Institut d'Astrophysique Spatiale, Orsay, France
University of Michigan, Ann Arbor, MI 48104 USA
Imperial College, London, UK
DESPA, Observatoire de Meudon, Paris, France
University College London, London, UK
W. H. Ip
Max-Planck Institut für Aeronomie, Katlenburg-Lindau, Germany
Institut d'Astrophysique de Paris, Paris, France on leave from Institut d'Astrophysique Spatiale, Orsay, France
Keywords: Jupiter, aurorae, magnetosphere, UV
Jupiter's strong magnetic field sustains a giant magnetosphere in which most of the basic plasma processes are similar to those acting in the Earth's magnetosphere (driven by the solar-wind interaction and by processes of internal origin), but differences such as its size, rotation period, magnetic field strength, plasma sources and distance to the Sun give rise to different boundary conditions and make the comparison of the processes effective in each magnetosphere very exciting. One of the specificities of Jupiter's magnetospheric activity is the very powerful aurorae which result from the magnetosphere/atmosphere coupling, delivering several 10 watts into each polar region (Herbert et al. 1987, Livengood et al. 1992), more than 100 times the average Earth aurora energy. This auroral energy input also significantly exceeds the solar UV input, and is suspected to dominate the atmospheric dynamics and energetics on a planetary scale (Sommeria et al. 1995). The aurorae are due to precipitation along high latitude magnetic field-lines of energetic charged particles of magnetospheric origin, resulting in collisional excitation of the main atmospheric constituents, H and H. These aurorae were discovered more than a decade ago in the FUV wavelength range (Broadfoot et al. 1979, Clarke et al. 1980) and monitored regularly since then. Accurate knowledge of the auroral morphology is essential to identify the magnetically conjugate active regions in the magnetosphere and even, hopefully, to characterize the physical plasma processes responsible for particle precipitation. Two years of imaging with the Faint Object Camera from 1992 to 1994, made a breakthrough in our vision of the FUV aurorae, even at the degraded resolution resulting from the telescope aberration (subarcsec, with variations depending on the extent of the feature). The complex morphology of the north was revealed, and its main features identified (e.g., Gérard et al. 1994 and references therein), (i) a narrow bright oval lying at the footprint of magnetic field lines crossing the equator in the distant magnetosphere, near 30 R, possibly at the boundary of the polar cap, (ii) some diffuse emission, longitudinally modulated (peaking near System III longitude 150--160 ), at the footprint of field lines connected to the outer Io torus (mainly detectable in the short wavelength H Werner bands and at Lyman ), (iii) a very broad and bright ``transpolar'' arc across the polar cap along the meridian 160 . By contrast, the south aurora was much less studied. An important finding was that all the structures were co-rotating with the planet, with little local time dependence, demonstrating that, by contrast with the Earth, the Jovian auroral processes are controlled by the structure of the magnetic field rather than by the solar wind dynamo.
New FUV images of the polar regions of Jupiter were taken with the Faint Object Camera during the Shoemaker-Levy 9 campaign, on July 13 and August 9, 1994. The upgrade of HST had restored the FOC nominal spatial resolution, 50 of the energy within a radius of 3 (0.014 arcsec)-pixels near 1550Å, the central wavelength used (filters F152M,F175W are used in combination). For 1-D elongated features, the transverse resolution (LSF) is still better, 0.030 arcsec (100 km on Jupiter). The small size of the pixels, which normally oversample the PSF, allows to full benefit of this resolution. The sensitivity is improved by about a factor of 3 compared to pre-COSTAR observations.
In both cases, we use two HST orbits, six hours apart, in order to get both faces of the planet (Jupiter's rotation period is 10 hours), with the north aurora best viewed on one images and the south aurora best viewed on the other (tilted magnetic axis) and during each orbit, we take one 11-minute image of each polar region, so that the north and south aurorae are observed quasi-simultaneously. Figure 1 shows the two pairs of images for August 9, each placed in a single frame and with an oblate spheroid overplotted to represent the planet limb. Due to good spatial resolution, sharp limb, and accurate offset between the images, the limb can be determined within about 200 km.
Figure: The north and south polar regions of Jupiter in the FUV, the aurorae appear in the H Lyman bands near 1550Å. Both faces of Jupiter are seen consecutively
The north aurora is consistent with the images obtained since 1992, except that it is much more in focus. What is really new is to see that the broad structure across the polar cap is structured, and that not only the bright auroral oval is very narrow, but that there are fainter concentric ovals at higher latitude, on what is considered as the polar cap. The good resolution finally allows us to see that the auroral emission overlaps the limb by a few pixels which means that the emission must arise from high altitude in the atmosphere, about 500 200 km above the limb. The south aurora, much more symmetric than the north, is less blurred by the rotation of the planet, so that it is even better suited to quantify the width of the auroral oval, in particular near the central meridian. For the first time, the instrument resolution is significantly smaller than the intrinsic width of the aurora. Figure 2 shows a north-south plot across the south oval, with the contribution of the disc background removed.
Figure: North-south plot across the south aurora. Left: one sees the peak of the high latitude bright oval, with the polar cap diffuse emission on one side, and an emission which continuously decreases on the other side, down to nearly the footprint of the Io torus. Right: magnified view of the narrow bright peak. Its latitudinal extent is 2.2 pixels FWHM, i.e., 250 km, out of which 170 km is LSF and 170 km is the intrinsic width of the oval.
The width of the arc, averaged over 2000 km in longitude does not exceed 170 km! This corresponds to localized active layers of a few 10 km only in the far magnetosphere.
Another very exciting feature is the small bright spot to the left of the south aurora. It has been identified as the FUV signature of Io (Prangé et al. 1996). Figure 3 shows a magnified view of the spot, elongated in longitude by about the distance the magnetic footprint of Io moved during the exposure. Quantitative estimates show that most of the emission comes from the projection of a region within a few Io radii of Io's surface.
Figure: The FUV signature of the Io-Jupiter electrodynamic circuit
Precise determination of the coordinates of the spot and of the theoretical footprint of the undisturbed magnetic field line (IFT) shows that the FUV spot is 9.6 2.5 ahead of the IFT in the direction of Io's motion. This differs from the value derived in the IR by Connerney et al. (1992), 15--20 , in exactly the same geometrical conditions. This gives the first evidence that this angle varies in time. Application of the Alfven-wave interaction theory suggests that this could be the consequence of local density variations in the Io torus (Prangé et al. 1996).
Finally, Figure 4 shows that the spot is very bright, as bright as the auroral emission. We have estimated the precipitated flux to be about 1/4 of the energy developed by the motion of Io across the magnetic field of Jupiter (2--3 10 watts). This, and the fact that we see a single spot, conflicts with the ``multiple arc'' structures of the Io-related decametric emissions.
Figure: Plot across the Io FUV footprint and the south auroral region of Jupiter
We are grateful to the STScI team, and especially to Alex Storrs for strong support in the preparation and execution of the program.
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