Institut d'Astrophysique Spatiale, CNRS/Paris XI, 91405 Orsay, France
L. Ben Jaffel
Institut d'Astrophysique de Paris, CNRS, 98 bis, boulevard Arago, 75014 Paris, France
Institut d'Astrophysique Spatiale, CNRS/Paris XI, 91405 Orsay, France
J. T. Clarke, G. Ballester
University of Michigan, Ann Arbor, MI 48109-2143
South Research Institute, San Antonio, TX 78238
Ecole Normale Supérieure, 46 allée d'Italie, 69364 Lyon, France
Research Associate at Institut d'Astrophysique de Paris, CNRS, 98 bis, boulevard Arago, 75014 Paris, France
Keywords: solar system, giant planets, atmospheres, Jupiter
To explain both an upper atmospheric thermal excess observed in the Jupiter atmosphere by the Voyager missions and a permanent enhancement of the H Ly- brightness observed near the equator (Clarke et al. 1980, Sandel et al. 1980, Dessler et al. 1981, Skinner et al. 1988, McGrath 1991), a scenario of supersonic jets emanating from the auroral zones, meeting at the equator, and inducing supersonic atmospheric turbulence in the Jovian Ly- bulge was recently proposed by Sommeria et al. (1995). The observations presented here were aimed at checking the validity of such a model.
The observations presented here were obtained during Cycle-4 and Cycle-5. The spectra were measured with the Echelle-A grating and the 1.7 arcsec square aperture. Since HST is orbiting in the atmosphere of the Earth, the measured spectra are the sum of the H Ly- emissions of Jupiter's atmosphere and of a sky-background due to the solar radiation reflected on the geocorona and the interplanetary hydrogen. These contributions are separated by Doppler shifts that depend on the line of sight directions and of the date of observation. The sky-background was measured in a separate sequence, by pointing the aperture off Jupiter's disc by arcmin. The Ly- emission of Jupiter is then deduced by scaling and subtracting this background from the measured spectra. The complete data reduction and calibration procedures are described in Emerich et al. (1996).
To check the jets' scenario, three main regions of the trajectories of the expected jets were investigated: the bulge region itself, a mid-latitude region at a longitude deduced from the Sommeria et al. (1995) model, and finally the source region (near the southern auroral zone).
1) Bulge Profiles: the mean profile shown in the left part of Figure 1, obtained for an exposure time of 16 minutes, presents a very disturbed shape. The sharp spikes and troughs observed in the intensity, narrower than the LSA response function (70mÅ), cannot be produced by thermal motions in a quiet atmosphere. These unusual features are distributed at Doppler shifts from the line center that correspond to velocities ranging from a few to a few tens of , directly evidencing for the first time the existence of supersonic velocities in the upper atmosphere of Jupiter.
Figure: On the left panel is shown one mean Ly- profile, measured in the region of the Ly- bulge, on May 27, 1994 (Cycle-4). The vertical bar indicates the theoretical central wavelength that corresponds to a quiet atmosphere. On the right is represented the time evolution of the previous profile split into four successive sub-spectra denoted from 1 to 4.
The previous spectrum was also split into four consecutive sub-spectra (4 minutes exposure time) which are represented in the right part of Figure 1. The variation of their shape shows evidence of very fast processes evolving on time scales of a few minutes.
2) Mid-latitude Profiles: one of them (Figure 2, left side), was obtained at 30^o South latitude, near the longitude predicted for the jets, and with an exposure time 20 minutes. This profile presents a global blue Doppler shift of about 14mÅ (5mÅ), relative to a quiet atmosphere. Furthermore, the global blue-shift of one of the corresponding sub-spectra (exposure time 5 minutes), reaches 20mÅ, implying mean velocities of the emitting gas up to 5 , projected on the line of sight. These Doppler shifts appear as being the signature of global H motions in the upper atmosphere of Jupiter.
3) Near Auroral Region Profiles (Figure 2, right side): the mean profiles, obtained around 58^o South latitude, with 20 minutes exposure times, do not present noticeable global Doppler shifts. However, they all present a strong double peaked feature on both sides of the line center, similar to those observed in 1994 for the north aurorae of Jupiter (Prangé et al. this issue). Such particular line shapes, observed for the first time in a planetary atmosphere, as well in the northern as in the southern auroral regions, could indicate a strong contribution from auroral particle precipitations. On the other hand, the varying dissymmetry observed between the `blue' and `red' peaks, might be due to an additional absorption by a hydrogen layer moving above the supposed auroral source.
Figure: On the left panel, is diplayed one midlatitude profile measured near the predicted jet's trajectory. On the right panel are shown three Ly- profiles observed near the southern auroral oval. Their respective intensities are 7.5, 11, and 16 kR. The vertical bars indicate the theoretical central wavelength that corresponds to a quiet atmosphere. These Cycle-5 observations were made between August 16, 1995, and September 7, 1995.
For three latitude locations observed on the Jovian disk, the H Ly- line profiles appear very complex. The unusual Ly- line shapes evidenced by these high resolution HST observations are the signature of a particularly active upper atmosphere. These observations directly confirm the existence of supersonic atmospheric turbulence in the Jovian bulge, as was suggested by Ben Jaffel et al. (1993), and reveal motions which could be explained by a global gas transport from the auroral zones toward the equator. However, to advance into the description and understanding of such a stormy and complex upper atmosphere, it is now necessary to obtain a complete map of Ly- profiles over the entire Jovian disc.
We acknowledge the whole STScI team, and particularly Alex Storrs and Steve Hulbert for helping us to optimize both the observation sequences and the data reduction process.
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