UVIS Saturn Atmosphere
Occultation Prospectus
D. Shemansky
J. Hallett
X. Liu
07/14/05
Solar vs Stellar Occultations
• Stellar occultations can be obtained using the FUV and EUV
spectrographs. This allows explorations of the transmission spectrum
in the range 91.3 – 190. nm, with sensitivity to H2 and hydrocarbon
absorbers. Major disadvantage: Cannot directly measure atomic
hydrogen.
• Solar occultations are obtained in the EUV spectrograph in the spectral
range 55. – 115. nm. Major advantage: Obtains absorption in the
ionization continuum of all species, including atomic hydrogen. Major
disadvantage: Differentiation of hydrocarbon species is difficult in the
EUV.
• Both kinds of occultations at similar latitudes are desirable.
Analytic capability
• A stellar occultation has been obtained in
Rev 6, providing the first direct test of
measurement capability for a hydrogen
dominated atmosphere for the UVIS
experiment. Previous stellar occultations
have been obtained at Titan [Shemansky et
al., Science, 308, 978, 2005].
The Rev 6 Saturn Occultation
• This occultation was under reaction wheel control.
The data shows no indication of pointing drift, and
provides high quality results.
• The analysis to date has been confined to the
region above the hydrocarbon homopause.
Analysis at lower impact parameters is deferred
until the upper thermospheric structure is fully
understood.
1.2
Alt: 855 km
T = 190 K; C (H 2) = 6.x1020 cm -2
1.0
Vib. Distribution: F13
0.8
0.6
0.4
0.2
0.0
900
950
1000
1050
Wavelength (D)
1100
1150
EUV2005_DELTOR_14REC_FIT1080_v1_a_1
1.2
1.1
H = 1080
T = 270 K, C(H2) = 3.5x1019 cm-2
1.0
0.9
I/I0
0.8
0.7
0.6
0.5
0.4
Same Vib Dist. as 1020 km
0.3
0.2
900
950
1000
1050
1100
1150
 (A)
Rev 6 EUV transmission spectrum at 1080 km vs H2 model.
Model constructed with individual rotation line profiles,
of order 10000 absorption lines in this spectrum.
VIB_EUV2005_FIT_all_a
100
h = 1020 km fit
H = 850 km
H = 795 km
H = 965 km and 907 km
10-1
10-2
10-3
10-4
10-5
10-6
10-7
10-8
10-9
0
2
4
6
8
10
Vibrational quantum level
12
14
Capability for determination of
the state of the hydrogen gas
• The Rev 6 transmission data contains no
detectable absorbers other than hydrogen down to
an altitude of 800 km.
• The absorption spectrum is analyzed using a
hydrogen physical model developed at USC. The
model includes line shape dependence on
perturbations, and has an estimated accuracy of
5% in fundamental parameters [Hallett,
Shemansky, and Liu. Ap. J., 624, 448, 2005].
Hydrogen is the most accurately known absorber
of all species involved in these occultations.
Extraction of the state of the H2
gas
• Direct experience with the experimental
data shows sensitivity to rotational
temperature and vibrational population
distribution. This allows two methods of
extracting gas kinetic temperature.
• The reduction process requires iterative
modeling of the transmission spectra, and is
labor intensive.
H2_syn_comp_04
Derived H2 densities from rev 6 stellar occultation
2000
H2_SYN_COMP_04
LINDAL_85_F3_EGR
smith_etal_83
FESTOU_ATREYA_82_N
1800
1600
h (km)
1400
1200
1000
800
600
400
200
0
7
9
11
13
15
17
19
21
log([H2])
Most recent iteration of hydrostatic model fit to Rev 6
occultation data. Strong heating is indicated above 1000 km.
H2_syn_comp_04_T
Derived temperature distribution from rev 6 occultation
H2_SYN_COMP_04
smith_etal_83
LINDAL_85_F4_ING
LINDAL_85_F4_EGR
FESTOU_ATREYA_82_T
2000
1800
1600
h (km)
1400
1200
1000
800
600
400
200
0
0
100
200
300
400
500
T (K)
Derived temperature distribution from rev 6 occultation
merged into Lindal et al. (1985) below 400 km.
Conclusions to date from Rev 6:
Upper thermosphere
• H2 X(v) is non-LTE by multiple orders of
magnitude. Rotation populations are thermal
below the exobase. Excitation of H2 X(v) by hot H
is required to explain large non-LTE excursion
[Hallett, Shemansky, & Liu, in prep].
• Strong heating above 1000 km is caused by
electron impact dissociation of H2 followed by
heat transfer from 60000 K atomic hydrogen
product [ Shemansky, Hallett, & Liu, in prep].
• Atomic hydrogen from top of Saturn atmosphere
is flooding the magnetosphere.
Expected primary results from
solar occultation
• Determination of upper thermosphere
atomic hydrogen distribution; Location of
diffusive separation.
• Definitive determination of energy
deposition magnitude and distribution.
• Loss rate of atomic hydrogen into the
magnetosphere.
Current limitations
• Knowledge of hydrocarbon absorption cross
section temperature dependence is limited,
compromising determination of accurate
absolute abundance and abundance profiles
[see Shemansky et al., Science, 308, 978,
2005]