Upload
mandy-hammersley
View
214
Download
1
Tags:
Embed Size (px)
Citation preview
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)
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 ?