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Spectro-imaging observations of Jupiter's 2-μm auroral emission. I. H3 distribution and temperature |
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| Authors: | E Raynaud J-P Maillard JH Waite Jr P Drossart |
| Institution: | a Lesia, Observatoire de Paris, F-92195 Meudon, France b Institut d'Astrophysique de Paris, F-75014 Paris, France c Southwest Research Institute, San Antonio, TX 78228, USA d University of Michigan, Ann Arbor, MI 48109, USA |
| Abstract: | We report on spectro-imaging infrared observations of Jupiter's auroral zones, acquired in October 1999 and October 2000 with the FTS/BEAR instrument at the Canada-France-Hawaii Telescope. The use of narrow-band filters at 2.09 and 2.12 μm, combined with high spectral resolution (0.2 cm−1), allowed us to map emission from the H2S1(1) quadrupole line and from several H3+ lines. The H2 and H3+ emission appears to be morphologically different, especially in the north, where the latter notably exhibits a “hot spot” near 150°-170° System III longitude. This hot spot coincides in position with the region of increased and variable hydrocarbon, FUV and X-ray emission, but is not seen in the more uniform H2S1(1) emission. We also present the first images of the H2 emission in the southern polar region. The spectra include a total of 14 H3+ lines, including two hot lines from the 3ν2-ν2 band, detected on Jupiter for the first time. They can be used to determine H3+ column densities, rotational (Trot) and vibrational (Tvib) temperatures. We find the mean Tvib of the v2=3 state to be lower (960±50 K) than the mean Trot in v2=2 (1170±75 K), indicating an underpopulation of the v2=3 level with respect to local thermodynamical equilibrium. Rotational temperatures and associated column densities are generally higher and lower, respectively, than inferred previously from ν2 observations. This is a likely consequence of a large positive temperature gradient in the sub-microbar auroral atmosphere. While the signal-to-noise is not sufficient to take full advantage of the 2-D capabilities of the observations, the search for correlations between line intensities, Tvib and column densities, indicates that variations in line intensities are mostly due to correlated variations in the H3+ column densities. The thermostatic role played by H3+ at ionospheric levels may provide an explanation. The exception is the northern “hot spot,” which exhibits a Tvib about 250 K higher than other regions. A partial explanation might invoke a homopause elevation in this region, but a fully consistent scenario is not yet available. The different distributions of the H2 and H3+ emission are equally difficult to explain. |
| Keywords: | Jupiter Jupiter/atmosphere Ionospheres infrared observations |
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