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Spectro-imaging observations of H 3 + on Jupiter Observatoire de Paris, France Emmanuel Lellouch.

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Presentation on theme: "Spectro-imaging observations of H 3 + on Jupiter Observatoire de Paris, France Emmanuel Lellouch."— Presentation transcript:

1 Spectro-imaging observations of H 3 + on Jupiter Observatoire de Paris, France Emmanuel Lellouch

2 H 3 + in planetary atmospheres 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) –Characterize the morphology and variability of the emission –Interpret the emission rates in terms of H 3 + column density and temperature –Measure winds from Doppler shifts on H 3 + emissions Bands observed so far on Jupiter:  2, 2 2,  2  H 3 + traces and drives the energetics and dynamics of Giant Planets upper atmospheres (see Steve Miller’s talk)

3 Outline 1.Detection of the 3  2 -  2 band on Jupiter 2.The question of LTE and the multiplicity of H 3 + « temperatures » 3.The spatial variations of the H 3 + emission: do they trace variations in the H 3 + column or « temperature »?

4 Spectro-imaging observations Imaging FTS « BEAR » at Canada-France-Hawaii Telescope Sept & Oct filters –2.09 µm : cm -1 –2.12 µm : cm 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)

5 Example of spectrum: 2.09 µm range 2 2 band

6 Example of spectrum: 2.12 µm range 2 2 and bands H 2 S 1 (1)

7 The two 3  lines Obs. freq. (cm -1 ) Calc. Freq. (cm -1 ) Assignment E’ (cm -1 ) (Neale et al. 1996) R(6,7) R(5,6)

8 Temperature/column density determinations Classical LTE formulation This effectively assumes complete LTE (T rot = T vib = T kin ) –Justified for rotational distribution (  rad ~ 1000 sec,  coll ~ sec) –For vibrational distribution,  rad ~ sec  non-LTE; however earlier studies (Y.H. Kim et al. 1992) have found that: All the nv 2 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 T rot in v 2 =2 level – 2.12 µm observations (including the lines) determine T vib (relative populations of v 2 =2 and v 2 = 3 levels)

9 Temperature results In average, T rot = 1170+/-75 K > T vib = 960 +/- 50 K  underpopulation of v 2 =3 relative to 2 = 2 with respect to LTE case  T vib  T rot Central meridian longitude

10 T rot and N (H 3 +) results We get T rot (2 2 ) = 1170+/-75 K, and N = (4-8)x10 10 cm -2 Quite different from Lam et al. 1997: T rot ( 2 ) = K, and N ~ cm -2 Interpretation: The 2 bands probe different levels in an atmosphere with strong positive temperature gradient Grodent et al. 2001

11 A non-LTE H 3 + model (Melin et al. 2005) Melin et al Q(1,0) 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 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

12 T rot vs. T vib Melin et al R(6,6) R(5,6) – We find T rot = 1170+/-75 K > T vib = 960 +/- 50 K, i.e. an underpopulation of v 2 =3 relative to v 2 =2 with respect to LTE case – Interpretation: – non-LTE effects are even more significant for (and higher overtones) than for 2 2 lines  the temperature determined by assuming QLTE for v 2 =2 and v 2 =3 underestimates T kin Melin et al. 2005

13 Beyond the QLTE hypothesis – Main conclusion: non-LTE effects are severe  May induce large errors in T and especially N (H 3 + ) determined from overtone and hot bands (up to ~2 orders of magnitude underestimate for N (H 3 + ) !) – Future studies: rather than « temperature/column density retrievals », better to perform forward modelling, i.e. test T kin (z), n(H 3 + ) profiles directly against data

14 Spatial distribution of emissions: H 2 vs. H 3 + H R(7,7): 4732 cm -1 H R(6,7): 4722 cm -1 H 2 S 1 (1): 4712 cm -1 « Hot spot » near = 70°N, L III = 160°

15 Variations in H 3 + emission rates : variations in temperature or column density ? Except in « hot spot », emission variations mostly due to variations in N(H 3 +) Confirmed by search for correlations between intensities/temperatures/columns on individual pixels 1-5 = five bins of increasing emission (5 = « hot spot » near L III = 160°)

16 Emission variations are mostly due to variations in N(H 3 +) In agreement with Stallard et al Little or no temperature variations: thermostatic effect from H 3 + –H 3 + cooling dominant above homopause e.g. T(0.01 µbar) ~1300 K. Would be 4800 K if no H 3 + 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

17 Conclusions We have detected the band of H 3 + 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 H 3 + emission generally trace variations in H 3 + columns and not in temperature The increased temperature in the « hot spot » (also visible at other wavelengths but NOT in H 2 emission) remains a mystery

18 The H 3 + Northern auroral « hot spot » Located at ~70°N, L III ~ 160 Has T vib ~ 250 K warmer than other regions Region peculiar at other : –Thermal IR emission of several hydrocarbons –Far UV (footprint of polar cusp) –X-ray emission –But not in H 2 S 1 (1) ! Origin? –Impact of very energetic (> 100 keV) electron (cf origin of FUV features)? Would rather produce ‘deep’ (cold) H 3 + –Increased vertical mixing due to increased precipitation, resulting in elevated homopause? Increased CH 4  reduces the deep cold H 3 + component  increase of mean H 3 + temperature But is it consistent with increase of H 3 + emission? –Why not seen in H 2 ?


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