Ultraviolet spectroscopy of giant planets’s aurora
with
HST/STIS, FUSE and Cassini/UVIS
J. Gustin, J.-C.Gérard, W. Pryor, P.D. Feldman, D. Grodent and B. Bonfond
EUROPLANET meeting – July 6th 2006
What can be learned from auroral UV spectroscopy?
* Infos on the auroral atmosphere:
- nature and amount of HCs above the auroral emission
- physical conditions at (and above) the altitude of the emission
(T(H2) and ETL conditions)
* Infos on the energy and flux of the precipitating particles
How?
* Comparisons between observed and synthetic spectra: model parameters
that  bestfit = auroral parameters
* 2 physical processes : Color Ratio (HC absorption) & Self-Absorption (H2
absorption)
Synthetic spectra generator: overview
eH2
Initial H2 is ETM
Excited H2:
( B, B ' , B")1  u

X 1g
H2
*
v”=0
E, F 1g
C , D , D ' 1 u
UV photons
Vibronic transitions

Rydberg transitions
 [750, 1750 Å]
<1200 Å: EUV
>1200 Å: FUV
1. COLOR RATIO
σ(CH4)
Measure of the HC absorption:
modeled or lab H2 spectrum (pure )
Color ratio:
(CR)
I (1550  1620Å)
I (1230  1300Å)
= unabsorbed auroral spectrum
0  
hydrocarbons (CH4)  partial absorption
Beert-lambert – type law: I   I  e
Unabsorbed spectrum: CR = 1.1
Interests of CR:
. CH4 column
. model atmosphere: CH4  H2
(Jupiter: Grodent et al., 2001 (aur))
altitude of the auroral emission peak
(Saturn: Moses et al.,2000 (equ))
. Jupiter: model of e- degradation
(Grodent et al., 2001)
. Saturn: stopping power table (e- in H2)
CR  <E>
mean energy <E>
of the precipitating
primary e-
Application to spatially resolved STIS spectra of Jovian
aurora (>100 spectra - 1, 5 and 12 Å resolution)
slit
limb
polar cap
main oval
extraction
Io footprint & trail: <E> ~ 50 keV (very stable)
Main oval: <E> ~ [40 – 210 keV]
Polar regions: <E> ~ [50 – 150 keV]
From 1 Å-res spectra: T ~ [320 – 650K]; T  with CR
Application to STIS spectra of Saturn aurora
(6 spectra available, at 12 Å resolution)
Mean CR: 1.39 (CH4 ~ 4.5 x1015 cm-2)  P ~0.2 μbar
 <E> ~12 keV
2. SELF-ABSORPTION
Auroral absorption by H2 above the emission (fluorescence)
 redistribution of energy at longer 
- Each ro-vibrational level of ground H2 can absorb: optical depth defined for each
transition (v”, J”)  (v, J):
s= popX(v”, J”) * H2 column
*

 SA gives infos on the absorbing T & on the depth of the emission (and thus <E>)
- In the synthetic spectrum:
transmission function Tr()
for each transition
No need of an atmospheric model to find <E> (stopping power table)
Application to FUSE (Far Ultraviolet Spectroscopic
Explorer) observations
- higher  resolution (~0.2 Å)
Compared to STIS :
- access to EUV (900-1200 Å )  SA
- spectra not spatially resolved
1. FUSE spectra of Jovian aurora (1030-1080 Å):
(1030-1080 Å)
T(H2): 800K
H2 col: 1.5 x 1020 cm-2
Pressure: 1 bar
2. FUSE spectra of Saturn aurora (1030-1080 Å) & (1090-1180 Å ):
(1030-1080 Å)
T(H2): 400K (res)
H2Col: 2.5x 1019 cm-2
(1090-1180 Å):
. No self-absorption
(T & colH2 < Jupiter
and v”=2)
. T(H2): 380K
. Sat. results not published
Application of SA to Cassini/UVIS Saturn observation
UVIS combines some of STIS & FUSE advantages:
. access to FUV & EUV  CR & SA at the same time
. spatially resolved spectra
. medium  resolution (5 Å)
STIS
FOV:
FUSE
UVIS
UVIS auroral spectrum of Saturn (EUV):
Results are close to FUSE parameters, but T & H2 col are not uniquely determined
STIS: P~0.2 b
 <E>~12 keV
FUSE: colH2 ~2-3x 1019 cm-2
 <E>~8-10.5 KeV
UVIS: colH2 ~5x 1019 cm-2
 <E>~14 Kev