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C. Emerich
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
R. Prangé
Institut d'Astrophysique Spatiale,
CNRS/Paris XI, 91405 Orsay, France
J. T. Clarke, G. Ballester
University of Michigan, Ann Arbor, MI 48109-2143
R. Gladstone
South Research Institute, San Antonio, TX 78238
J. Sommeria
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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