Spectro-imaging observations
of H3+ on Jupiter
Emmanuel Lellouch
Observatoire de Paris, France
H3+ in planetary atmospheres
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Discovered in 1988 in Jupiter’s auroral regions (Drossart
et al. 1989) from its 22 emission
Since then, also discovered in Saturn and Uranus, and in
Jupiter’s low latitude regions
General goals on the observations (esp. Jupiter)
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Characterize the morphology and variability of the emission
Interpret the emission rates in terms of H3+ column density and
temperature
Measure winds from Doppler shifts on H3+ emissions
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Bands observed so far on Jupiter: 2, 22, 22 - 2

H3+ traces and drives the energetics and dynamics of Giant
Planets upper atmospheres
(see Steve Miller’s talk)
Outline
1.
2.
3.
Detection of the 3 2 - 2 band on Jupiter
The question of LTE and the multiplicity of H3+
« temperatures »
The spatial variations of the H3+ emission: do they trace
variations in the H3+ column or « temperature »?
Spectro-imaging observations
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Imaging FTS « BEAR » at Canada-France-Hawaii
Telescope
Sept. 1999 & Oct. 2000
2 filters
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2.09 µm : 4760-4805 cm-1
2.12 µm : 4698-4752 cm-1
14 data cubes (2 spatial, 1 spectral dimension) sampling
either the Northern or the Southern auroral region of
Jupiter at various longitudes
See detailed report in Raynaud et al. Icarus, 171, 133
(2004)
Example of spectrum: 2.09 µm range
22 band
Example of spectrum: 2.12 µm range
22 and 32-2 bands
H2 S1(1)
The two 32 - 2 lines
Obs. freq. (cm-1) Calc. Freq. (cm-1) Assignment E’ (cm-1)
(Neale et al. 1996)
--------------------------------------------------------------------------------4721.79
4722.383
323 - 2 R(6,7) 7993.60
4749.66
4749.937
323 - 2 R(5,6) 7998.64
---------------------------------------------------------------------------------
Temperature/column density determinations
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Classical LTE formulation
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This effectively assumes complete LTE (Trot = Tvib= Tkin)
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Justified for rotational distribution (rad ~ 1000 sec, coll ~ 10-3 sec)
For vibrational distribution, rad ~ 10-2 sec  non-LTE; however
earlier studies (Y.H. Kim et al. 1992) have found that:
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All the nv2 levels are underpopulated w.r.t. ground state
But their relative populations are close to LTE distribution : « quasi-LTE »
distribution
Here:
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2.09 µm observations (2 2 lines only) determine Trot in v2 =2 level
2.12 µm observations (including the 3 2 - 2 lines) determine Tvib
(relative populations of v2 =2 and v2 = 3 levels)
Temperature results
Tvib
 Trot
Central meridian longitude
In average, Trot = 1170+/-75 K > Tvib = 960 +/- 50 K
 underpopulation of v2 =3 relative to 2 = 2 with respect to LTE case
Trot and N (H3+) results
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We get Trot (22) = 1170+/-75 K, and N = (4-8)x1010 cm-2
Quite different from Lam et al. 1997: Trot (2) = 700-1000
K, and N ~ 1012 cm-2
Interpretation:
The 2 bands probe different levels in an atmosphere with strong
positive temperature gradient
Grodent et al. 2001
A non-LTE H3+ model (Melin et al. 2005)
– Based on detailed balance calculations (Oka and Epp 2004) and a
physical (temperature,density) model of Jupiter’s upper atmosphere
– Calculate line production profile in atmosphere and compare with
LTE situation
– Results:
–The 22 band probes higher and hotter atmospheric levels than 2
– Non-LTE effects are minor for 2 but much more significant for 2 2 lines
– Thus, using 2 2 lines leads to too low a column density
2 Q(1,0)
222 R(6,6)
Melin et al. 2005
Trot vs. Tvib
– We find Trot = 1170+/-75 K > Tvib = 960 +/- 50 K, i.e. an underpopulation
of v2 =3 relative to v2 =2 with respect to LTE case
– Interpretation:
– non-LTE effects are even more significant for 32 - 2 (and higher
overtones) than for 22 lines  the temperature determined by
assuming QLTE for v2 =2 and v2 =3 underestimates Tkin
323- 2 R(5,6)
222 R(6,6)
Melin et al. 2005
Melin et al.
2005
Beyond the QLTE hypothesis
– Main conclusion: non-LTE effects are severe
 May induce large errors in T and especially N (H3+) determined
from overtone and hot bands (up to ~2 orders of magnitude
underestimate for N (H3+) !)
– Future studies: rather than « temperature/column density retrievals »,
better to perform forward modelling , i.e. test Tkin(z), n(H3+) profiles
directly against data
Spatial distribution of emissions: H2 vs. H3+
H3+ 222 R(7,7): 4732 cm-1
H2 S1(1): 4712 cm-1
H3+ 323- 2 R(6,7): 4722 cm-1
« Hot spot » near
 = 70°N, LIII = 160°
Variations in H3+ emission rates :
variations in temperature or column density ?
1-5 = five bins of increasing emission
(5 = « hot spot » near LIII = 160°)
• Except in « hot spot », emission variations mostly due to
variations in N(H3+)
• Confirmed by search for correlations between
intensities/temperatures/columns on individual pixels
Emission variations are mostly due to
variations in N(H3+)
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In agreement with Stallard et al. 2002
Little or no temperature variations: thermostatic
effect from H3+
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H3+ cooling dominant above homopause
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e.g. T(0.01 µbar) ~1300 K. Would be 4800 K if no H3+ cooling
(Grodent et al. 2001)
Large variations of input energy radiated by modest increases
of temperature
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E.g. « diffuse » and « discrete » aurora (differing by amount of
hard electron precipitation) have similar (within ~100 K)
temperatures
Exception: the « hot spot », actually hotter than
other regions by ~250 K
Conclusions
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We have detected the 32 - 2 band of H3+ on Jupiter
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Our temperature/column density determinations differ
with those obtained from other bands. This can be
understood from:
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Strong non-LTE effects on combination and overtone bands
The temperature profile in Jupiter’s auroral ionosphere
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Spatial variations in the H3+ emission generally trace
variations in H3+ columns and not in temperature
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The increased temperature in the « hot spot » (also
visible at other wavelengths but NOT in H2 emission)
remains a mystery
The H3+ Northern auroral « hot spot »
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Located at ~70°N, LIII ~ 160
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Has Tvib ~ 250 K warmer than
other regions
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Region peculiar at other :
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Thermal IR emission of several
hydrocarbons
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Far UV (footprint of polar cusp)
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X-ray emission
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But not in H2 S1(1) !
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Origin?
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Impact of very energetic (> 100 keV) electron (cf origin of FUV features)?
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Increased vertical mixing due to increased precipitation, resulting in
elevated homopause?
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Would rather produce ‘deep’ (cold) H3+
Increased CH4  reduces the deep cold H3+ component  increase of mean
H3+ temperature
But is it consistent with increase of H3+ emission?
Why not seen in H2 ?