Spectro-imaging observations of H3

+ on Jupiter

Observatoire de Paris, France

Emmanuel Lellouch

H3+ in planetary atmospheres

• Discovered in 1988 in Jupiter’s auroral regions (Drossart et al. 1989) from its 22 emission

• Since then, also discovered in Saturn and Uranus, and in Jupiter’s low latitude regions

• General goals on the observations (esp. Jupiter)– 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

• Bands observed so far on Jupiter: 2, 22, 22 - 2

H3+ traces and drives the energetics and dynamics of Giant

Planets upper atmospheres

(see Steve Miller’s talk)

Outline

1. Detection of the 3 2 - 2 band on Jupiter

2. The question of LTE and the multiplicity of H3+

« temperatures »

3. The spatial variations of the H3+ emission: do they trace

variations in the H3+ column or « temperature »?

Spectro-imaging observations

• Imaging FTS « BEAR » at Canada-France-Hawaii Telescope

• Sept. 1999 & Oct. 2000• 2 filters

– 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

22 band

Example of spectrum: 2.12 µm range

22 and 32-2 bands

H2 S1(1)

The two 32 - 2 lines

Obs. freq. (cm-1) Calc. Freq. (cm-1) Assignment E’ (cm-1)

(Neale et al. 1996)

---------------------------------------------------------------------------------

4721.79 4722.383 323 - 2 R(6,7) 7993.60

4749.66 4749.937 323 - 2 R(5,6) 7998.64

---------------------------------------------------------------------------------

Temperature/column density determinations

• Classical LTE formulation

• This effectively assumes complete LTE (Trot = Tvib= Tkin)

– 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:

• All the nv2 levels are underpopulated w.r.t. ground state

• But their relative populations are close to LTE distribution : « quasi-LTE » distribution

• Here:– 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

In average, Trot = 1170+/-75 K > Tvib = 960 +/- 50 K underpopulation of v2 =3 relative to 2 = 2 with respect to LTE case

Tvib

Trot

Central meridian longitude

Trot and N (H3+) results

• We get Trot (22) = 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)

Melin et al. 2005

2 Q(1,0) 222 R(6,6)

– 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 22 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

Trot vs. Tvib

Melin et al. 2005

222 R(6,6)32

3- 2 R(5,6)

– 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 32 - 2 (and higher overtones) than for 22 lines the temperature determined by assuming QLTE for v2 =2 and v2 =3 underestimates Tkin

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+ 22

2 R(7,7): 4732 cm-1

H3+ 32

3- 2 R(6,7): 4722 cm-1

H2 S1(1): 4712 cm-1

« Hot spot » near = 70°N, LIII = 160°

Variations in H3+ emission rates :

variations in temperature or column density ?

• 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

1-5 = five bins of increasing emission (5 = « hot spot » near LIII = 160°)

Emission variations are mostly due to variations in N(H3+)

• In agreement with Stallard et al. 2002 • Little or no temperature variations: thermostatic

effect from H3+

– H3+ cooling dominant above homopause

• 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– 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

• We have detected the 32 - 2 band of H3+ on Jupiter

• Our temperature/column density determinations differ with those obtained from other bands. This can be understood from:

– Strong non-LTE effects on combination and overtone bands– The temperature profile in Jupiter’s auroral ionosphere

• Spatial variations in the H3+ emission generally trace

variations in H3+ columns and not in temperature

• 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 »

• Located at ~70°N, LIII ~ 160 • Has Tvib ~ 250 K warmer than other regions• Region peculiar at other :

– Thermal IR emission of severalhydrocarbons– Far UV (footprint of polar cusp)– X-ray emission– But not in H2 S1(1) !

• Origin?– Impact of very energetic (> 100 keV) electron (cf origin of FUV features)?

• Would rather produce ‘deep’ (cold) H3+

– Increased vertical mixing due to increased precipitation, resulting in elevated homopause?

• 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 ?