectrum. We then removed various fractions of airglow from the auroral spectrum, and applied our fitting
procedure to the spectrum so obtained. We concluded from this
technique that our best fit parameters are not significantly influenced by the removal of various fractions of airglow emission. The
H2 column obtained from the best fit is 3.0 × 1019 cm−2 , which
places the bulk of the energy deposition near 0.1 μbar. This corresponds to precipitating electrons with a mean energy of 10.1 keV.
The second FUSE auroral spectrum we analyzed is the LiF2a,
shown by the black line in Fig. 7. A few lines due to pumped Ly
β and solar NIII and OVI are clearly identified in the spectrum,
as considered by Liu and Dalgarno (1996). The best fit obtained
from several series of synthetic spectra is shown by the red line in
Fig. 7. It is characterized by a rovibrational temperature of 400 K
and a slant H2 column of 1 × 1020 cm−2 , corresponding to a vertical H2 column of 6 × 1019 cm−2 . The fit is excellent and the main
discrepancies are explained by the fluorescence lines present in
the spectrum. Examination of the χ 2 variations show that in the
100–500 K range, the regression procedure is much more sensitive to temperature variations than to H2 column variations. This
is confirmed by Fig. 15, panel e of Gustin et al. (2004a), which
shows that at 400 K, the brightness of lines connecting to v = 0,
1, 2 and 3 does not vary significantly in the 1090–1180 Å window, when the H2 column does not exceed 1 × 1020 cm−2 . Indeed,
line to line comparisons between models at 400 K at various H2
column values (1 × 1020 cm−2 ) did not show substantial deviations. Comparisons between the observation and models started
to exhibit discrepancies for columns higher than 1 × 1020 cm−2 ,
increasing the χ 2 values at the same time. This reasoning demonstrates that H2 models in this bandwidth are not sensitive to H2
columns 1 × 1020 cm−2 . The slant H2 column of 1 × 1020 cm−2
of the best fit should then be considered as an upper limit of the
absorbing H2 column. It should be said that this reasoning is valid
for temperatures near 400 K. Fig. 15 of Gustin et al. (2004a) clearly
shows that at 900 K, the H2 column starts to change the spectrum
shape for H2 columns near 1 × 1017 cm−2 . We also considered
models for which the temperature of the emitting and absorbing
5. Summary and conclusions
Comparisons between modeled and observed H2 spectra are
an effective tool which provides qualitative and quantitative information on the auroral characteristics. Two atmospheric processes
have been used to derive the energy of the precipitating electrons in Saturn’s atmosphere: absorption by hydrocarbons through
FUV spectra, and self-absorption through EUV spectra. The temperature of the absorbing H2 molecules can also be derived from
self-absorption, provided the spectral resolution is sufficiently high,
which is the case with the FUSE observations.
STIS and UVIS FUV spectra were found to be very little absorbed by methane, with vertical CH4 columns less than or equal
to 1.2 × 1016 cm−2 . Using the low-latitude atmospheric model of
Moses et al. (2000), the CH4 column derived from STIS and UVIS
FUV have been used to estimate the H2 column overlying the emission hence the depth of the auroral emission. From a relationship
between electron energy and H2 column, we find that the energy
of the primary electrons lies in the range ∼10–18 keV. This is
consistent with the value of ∼10 keV obtained by Sandel et al.
(1982) from a particularly bright H2 spectrum of Saturn’s aurora
obtained with Voyager UVS, absorbed by a methane column of
8 × 1015 cm−2 . Besides, the other, weaker UVS spectra they analyzed did not require any CH4 absorption in the modeling process.
This suggests that the peak of the UV auroral emission originates
above but close to the methane homopause, and primary energies
near ∼18 keV should be seen as an upper limit value. The variability of the methane absorption from one case to another can be
explained by the intrinsic variability of the aurora, as described in
the introduction.
In the EUV, FUSE and UVIS spectra clearly exhibit self-absorbed
H2 bands. Comparison between the un-self-absorbed model and
the UVIS EUV spectrum we analyzed demonstrates strong attenuation of lines connecting to vibrational levels 0 and 1 of X 1 Σ g+ , and
an intensification of lines connecting to v > 1. The model best
fitting the UVIS EUV spectrum is characterized by a temperature–
vertical H2 column pair of 700 K–2.4 × 1020 cm−2 . Because of its
relatively low spectral resolution (∼5.5 Å), this spectrum does not
resolve the H2 rotational lines and the T –H2 column pair providing the best fit to UVIS EUV is not unique. The χ 2 boundaries used
to limit the temperature and H2 column ranges were found to be
insufficient to constrain their extent to a satisfactory level and provide valuable quantitative results.
Alternatively, the high resolution of the FUSE EUV spectra allows the determination of a H2 rotational temperature. The LiF1a
spectrum qualitatively supports conclusions based on UVIS EUV
data, with attenuation of lines connecting to v = 0 and 1. It is
best fitted with a model at 500 K and 3.0 × 1019 cm−2 of H2 .
The LiF2a spectrum does not reveal self-absorbed lines and is best
fitted with a model characterized by a H2 temperature of 400 K.
Most of the lines within the LiF2a bandwidth connect to v = 2,
and the absence of self-absorption indicates that this vibrational
level is virtually not populated at 400 K, as expected from the
Maxwell–Boltzmann vibrational distribution used in this study. At
400 K, self-absorption is detectable in the LiF2a segment for a H2
column of 6.0 × 1019 cm−2 , which sets the H2 column upper limit
to this latter value. The H2 columns deduced from FUSE LiF1a and
LiF2a lead to primary auroral electron energies of 10 and 15 keV,
respectively. The best fits to the FUSE spectra were obtained assuming LTE, and no evidence of departure from thermodynamical
equilibrium has been detected. This result is in agreement with
Melin et al. (2007), who fitted H+
3 infrared (IR) spectra adopting
UV spectroscopy of Saturn’s aurora with STIS, FUSE and UVIS
185
Fig. 7. FUSE LiF2a spectrum and best fit model at T = 400 K without self-absorption. A H2 vertical column of 6 × 1019 cm−2 establishes its upper limit, since models near
400 K are not significantly sensitive to H2 vertical column below this value. Pumped lines at 1118.61, 1119.08, 1163.80 and 166.76 Å are identified as L (6, 2) P (1), W (1, 3) Q (3),
W (1, 4) Q (3) and L (6.3) P (1), respectively (Liu and Dalgarno, 1996).
LTE. We were able to obtain good fits assuming that the temperature of the emitting and absorbing molecules are equal. This
suggests that self-absorption occurs at the altitude of the emission.
Owing to the absence of self-absorption in the LiF2a spectrum,
this segment is ideal to derive the H2 temperature. The 400 K we
find is in accord with the 350–500 K obtained from H+
3 IR spectra, measured at the H+
3 peak altitude by Melin et al. (2007). These
values are significantly higher than the ∼125 K expected from the
Moses et al. (2000) equatorial model near 0.15 μbar, which suggests that an equatorial temperature profile is not suited to polar
latitudes. This can be explained by additional heat sources expected at the poles, such as Joule heating, particle precipitation
heating and ion drag heating, in addition to the solar EUV input.
This scenario has been studied by Smith et al. (2005). From a 3-D
general circulation model, they were able to reproduce both the
exospheric temperature of ∼400 K at 30◦ N derived from Voyager
solar occultation measurement (Smith et al., 1983) and the auroral H+
3 temperature of 350–500 K obtained by Melin et al. (2007),
assuming a polar heating input of ∼8 TW peaking at ∼60 nbars.
Our analysis shows that methane absorption in the FUV and
self-absorption in the EUV, which are independent processes, both
provide the same range of primary electron mean energies, from
∼10 to ∼18 keV, corresponding to an auroral emitting layer located between 0.10 and 0.28 μbar as illustrated in Fig. 8. This
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J. Gustin et al. / Icarus 200 (2009) 176–187
Fig. 8. Equatorial atmospheric model of Moses et al. (2000). The horizontal lines show the pressure level of the auroral emission peak as listed in Table 1, determined from
UVIS-FUV, STIS and FUSE spectra. The two dashed lines corresponding to STIS represent the minimum and maximum pressure levels deduced from the six STIS spectra
analyzed. Because of the too broad range of H2 columns derived from the UVIS-EUV spectrum, a precise pressure level has not been obtained.
agreement suggests that the methane mixing ratio profile from
the Moses et al. (2000) equatorial model is, to a first approximation, relevant to describe the polar atmosphere. On the other
hand, the auroral temperatures we obtain suggest that the Moses
et al. (2000) temperature profile is not appropriate for polar regions. This aspect is important, since higher temperatures not only
affect the hydrocarbon chemistry but also the atmospheric structure and homopause level. It is difficult to infer how a realistic
polar atmospheric model would modify the primary electron energy derived from methane absorption, since multiple parameters
(that may have opposite effects) are involved. This facet of the
problem is beyond the scope of this paper, and it should be emphasized that FUV results are tied to a given methane mixing ratio
profile, which is not well known in polar regions.
An estimate of the total unabsorbed H2 brightness in the 800–
1700 Å bandwidth has been derived from the synthetic spectra
that best fit the observations. We obtain spatial and temporal averaged brightnesses from 2.9 to 14 kR for STIS and UVIS, in the
range of the 1 to 100 kR determined by Clarke et al. (2005) from
STIS FUV images. Higher brightnesses are derived from FUSE spectra (129 and 139 kR for LiF1a and LiF2a, respectively), similar to
the 150 kR value obtained by Gérard et al. (1995) using the Faint
Object Camera (FOC).
When compared to results obtained from the analysis of STIS
(Gérard et al., 2003) or FUSE (Gustin et al., 2004a) spectra of jovian
aurora, the present study shows that the H2 columns at the auroral
peak are one order of magnitude lower for Saturn, with a bulk of
the energy deposition situated higher in the atmosphere. Indeed,
the energy of the precipitated electrons in the case of Saturn is
modest, compared to the 40–210 keV determined for Jupiter. This
is consistent with theoretical studies, predicting accelerated magnetospheric electrons of a few keV to a few tens of keV in the case
of Saturn (Cowley et al., 2004b, 2004a) and primary electron energies of order 50–100 keV in the case of Jupiter (Cowley and Bunce,
2001; Cowley et al., 2005). It should be noted that our limited set
of observations, with a total of 8 spectra from the south dayside
aurora and 2 spectra from the north nightside aurora, does not allow us to formulate a statistically significant picture of the auroral
characteristics with respect to the location (north/south, day/night)
and time of the emission. The variations of the auroral character-
istics with time and location is an important point that should be
examined in a near future with the help of more recent observations made by the UVIS spectrometer.
Acknowledgments
We thank the two referees for their valuable comments and
suggestions.
This work is based on observations with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute
(STScI), which is operated by the AURA, Inc., for NASA, observations by the NASA–CNES–CSA FUSE mission operated by the Johns
Hopkins University and observations by the Cassini mission. Financial support to US participants was provided by NASA Contract
NAS5-32985 and with the Cassini Project. J.-C.G. and D.G. acknowledge support from the Belgian Fund for Scientific Research (FNRS).
The PRODEX program of ESA provided financial support for this
research. We also thank Julianne Moses for providing us with an
electronic version of her model of Saturn’s atmosphere, Janet Hallet for providing us with a synthetic spectrum of H2 airglow and
Kristopher Larsen for the help in Fig. 2.
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