More about the structure of the high latitude Jovian aurorae

Planetary and Space Science 49 (2001) 1159–1173

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More about the structure of the high latitude Jovian aurorae Laurent Pallier, Ren*ee Prang*e ∗ Institut d’Astrophysique Spatiale, Batiment 121, CNRS-Universite Paris XI, 91405 Orsay Cedex, France Received 9 August 2000; received in revised form 22 November 2000; accepted 7 December 2000

Abstract This study is based on the determination of a ‘reference’ main oval for Jupiter’s aurora from a series of high-resolution images taken 9 We have taken with the Faint Object Camera on board the Hubble Space Telescope in the H2 Lyman bands centered near 1550 A. advantage of the visibility of the northern auroral oval over a large range of longitudes on June 24, 1994, especially for longitudes smaller ◦ than 170 where it is generally very faint and thus undetectable. In the south, where there is no such problem, we combined images taken from various points of view between June 1994 and September 1996. We =nd that the northern main oval is consistent in size and in general aspect with the footprint locus, in the Connerney (1998)’s VIP4 model, of magnetic =eld lines crossing the equator near 20RJ . However, the precise shape of this oval diBers from the model (and from previous ‘reference’ main ovals) in that it exhibits a ‘bean-like’ ◦ aspect with excursions toward lower latitudes in the SIII longitude range 190–240 (an already reported feature), and toward higher ◦ latitudes in the poorly documented 120–150 range. In the south, our reference oval covers an area about that of the 30RJ VIP4 model. ◦ As in the north, it is shifted from a VIP4 model oval, toward lower latitudes from 110 to 200 and toward higher latitudes from 310 to ◦ 100 . The accurate de=nition of these (magnetically conjugate) oval loci puts additional strong constraints on magnetic =eld models at high latitude. Based on our reference main oval, we have then extrapolated still higher latitude ovals. Very interestingly, we =nd that we ◦ can =t (i) the highest latitude arc of oval detected well inside the main oval at longitudes greater than 170 , and (ii) the high latitude ◦ edge of what we had previously named the ‘transpolar emission’ at longitudes less than 170 (both also detected on images taken at other dates), by a single empirical oval. We suggest that this oval indicates the location of the northern polar cap boundary, within the uncertainty related to the temporal variability of the actual emission features. We can also =t another secondary arc of oval and a second branch of our ‘transpolar emission’ by a slightly larger empirical oval, presumably connected to the outer magnetosphere. This implies that the region of permanent high latitude diBuse emission seen inside the main oval in the dusk sector must be at the footprint of closed =eld lines connected all the way out from the middle magnetosphere to the magnetopause. Finally a bright spot is sometimes observed just equatorward of the northern polar cap boundary with an average brightness of 0.5-1 MR (comparing with a bright main oval). We establish that this spot does not rotate with the planet, but rather remains close to magnetic noon. We thus tentitatively identify it, by reference to the Earth aurora, with the footprint of the northern Jovian polar cusp, or with a transient dayside aurora. We also highlight c 2001 Elsevier Science Ltd. All rights reserved. the diBerences observed in the southern high latitude structure. 

1. Introduction The Jovian auroral emissions, which result from the precipitation of energetic charged particles into the planet’s upper atmosphere along magnetic =eld lines surrounding the poles, represent the observable signature of auroral magnetospheric processes. As such, they have been studied for about 50 years in the radio frequency range, and for almost two decades at shorter wavelengths where spatial information is available. The far ultraviolet (FUV) auroral emissions, discovered in 1979 with Voyager and IUE (Broadfoot et al., 1979; Clarke et al., 1980), and due to collisional ∗

Corresponding author. Tel.: 33-1-69-85-8582; fax: 33-1-69-85-8675. E-mail address: [email protected] (R. Prang*e).

excitation of atmospheric species, have been the only direct signature of magnetospheric precipitating beams until Galileo discovered auroral emission at visible wavelengths with the solid state imaging instrument (SSI) (Ingersoll et al., 1998). FUV images were not obtained until the early 90s, with imagers on board the Hubble space telescope (HST), =rst with the faint object camera (FOC) (Dols et al., 1992), then the wide =eld planetary camera 2 (WFPC2; Clarke et al., 1996), and now with the space telescope imaging spectrograph (STIS). These instruments have provided high-resolution images with which to study the morphology of Jupiter’s aurorae. This in turn allows to identify active regions in the magnetosphere by mapping the auroral emission loci up the magnetic =eld lines to the equatorial plane. Previous studies have revealed that the Jovian aurorae

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Fig. 1. A typical Jovian aurora as seen with the HST faint object camera on August 9, 1994. It illustrates the main auroral structures derived from Prang*e et al. (1998) and from other studies mentioned in the introduction. The most prominent feature is a bright narrow ring in each hemisphere, generally referred to as the ‘main oval’. Fainter emissions surround the main oval at lower latitude, whereas two, apparently diBerent, structures are identi=ed inside the northern main oval: westward of the SIII longitude ◦ 170 , one can see two faint arcs of ovals at high latitude, and a broad ◦ bright ‘stripe’ crossing the main oval interior almost along the 160 meridian (Prang*e et al. (1998)) called this feature ‘transpolar emission’). Finally there is also a faint diBuse emission layer which covers the dusk sector (right side on the images) in both hemispheres. This =gure also ◦ illustrates the usual fading of the northern main oval eastward of ∼ 170 where it seems to merge into the ‘transpolar emission’.

consist of a complex, but rather stable, pattern of features illustrated on Fig. 1, at the footprints of magnetic shells crossing the equatorial plane at various distances, ranging roughly form the orbit of Io at 5.9 Jovian radii (RJ ) from the planet center in the inner magnetosphere, out to the magnetopause (e.g. Clarke et al., 1996; Grodent et al., 1997; Prang*e et al., 1998). Among these features, the ‘main oval’ seems to be the most permanent feature, and very often the brightest one, i.e. the one corresponding to the most eLcient precipitation process in the magnetosphere. It has also been the most thoroughly studied from the analysis of various FUV datasets, and of near infrared (IR) images of the thermal H3+ emission at somewhat lower spatial resolution, where it is the dominant feature (Kim et al., 1994; Connerney et al., 1996). The main oval is a very narrow structure, 50–500 km across at half-maximum (Tsurutani et al., 1997; Prang*e et al., 1998; Ingersoll et al., 1998) which surrounds each pole of Jupiter approximately along the footprints of magnetic =eld lines crossing the equator in the middle magnetosphere near 20–30RJ on the dayside (G*erard et al., 1994a; Ballester et al., 1996; Clarke et al., 1996; Satoh et al., 1996; Prang*e

et al., 1998), and maybe somewhat closer in on the nightside (Vasavada et al., 1999). It has long been recognized that the locus of the main ovals slightly departs from the surface footprint of a constant model magnetic shell, especially in the north where the oval extends at lower latitude ◦ ◦ in the System III (1965) longitude range 180 6 III 6 300 (e.g. G*erard et al., 1994a; Connerney et al., 1996), even using the latest improved magnetic =eld models (Connerney et al., 1998). This departure has been identi=ed as a longitude, rather than a local time, eBect by various independent studies, including Galileo observations from the nightside (Satoh et al., 1996; Prang*e et al., 1998; Vasavada et al., 1999). The ovals also exhibit longitudinal structures at various spatial scales (Ballester et al., 1996; Prang*e et al., 1998). At the largest scale, it has long been noted that the north oval was generally bright and well de=ned for longitudes ◦ III ¿ 170 and that it became faint or even disappeared at smaller longitudes (e.g. G*erard et al., 1993, 1994a). More systematic studies have identi=ed a longitudinal asymmetry both in the north and in the south, with the brightest parts of the northern and southern ovals being out of phase (Satoh et al., 1996; Prang*e et al., 1998; Satoh and Connerney, 1999). This was interpreted as the combined eBects of a particular precipitating process in the magnetosphere, pitch angle scattering which produces energetic charged particle losses toward the =eld line footprint where the surface =eld strength is smallest, and of the azimuthal asymmetry of the Jovian magnetic =eld structure (Prang*e and Elkhamsi, 1991). This eBect is best visible in the north and the locus of the main north oval is most often impossible to de=ne ◦ below III ∼ 170 . Although very stable in shape, the main oval is subject to signi=cant temporal intensity variations. Some seem to aBect the feature as a whole (Prang*e et al., 1998), accounting for the variability of the global auroral activity over a few days identi=ed with IUE, and to be related to dynamical processes in the middle magnetosphere (Prang*e and Livengood, 1998; Prang*e et al., 2001). Others, observed a few times so far, consist of sudden brightenings of the main oval, very localized in azimuth near the dawn limb, and fading within a few hours (G*erard et al., 1994b; Ballester et al., 1996). Finally, it happens at times that the dim-to-undetectable segment of the north oval, eastward of ◦ 170 brightens temporarily. An event of this type has been discussed in Dougherty et al. (1998), and we have shown that the brightened part of the oval was magnetically connected to a transient layer of =eld-aligned currents detected by the magnetometer on board the Ulysses spacecraft. Such transient brightenings provide the only opportunities to map this part of the northern main oval with some degree of accuracy. It has not been possible to use the event mentioned above to map this region of the main oval, because it was obtained in 1992 during a pioneer observing program with the HST=FOC which suBered from a rather poor signal-to noise ratio and spatial resolution (Dols et al., 1992). In 1994, just after the HST spherical aberration was corrected, we performed a new series of observations with FOC

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when its nominal resolution was restored. The images, taken in support of a series of spectra of the auroral emissions at central meridian longitudes (CML) spanning the range ◦ 105–227 ; had not been analyzed so far because the auroral emission was not very bright. Indeed, the auroral activity at that date, as derived from simultaneous IUE observations (see Fig. 3 in Prang*e et al., 2001) was below average. How◦ ever, we noted that the III 6 170 segment of northern main oval was brighter than the rest of the oval and clearly visible, in particular in a few images in which it was enhanced ◦ by line of sight eBects (CML ∼ 150 ). The analysis of this set of images allows us to derive a reference northern auroral oval. As we had only a single image of the southern oval at that time, we have used a series of data taken at other dates and covering a large range of CML to similarly derive a southern reference main oval. In Section 2, we describe the data, the observing conditions, and the procedures used to improve the visibility of auroral features. In Section 3, we explain how our reference main ovals are de=ned, we establish their geometry, and compare them with previous determinations of the auroral ovals. Our reference main oval is then used in Section 4 to derive a best =t oval to higher latitude features, which we suggest could be the polar cap, and we infer a possible signature of the northern polar cusp. Finally, the results are summarized and discussed in Section 5. 2. Observations and image processing The images used for the northern Jovian aurora in this study were the =rst ones obtained with FOC after correction of the HST spherical aberration by the COSTAR device. They had been designed to provide the global auroral morphology context for a series of spatially resolved, high spectral resolution, FUV spectra (Prang*e et al., 1997). As in previous pre-COSTAR observations, the combination of the UV =lters F152M–F175W allowed us to isolate the spec9 which includes the brightest emistral range 1460 –1670 A, sions of the H2 Lyman bands, and to limit the contribution from the solar-reOected Oux through the red wing of the =lters. As discussed in more detail in Tsurutani et al. (1997) and Prang*e et al. (1998), the COSTAR implementation has fully restored the FOC nominal spatial resolution, and the transverse resolution for 1-D sources (line spread function or LSF) could reach ∼ 0:04 arcsec at half-maximum, i.e. ∼ 100–150 km projected on the disc of Jupiter, well oversampled by the new pixel size, 0:014 × 0:014 arcsec. However, due to an erroneous requirement during the preparation phase, we obtained (i) a =eld of view half of what we expected (7:5 × 15 arcsec), just enough to include the whole auroral zones, but increasing the diLculty in de=ning the planetary limb, and (ii) a smaller than expected sensitivity, whereas, the auroral activity itself was faint. The images thus remained unexplored pending the development of eLcient image processing methods, although it was obvi-

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ous, even on the raw data, that the auroral morphology was unusual and exhibited a relatively strong emission at longi◦ tudes smaller than ∼ 160 . This localized brightening was also observed in the spectra taken within one day after the images were obtained during the same program (see Fig. 3 in Prang*e et al., 1997). The images were taken on June 24, 1994, using three consecutive HST orbits, during the course of which the Jovian ◦ ◦ CML varied from 105 to 227 . One image of the northern aurora was taken during the =rst orbit, and three more during each of the last two orbits, thus evenly covering the range of observing geometries where northern aurorae are at lowest latitude and best visible from Earth. Following previous studies, we consider that during the ∼ 3:5 h that the observation lasted, the auroral morphology did not vary signi=cantly, so that the combination of these seven images provide an accurate mapping along most of the auroral oval at that particular date. The exposure times were ∼ 9 min ◦ during which Jupiter rotated by only 5:4 . However, this exposure time, shorter than usual, also contributed to a smaller signal-to-noise ratio. During this observing program, the south aurora was im◦ aged only once at CML= 96 . Although this geometry provides in principle a good view of the aurora, the emissions were so weak along most of the oval that only a very lim◦ ited longitude range around III = 0 could be mapped. We used, therefore, a series of images of the south aurora, performed at other dates and various CMLs in similar conditions (same camera, same =lter combination, but a larger =eld of view ∼ 15×15 arcsec). A veri=cation of the consistency of this data set extending over a couple of years and diBerent weak-to-moderate auroral activity conditions, was provided a posteriori by the consistency of the resulting mosaic map, since each individual map widely overlaps its two neighboring ones. We have used data from June 24, 1994 (one image), July 13 and 18, 1994 (two and one images, respectively), August 9, 1994 (two images), and September 5, 1996 (three images; Table 1). The July 13, August 9, and September 5 data have already been analyzed and published in Prang*e et al. (1998). As already mentioned, the June 24, 1994 images suffer from a weak signal-to-noise ratio, and in order to improve the visibility of auroral structures, we have developed a special processing which includes the application of a 2-D Morlet sine wavelet, and of a 2-D spatial =ltering code. The wavelet processing allows to highlight structures of a given size. In mathematical terms, it acts like a spatial =lter at given scales on the image. The wavelet is a nul integral function (in 1-D) or surface (in 2-D) characterized by a scale parameter. By convolving the 2-D wavelet and the image in the Fourier space, one can amplify structures of the same spatial size as the scale parameter of the wavelet, and eliminate structures of other sizes. Convolutions with the same wavelet at increasing spatial scale allow to isolate larger structures. We have selected the 2-D Morlet sine wavelet rather than others for its best eLciency

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Fig. 2

Fig. 5 9 after the contrast-improving procedure and planetary background Fig. 2 An overview of the June 24, 1994 HST=FOC image set at 1460 –1670 A subtraction (see text). A grid of planetocentric coordinates has been overplotted, as well as the surface footprint of magnetic =eld lines crossing the equator at 5:9RJ (at the orbit of Io) and 30RJ using the VIP4 model of Connerney et al. (1998). The image characteristics are detailed in Table 1. Fig. 5. Same as Fig. 3, but for both hemispheres we have superimposed the ‘reference main oval’ derived from a series of pro=les as on Fig. 4, taken ◦ every 2 . The uncertainty in the determination of the reference oval location is of the order of the square size. Two additional ‘reduced reference ovals’ have been overplotted on the north projection (see section 4).

L. Pallier, R. Prange / Planetary and Space Science 49 (2001) 1159–1173 Table 1 HST=FOC images of the Jovian aurorae used in this studya Date

Hour UT

Exp.time (s)

CML

Hemisphere

2 February 1992 04:25 600 174 South 24 June 1994 02:37 536 96 South 24 June 1994 02:52 536 105 North 24 June 1994 04:10 536 152 North 24 June 1994 04:23 536 160 North 24 June 1994 04:38 536 169 North 24 June 1994 05:46 536 210 North 24 June 1994 05:59 536 218 North 24 June 1994 06:14 536 227 North 13 July 1994 14:59 776 160 South 13 July 1994 15:19 776 172 North 13 July 1994 21:23 776 32 South (13 July 1994) 21:43 776 44 North 18 July 1994 06:05 780 234 South 9 August 1994 02:27 716 166 North 9 August 1994 02:47 663 179 South 9 August 1994 08:49 663 37 South (9 August 1994) 09:10 663 49 North 5 September 1996 09:53 716 05 South 5 September 1996 11:51 896 61 South 5 September 1996 13:02 956 120 South 5 September 1996 13:28 1172 136 North a The parentheses indicate exposures used for disc background subtraction.

especially for a-few-pixel elongated features. However, we have modi=ed its shape in order to amplify elongated structures in our images. The wavelet has been stretched along the x-dimension and compressed along the y-dimension. The noise-=ltering procedure, already used by Prang*e et al. (1998), compares the excess brightness of any given pixel relative to the average in a sliding 2-D test box around it, to the statistical noise in the box. Above an adjustable threshold, the excess is considered as real, below this threshold it is considered as noise and decreased by a given amount. As for the wavelet, we use rectangular test boxes (9 × 3 pixels) which emphasize elongated structures aligned with the long axis. Varying the direction of the box axis (conversely, the wavelet x-axis) allows us to signi=cantly improve the contrast of the auroral features over the disc background emission whenever the box (the wavelet x-axis) is aligned along the direction of the feature. However, in order to avoid arti=cially created features, we only consider as real features which can be distinguished on the raw images and which remain visible for all orientations of the test box. Since the various auroral features on a given image may correspond to diBerent preferential directions, we realize a =nal mosaic image for each exposure from partial images processed along these particular directions. We then rebin the images (rebinned pixel size: 0:028 × 0:028 arcsec) and we insert them into a standard 512 × 512 square array, in which we determine the location of the planetary limb by =tting an oblate spheroid to the sharp edge of the solar Oux reOected by the planetary disc. We

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estimate the accuracy of our best =t to ±3 pixels for the June 1994 images and to ±2 pixels for the other images. A grid of coordinates (parallels and meridians) is overplotted on the spheroid. The next step consists of removing the solar reOected disc contribution. Two images taken on July 13 and August 9, 1994 at CMLs where we did not =nd auroral emission at any detectable level are used as an empirical model of the disc background. These images are scaled to the size of the auroral images and to the same exposure time, they are shifted until the limbs coincide, and they are subtracted from the auroral images. The remaining features are then of auroral origin (Fig. 2). The =nal step of the processing consists of a projection of each image over a polar view of the oblate spheroid. The projection is executed assuming that the emission is con=ned within a thin layer centered at 400 km (±1 pixel, i.e. ±100 km) above the 1-bar level, in agreement with the =ndings of Prang*e et al. (1998) in the same spectral range. This spectral range is characterized by a total lack of contamination by the vertically extended Lyman emission (whatever its relative brightness) which could bias the vertical distribution of the emitting layer. The projection process spreads square pixels, especially pixels near the limb, into elongated features aligned along the line of sight (once projected, the last disc pixel covers ∼ 15 pixels normal to the limb projection). The spatial resolution on the projected image is thus degraded near the limb in the perpendicular direction. But this only aBects the retrieval of the emission locus when the emission more or less follows the limb, which occurs only for portions of three images in the south (July 13, 18 and August 9). Since we have obtained, for each hemisphere, a series of overlapping polar projections for various Jovian rotational phase angles, the latter eBect is signi=cantly attenuated. Once co-added, these projections produce a composite polar projection for each hemisphere where most of each auroral zone is visible (quasi-simultaneously in the north), as displayed in Fig. 3. 3. Determination of a reference main oval 3.1. Determination of the main oval locus The range of system III (SIII ) longitudes covered by our ◦ ◦ images at auroral latitudes extends from 90 to 295 for ◦ ◦ ◦ the north, and from −70 (290) to 260 for the south, but this includes most of the auroral zone because the unexplored part corresponds to the short polarmost segment of the asymmetric ovals (Figs. 3 and 5). In both hemispheres, we identify a bright ring which corresponds to the ‘main oval’ as described in Clarke et al. (1996), and in Prang*e et al. (1998) and marked as (M) in Fig. 3. Inside this ring, there are signi=cant structures and diBuse emissions at higher latitude in the northern hemisphere. We can distinguish (i) a widespread diBuse emission =lling a large part of the area limited by the main oval in both hemispheres, with, in the

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Fig. 3. Composite polar projections of the northern and southern Jovian aurorae with grids of planetocentric coordinates. The north projection comes from the June 24, 1994 data only. The south projection is based on observations extending from 1994 to 1996 (see text). For clarity purposes, we have ◦ intensi=ed the north projection west of III = 180 and the upper half of the south projection by a factor 1.6. Arrows have been overplotted to indicate the main features discussed in the text: main oval (M), faint inner arcs (I1 and I2), and former ‘transpolar emission’ (T). The stripe across the aurora, limited by the white lines, is used to derive the brightness pro=le shown in Fig. 4.

north only, (ii) a brighter broad region extending roughly ◦ along the meridian III ∼ 160 (T, see below for meaning) and which can be best identi=ed with the viewing geometry of Fig. 2c, and (iii) one (maybe two) faint inner arcs of ovals ◦ at longitudes ¿ 170 (I1,I2). Looking into more details at the individual projections, we can identify (i) as a combination of the high latitude diBuse duskside emission described in previous studies widely blurred in longitude because it is seen from various CMLs, (ii) as a ‘transpolar-type emission’ as de=ned in Prang*e et al. (1998) and illustrated in Fig. 1, nearly =xed in longitude, and (iii) as the high latitude secondary ovals discussed in Prang*e et al. (1998) and visible on the left in Fig. 1. In the south by contrast, there is little emission inside the south main oval, except the blurred diBuse duskside ◦ ◦ emission from −70(290) –120 . In the following, we will focus on the main oval. Its intensity varies with longitude, but in the north, it does not follow the usual scheme. In most cases, the main oval is bright and ◦ very narrow for III 6 170 ; and weak to undetectable for ◦ III ¿ 170 where it merges with the high latitude diBuse emission. Figs. 2 and 3 show that, on June 24, the western ◦ part of the oval (III ¿ 170 ) is still narrow, but signi=cantly fainter than usual. A strong brightening, with a double-arc ◦ ◦ structure in several images, extends from 140 to 175 and roughly rotates with the planet. It is prolongated eastward ◦ of 140 by another narrow, moderately bright segment of arc. This part of the oval is especially obvious between the ‘transpolar emission’ and the 30RJ VIP4 oval, in Fig. 2 for ◦ CML = 152 where the structure, just aligned with the line of sight, is arti=cially enhanced, and to a lesser extent for

Fig. 4. Transverse cut across the north polar region of the June 24, 1994 composite projection in Fig. 3. where the cut is indicated by the white lines. The location of the footprint of the orbit of Io, of the main reference oval and of the reduced reference ovals (1 and 2) are indicated.

◦

CML = 160 . We thus take advantage of this unusual situation where the whole northern main oval is detectable to de=ne a ‘reference main oval’ from this series of images. In order to de=ne its locus as accurately as possible, we use a series of brightness pro=les across the main oval (Fig. 4). To derive a pro=le, we draw a stripe from the

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center of the oval as perpendicular as possible to the oval (see Fig. 3), and we sum the counts across the stripe. To improve the signal-to-noise ratio, the stripe must be as wide as possible, in so far as the location of the peak emission does not vary by more than 2 to 3 pixels across the stripe. In the example shown in Figs. 3 and 4, the stripe is 15 pixels wide, the maximum rectilinear extent of the main oval across the stripe for this orientation, and it provides a main oval full-width at half-maximum (FWHM) of ∼ 1250 km. This is rather broad, but this stripe intersects the brightest part of the oval where the individual images show a double arc structure. The thinnest segment of arc, 250 km at FWHM (2.5 pixels), has been derived from a 90-pixel wide ◦ stripe along the 210 meridian. This compares quite well with the values found by Prang*e et al. (1998) for narrow segments of the southern oval, and it con=rms that our integration across the stripe does not arti=cially broaden (and thus possibly displace) the oval peak. Each pro=le is then =tted by a third degree polynom in the vicinity of the oval in order to precisely de=ne the emission peak along the direction of the cut (within ∼ 2 pixels). Finally, we retrieve the 2-D coordinates of the peak emission corresponding to each given direction. Since the sampling of the main oval is uneven, we regularize it by a linear interpolation every 2 degrees in longitude, and we de=ne our ‘reference main oval’ displayed in Fig. 5. As mentioned above, we do not have any observation in ◦ the range 295 –90 : the main oval there closely approaches the northern rotational pole which, at that time, was not visible from Earth because the rotational axis was tilted ◦ ∼ 3 away from the normal to the line of sight. Even though the longitude range seems large, it only corresponds to a short segment of the oval. Assuming spatial stability of the main oval (an assumption which is validated in the following), we have tried to constrain its location in this region, using three images from other FOC programs which showed the other side of the planet. On these images, there is no sign of any auroral feature, so that the planetary limb constrains the equatorward boundary of the oval. The =rst image, taken on August 9, 1994, ◦ at CML = 49 , is the one used to remove the planetary background emission. The other two have been taken ◦ ◦ at CML = 25 and 330 on February 15, 1993, before the correction of the telescope aberration (so that their spatial resolution and their contrast above the disc background are not as good). In Fig. 6, we have overplotted the projection of a 1-pixel wide ring at the limb (black bars for February 15, 1993, and grey bars for August 9, 1994) which constrains the oval equatorial boundary. However, more observations with appropriate observing conditions will be necessary to precisely de=ne this segment of the main oval. Note that the last points eBectively ◦ ◦ observed on the frontside, at III 6 120 and ¿ 270 are located within this limb pixel. This is thus quite consistent with the fact that they are not detected from the farside since the emission would be washed out by line

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of sight eBects (Prang*e et al., 1998 have shown that the emission in this spectral range is limb darkened in the last pixel). The same procedure has been applied to the composite southern auroral image. The longitudinal coverage of the main oval is nearly complete, and a mere interpolation allows to de=ne an entire southern reference oval. 3.2. Geometry of the reference main ovals In Figs. 6a and b, we have plotted our northern and southern reference main ovals together with the locus footprint of the magnetic =eld lines connected to the middle magnetosphere, computed with the VIP4 model (Connerney et al., 1998). In the north, our reference oval is closely consistent in size with the 20RJ VIP4 model oval, but it displays ◦ a ‘bean-like’ aspect with excursions up to 3 toward lower ◦ latitudes in the SIII longitude range 190 –240 (a frequently ◦ reported feature already), and up to ∼ 5 toward higher lati◦ tudes in the poorly documented 120 –145 range. Near both ◦ ends of our incomplete observational oval, between ∼ 250 ◦ ◦ ◦ and 280 , and between 115 and 120 , our reference oval coincides with the VIP4 oval. In-between, the model oval lies slightly poleward of our equatorial limit, so that it does not disagree with the observations. In the south, our reference oval is extremely similar in size and shape with the 30RJ VIP4 model. Since the main ovals have been suspected to be at the footprint of closed =eld lines (Ballester et al., 1996; Dougherty et al., 1998; Prang*e et al., 1998, 2000), this slight discrepancy between the northern and southern oval =ts suggest that the surface magnetic =eld is still uncertain at these very high latitudes. As in the north, our reference oval departs from any VIP4 ◦ oval model, being shifted by up to 3 toward lower latitudes ◦ over a large range of longitude (90 –300 ), and to the same ◦ amount toward higher latitudes for (300 –100 ). Other studies before ours have dealt with the determination of the auroral oval locations and shapes. Among those, Clarke et al. (1996) have de=ned a ‘WFPC2 reference oval’, often used for comparisons in further studies. Figs. 6c and d reveal a global agreement between both sets of reference ovals. However, we also =nd some diBerences. In the north, ◦ both reference ovals practically coincide from III = 140 to ◦ III = 220 , and they both shift equatorward of the model ◦ ◦ ◦ VIP4 oval westward of 190 . From 190 to ∼ 290 , our reference oval is slightly more shifted than the Clarke et al.’s ◦ one (by 1–1:5 ). We believe that this may somewhat exceed somewhat the uncertainties of both observations. In the ◦ range 290 –80 , the WFPC2 reference oval lies poleward of the limit we have established, as the VIP4 oval does, so that they are thus equally reliable. However, it seems that the VIP4 oval is more consistent with ours than the WFPC2 ref◦ ◦ ◦ erence oval near III = 90–110 . Finally from 110 to 140 , the WFPC2 reference oval is found at a signi=cantly lower latitude than our reference oval on the other side (equatorward) of the VIP4 oval.

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Fig. 6. (a) The northern reference main oval determined from our images is plotted as open squares on a polar view upon the same grid of coordinates as in Fig. 2, and compared with the footprints of the magnetic =eld lines crossing the equatorial plane at 5.9 and 20Rj (north) or 30RJ (south) in the Connerney (1998)’s VIP4 model (small solid squares). Bars in the northern map represent the projection of the limb pixel in FOC images mapping the aurora-free hemisphere (see text); (b) same as (a), but for the southern hemisphere; (c) and (d) same as (a) and (b), but comparison with the WFPC2 reference ovals; (e) comparison of our northern reference oval with the UVS Voyager data (solid circles: auroral emission detected, open circles: no auroral emission detected), the small squares is the VIP4 5:9RJ oval; (f) comparison with the visible Galileo SSI ‘primary oval’ detected on the nightside (small squares, derived from Vasavada et al., 1999).

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In order to understand this diBerence, we have carefully studied the procedure of Clarke et al. (1996). Their reference oval is a truly ‘instantaneous’ one, based on a single ◦ image taken at CML ∼ 190 . Thus, in the longitude range in question, the WFPC2 emission is seen at or above the ◦ ◦ planetary limb for ∼ 280 6 III 6 ∼ 130 . By contrast, this part of the FOC oval is seen in very good conditions ◦ across the body of the planet at CML = 105–169 . Near the limb, it is quite diLcult to perform any accurate polar projection, because the emission may originate from anywhere between the entry of the line of sight into the emitting layer on the front side of the disc to its exit from the layer behind the limb. In addition, these two limits depend on the altitude and vertical extent of the emitting layer. Although the assumptions used for the projection are not provided by the authors, the shape of the projected oval suggests that they have considered emissions from behind the limb: simulations show that the departure between our segment of oval and Clarke et al.’s one is of the order of the distance between frontside and farside projections for an emission at 400 km. Note that it is also almost of the order of any inaccuracy of the limb =tting by 0:1 arcsec (one WFPC2 pixel). But there exists also another most presumable ex◦ planation: the main oval was not active in the ∼ 90–150 longitude range at the time of the observations (see Fig. 1 in Clarke et al., 1996) and the image shows increased diBuse emission all the way down from high latitude to the footprint of the Io torus. With the viewing geometry of the WFPC2 image, this diBuse emission, extending beyond the limb, is brightened along the line of sight and can mimic a narrow oval following the limb, as shown by Grodent et al. (1997). We =nd a con=rmation of our main oval locus in this longitude range with the Galileo SSI visible images. Vasavada et al. (1999) conclude that the locus of their northern ‘primary oval’ parallels the WFPC2 reference oval (and consequently our reference oval as well) in the longitude ◦ range 155 –210 even where they both diverge from model magnetic =eld contours. But they note a poleward deOection ◦ at III ¡ 155 , which they tentatively identify to Prang*e ◦ et al. (1998)’s III = 155–160 ‘transpolar emission’. In fact, a more careful consideration of their Plate 1, reproduced in Fig. 6f, indicates that, even though the oval starts to deviate slowly from the WFPC2 and model oval ◦ shapes east of 155 , the most striking deOection occurs ◦ near III = 145 (see Galileo E11 data) where the visible aurora abruptly turns polewards and continues along the ◦ ◦ meridian 145 up to the limit of the map at ∼ 70 N. This is exactly what we observe, except that we follow this ◦ branch of the oval up to more than 80 N, and that the ◦ deOection point is closer to the 140 meridian. We thus infer that SSI has also captured this segment of the main oval while it was active and that it con=rms the distorted shape of our reference oval, together with the poleward deviation of this segment from any VIP4 model. Note that there also exist a few WFPC2 images where the main

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oval was brightened in this longitude range (e.g. Fig. 3H in Clarke et al., 1996), and this probably explains part of the ‘kink’ mentioned in Ballester et al. (1996) (although not all of it, the rest being due to lower latitude diBuse emission brightened by line of sight eBect near the limb as discussed above). However, this has not been recognized so far as a part of the locus of the standard WFPC2 main oval. Out of curiosity, we have also considered the Voyager UVS auroral observations (Fig. 6e). Within the lon◦ gitude range ∼130–230 ; where the oval is well inside the planetary disc, the equatorward limit of the auroral emission detected by UVS is consistent with our reference oval within the (rather signi=cant) uncertainties ◦ ◦ on their early determination. Between 230 and 330 ; ◦ ◦ and between 70 and 130 , emission is seen equatorward of our reference oval, but inside the VIP4 footprint of the orbit of Io (which diBers signi=cantly from the O4 oval used in the early analyses), at longitudes where ‘low-latitude’ emission have been detected in the FUV and in the IR (Prang*e et al., 1998; Satoh et al., 1996). ◦ In the relatively limited remaining interval, ∼ 340–60 ; UVS seems to have detected emissions equatorward of the Io footprint, but these observations were so close to Jupiter’s limb that (i) their location is quite uncertain, and (ii) limb brightening eBects may have arti=cially brightened very faint emissions. It seems therefore, that the UVS observations, when compared to more recent auroral ovals, either observational or theoretical, no longer raise questions as puzzling as they used to be some years ago. Finally, we have veri=ed the location of our (quasi-) instantaneous north reference oval against FOC observations taken at diBerent epochs. Fig. 7 displays polar projections of individual images taken on June 24 and August 9, 1994 and September 5, 1996 (with the same =lters and the COSTAR device), and on February 9, 1992. In Fig. 7c, we have displayed the August 9, 1994 polar map, which exhibits the brightest main oval we ever observed with FOC, but, as in most cases, active only for ◦ III ¿ 170 (see Fig. 1). We have also overplotted our reference main oval, as derived from the June 24 composite map. ◦ ◦ In the longitude range 175 6 III 6 235 where the bright main oval is visible, it exactly coincides with the reference ◦ main oval (the maximum departure, near III = 200 , is of ◦ the order of the uncertainty on the reference oval, ∼ 0:5 ). ◦ ◦ Near 160 and 145 ; the reference oval crosses regions of ◦ ◦ faint secondary maxima. But from 140 to 120 ; the reference oval follows, closely as well, a region of absolute minimum (a valley between regions of moderate emission) which we interpret as a typical ‘extinct’ main oval segment. Fig. 7d presents a polar map obtained on September, 5, 1996, during a much fainter aurora. Despite a lower signal-to-noise ratio, it shows a similar agreement with the ◦ ◦ segment of main oval visible between 220 and 175 (max◦ ◦ ◦ imum departure ∼ 1–1:5 between 200 and 220 ), and

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Fig. 7

Fig. 8 Fig. 7 Four north polar projections derived from 1992-to-1996 individual HST=FOC exposures with our reference main oval and its two reduced reference ovals (see discussion in text). The very bright spot equatorward of the reference main oval in the upper left corner of Fig. 7a is not noise, but emission from the VIP4 5:9RJ oval, brightened by line of sight eBects. Fig. 8 Two examples of the June 24, 1994 individual polar maps showing a distinct bright spot at the dayside equatorward edge of the innermost auroral oval near magnetic noon (arrows).

L. Pallier, R. Prange / Planetary and Space Science 49 (2001) 1159–1173 ◦

◦

with a series of secondary minima between 160 and 145 . Eastward of its poleward bifurcation, the reference oval seems to follow a series of faint maxima equatorward of the edge of the brighter ‘transpolar emission’. It is presumable that this segment of the main oval was moderately active at that time, and that its detection became possible on this particular exposure, with the predicted morphology, ◦ owing to a favorable line of sight eBect (CML = 136 ). Finally, Fig. 7a is derived from an image taken before the COSTAR implementation, and at shorter wavelengths where the =lter transmission was lower, so that we had to smooth the projection signi=cantly to improve the signal-to-noise ratio. Nevertheless, it is of interest because it exhibits some similarities with the June 1994 data under study. It is characterized by a western main oval much fainter than usual (relative to the other auroral structures) and a very bright ◦ high latitude emission between III ∼ 160 and the limb ◦ (100 ). This unusual event was related to the presence of a layer of =eld aligned currents detected in the dusk side magnetosphere at the same time by the magnetometer on board Ulysses (Dougherty et al., 1998). Again, we =nd that ◦ the few regions of detectable main oval longward of 175 are reasonably well aligned with the western part of the ◦ reference oval (maximum departure ∼ 1:5 in latitude near ◦ 230 ), and that the latter follows quite well the segment of intense emission, which we can now identify as an activation episode of the eastern part of the main oval. We thus show that the main oval is quite stable in location along our instantaneous reference oval (if not in brightness dis◦ tribution along the oval), at longitudes larger than ∼ 170 where it is almost always visible, as well as at longitudes ◦ smaller than ∼ 170 where it is only activated occasionally. This stability also validates our option to de=ne a southern reference main oval from images taken at various diBerent dates. ◦ Note that the longitude range III ¡ 170 is also characterized, for all the maps studied here, by a signi=cant amount of ‘low latitude belt’ emission (e.g. Prang*e et al., 1998) between the main reference oval and the Io orbit footprint, in particular near the western limb in Fig. 7a and at lon◦ gitudes less than 160 in all the images. The latter are the diBuse emissions we suspect to account for the discrepancy between our reference oval and the WFPC2 one. In the southern hemisphere, the WFPC2 reference oval is in general quite close to ours, and they both diBer from the model VIP4 oval in a similar way. However, as in the north we also note some diBerences, presumably also due to the use of a single image for the WFPC2 ◦ oval. This image has been taken at CML ∼ 10 ; and not surprisingly, both reference ovals almost coincide for ◦ ◦ ◦ −55 (305) 6 III 6 85 (except for a very small poleward ◦ ◦ excursion of the WFPC2 oval 5 both sides of III = 0 ). By contrast, and as in the north, the WFPC2 reference oval is at lower latitude than ours on the opposite side, in the ◦ range 90–140 where it is seen from above the planetary limb.

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4. High latitude features 4.1. Model 9t of higher latitude ovals: the polar cap? As mentioned above, the region inside the main oval contains other auroral features. In both hemispheres, a diBuse emission =lls the evening sector. In the north, faint and partial inner ovals (‘arcs’) are seen at longitudes larger than ◦ III ∼ 180 (one or two are generally detected), and an ◦ elongated structure runs close to the meridian 155–160 . We called the latter ‘transpolar emission’ because it often crosses the polar region from one side of the main oval to the other (Prang*e et al., 1998). It is interesting to investigate whether the inner arcs can be organized with respect to high latitude model ovals consistent with our reference main oval. It appears that the polar projections of northern VIP4 model ovals at the footprint of increasing radial distances in the equatorial plane can, as a =rst approximation, be derived from one another by a mere scaling from the (approximately) common center of the ovals. This rule has been checked on 6RJ to 30RJ VIP4 ovals. We thus derive a family of increasingly small high latitude empirical ovals, by similarly scaling the polar projection of our reference main oval from its center located ◦ ◦ at III = 185 , = 73 . Such ‘reduced reference ovals’ are displayed in Fig. 5, and, within the uncertainties due to our crude approach and to the low level of the inner emissions, we =nd that there is indeed a reduced reference oval which reasonably =ts each of the faint partial inner archs of the composite June 24 map, especially the smallest one which is the brightest. The scaling factor applied for the largest reduced reference oval is the one needed to switch from a 30RJ to a 70RJ VIP4 oval. Although it is known that the VIP4 model is not applicable beyond 30RJ ; this indicates qualitatively that the inner arc is at the footprint of very distant magnetic =eld lines. The innermost arc is thus at the footprint of even farther =eld lines, maybe close to the magnetopause. At easternmost longitudes, where the reduced refer◦ ence ovals turn poleward near (III = 160–150 ); they again follow two lines of increased brightness. These bright elongated features correspond to meridional ‘branches’ spatially resolved by Prang*e et al. (1998) in their so-called ‘transpolar emission’ near the equatorial and polar edges. At intermediate longitudes, the reduced ovals (especially the largest one) also coincide with a series a bright features. In the same longitudinal range, a still brighter feature connects the eastern and western segments of the innermost reduced oval at higher latitude, and will be discussed below. The =t of the reduced reference ovals with emission peaks, along the inner ovals on one side, and on the edges of the transpolar emission on the other side, is also clear in transverse pro=les across the auroral region as the one displayed in Fig. 4. This =nding suggests that the inner arcs and the ‘transpolar emission’ are not distinct features. On the contrary, they seem to form a series of complete auroral ovals, at the footprint of =eld lines connected to the same region

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in the outer to the very outer magnetosphere. Their quite diBerent aspects, which confused us so far, maybe due to longitudinal eBects, or to local time eBects (for instance, the presence of an evening sector diBuse emission near the eastern segments). This result was so unexpected that we have veri=ed it with other images taken at various dates. In particular, the inner arcs and the ‘transpolar emission’ were much brighter ◦ in our 9 August, 1994 data (taken at CML = 166 ). In Fig. 7c, the August 9, 1994 map exhibits two clear inner ◦ arcs westward of ∼ 170 ; with the innermost one brighter than the other one (as in the June 24 case). The reduced reference ovals which are overplotted have not been recalculated, they are the ones de=ned to =t the June 24 arcs. Their =t to the faint arcs is amazingly good considering the crude approximation used for the reduced ovals and the plausibility of time variations at such high ◦ latitudes (both arcs are ∼ 1–1:5 north of the reduced ◦ reference ovals near III = 200 ; and coincide with them ◦ on both sides). Eastward of 150 ; they also follow the equatorward and poleward edges of the transpolar emission. A comparable agreement is found in Figs. 7a and d, although the inner arcs, much fainter, are more diLcult to visualize (especially the largest one which is always fainter). We are thus led to reinterpret what we had formerly called ‘transpolar emission’. Based on the shape of the model auroral ovals, including the most recent ones, this emission seemed to be located at a very high magnetic latitude, and had been tentatively related to the footprint of open =eld lines in the polar cap. This work establishes that the emission must be excited by precipitation from the same distant magnetic =eld lines, beyond ∼ 40–50RJ or more all the way to the magnetopause, which produces the inner arcs at westernmost longitudes. Its unusual shape, almost rectilinear along a meridian, is only coincidental, and results in fact from the magnetic =eld topology in this longitude range, where we have shown that our reference oval at lower latitude even reverses its curvature (e.g. its bean-like aspect). Inside reduced reference oval 2 (the innermost one), we generally note a rapid decrease of the auroral emissions, so that the poleward edge of the transpolar emission and the innermost inner oval generally resemble a high latitude limit for bright auroral emissions, and we suggest that they altogether represent west and east parts of the Jovian northern ◦ polar cap boundary. Around III = 160–170 ; in the region of the abrupt poleward bifurcation, the high latitude limit of bright emission is either along the reduced reference oval (Fig. 7c), or somewhat poleward but connected to it on both sides (Figs. 5 and 7a–c). Since we expect the polar cap shape and size to vary as a function of the solar wind and interplanetary magnetic =eld conditions, we suggest that the polar cap boundary is represented by the innermost reference oval when it is most extended, and that it is represented by the innermost bright emission locus when it diBers slightly

from the reduced reference oval. In this scenario, the polar cap variability seems to be highest in the longitude range ◦ 160 –180 . It is important to emphasize that we have not detected transpolar-like emissions nor inner ovals in any of our images of the southern aurora. The only feature inside the main southern oval is the diBuse emission located in the afternoon sector as reported already by Prang*e et al. (1998). This interhemisphere asymmetry may be due to some asymmetry in the interaction with the solar wind, perhaps related to the tilt of the magnetic axis with respect to the interplanetary magnetic =eld, and it would deserve particular attention from people involved in MHD modeling of the Jovian magnetosphere. 4.2. Tentative identi9cation of a northern polar cusp In a previous study, Prang*e et al. (1998) had attempted to characterize their ‘transpolar emission’ by investigating the motion of its equatormost region as Jupiter rotates, either =xed in longitude, or =xed in magnetic local time. Their result was inconclusive, and they attributed it to two reasons: =rst in the longitude range of interest, any given magnetic local time rotates with the planet (even not as fast), and second the range of CMLs available was not large enough to untangle the problem. We understand now that there was another, probably more critical, reason, namely, they had considered any bright emission at the south end of the ‘transpolar emission’ near or poleward of the main oval. The above study tells us that these emissions might thus originate indiBerently from closed =eld lines connected to the middle magnetosphere (near 30RJ ) from more distant ones all the way to the magnetopause, or even from open ones. Depending on their origin, they are not expected to behave similarly. Hence a large scatter of the data. In each of our June 24, 1994 images we can identify and resolve two diBerent types of very bright spots. The =rst one extends along, or just poleward of, the main oval ◦ near III = 150–160 . This is the double arc structure already mentioned. The second one, slightly westward of the =rst one, is located near the equatorward edge of our reduced reference oval 2 (Fig. 8). Its brightness, ∼ 0:5 − 1 MRayleigh of total H2 emission averaged over the exposure time, compares with the brightest parts of the main oval and would imply average precipitating energy Ouxes of the order a few tens to ∼ 100 ergs cm−2 s−1 . We have determined its SIII longitude in each of our seven images, and we have plotted it as a function of the subsolar point longitude in Fig. 9 (thick crosses). We have overplotted the longitude of three particular magnetic local times, magnetic noon ± 2 MLT, as Jupiter rotates. In this data set, the scatter of the data is signi=cantly reduced compared to the previous study owing to a better selection of the feature of interest and to larger ◦ CML range 105–227 . The plot de=nitely dismisses any

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Fig. 9. SIII longitude of the bright spot at the equatorward edge of the innermost auroral in the June 24, 1994 (thick crosses). The thin cross and star show the location of a similar bright spot on Figs. 7a and d. The abscissa is the SIII longitude of the subsolar point (CML ± solar phase angle).

corotation of the spot with Jupiter (at constant longitude), and the data are fairly well aligned along a magnetic local time curve between 10 and 12 MLT. Going back to our previous data, we have at times found a similar bright spot just equatorward of our reduced reference oval 2 (i.e. Fig. 7d and maybe 7a), and also located in the same prenoon sector. Within the uncertainty of the mapping, it is thus clear that this feature must be =xed in magnetic time, near noon. Following our interpretation of reduced reference oval 2 as the polar cap boundary, we thus suggest that this spot is at the footprint of the northern Jovian polar cusp where magnetic =eld lines are directly open to the solar wind, detected for the =rst time. Alternatively, solar wind ram pressure increases could, as on Earth, trigger a so-called ‘dayside aurora’, also centered near magnetic noon (Zhou and Tsurutani, 1999), and very close in latitude to the polar cusp (both structures are very diLcult to resolve on Earth (B. Tsurutani, personnal communication). In any case, both features are under solar wind control, which could explain why we do not detect the feature on all our images. A correlated study of the occurence of this feature with solar conditions is in progress. 5. Summary and discussion In this study, we have taken advantage of a particular set of images of the northern Jovian far ultraviolet aurorae to de=ne a reference main oval. These images have been taken with the Faint Object Camera onboard the Hubble Space Telescope at a time when the whole main auroral oval was ◦ active in the north, including at longitudes III 6 170 where it is most often very faint to undetectable. The images, taken ◦ ◦ from several points of view (CML from 105 to 227 ), allow us to determine the locus of emissions with good accuracy since almost the entire oval can be observed far enough from

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the planet limb on one or more of the images. Any temporal variation between the various overlapping oval segments is ruled out over the few-hour duration of the observations. 9 wide =lter centered near 1550 A 9 in the H2 Use of a ∼ 200 A Lyman bands rigorously prevented from any contamination by Lyman emission, whose distribution is quite extended in altitude, thus allowing for accurate polar projections including near the limb. As in previous studies, we =nd that our reference oval roughly follows the footprint of a magnetic shell connected to the middle magnetosphere. In the ◦ north its lowest and highest latitude regions (near III = 0 ◦ and 180 ) are well =tted by the 20RJ surface footprint of the Connerney’s VIP4 model. We also =nd the already described departure from the model ovals toward lower lati◦ tude for longitudes 190 –240 . The segment of the north oval most shifted maps up to a radial distance of ∼ 10RJ in the equatorial plane with the VIP4 model. This study additionally reveals that the main oval deviates, toward higher lati◦ tude, in the poorly documented longitude range 120 –150 , reaching latitudes connected to radial distances signi=cantly beyond 30RJ (limit of validity of this model). In this sector, the oval is almost rectilinear in polar projection, with even a reversed curvature leading to a typical ‘bean-like’ shape, so that it delineates an area of about equal surface than the model oval, its center being westward shifted by a few degrees. This instantaneous reference oval =ts quite well the whole set of FOC data we have studied in the past, an evidence that it is quite stable in time. In the absence of a similar data set available for the south, we had to combine a series of images taken at diBerent dates, also covering a large range of CML. The resulting reference southern main oval is very similar to the VIP4 30RJ , but it ◦ seems to be shifted by a few degrees along the 0−180 meridional plane. It lies at higher latitude in the longitude range ◦ ◦ ◦ 310-(0) –100 and at lower latitude for 110 6 III 6 300 . Following the VIP4 model, this would imply particle precipitations toward south as close as 15RJ near the longitude ◦ 200 in the equatorial plane, and, precipitations from further ◦ out than at least 40−50RJ near 0 In each hemisphere, our reference oval is consistent, within our error bars, with Clarke’s et al. (1996)’s WFPC2 reference oval in the longitude range where their oval is seen well inside the planet disc. However, it lies at signi=cantly higher latitude in the range where their auroral ◦ oval is seen above the limb: i.e. between 110 –130 where our southern reference oval more closely follows the VIP4 ◦ model oval, and shortward of 140 in the north where we establish the high latitude excursion of the main oval responsible for its bean-like shape. We assign these diBerences between Clarke et al.’s reference ovals and ours to the fact that the eastern part of the northern main oval was poorly active during their observations and to projection inaccuracies near the planet limb (Clarke et al. derived each of their reference ovals from a single exposure taken from a single point of view) possibly including eBects from a diffuse emission extending horizontally toward lower latitudes

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beyond the limb, and=or from some Lyman contribution extending vertically over thousands of kms. The high latitude excursion of our northern reference oval is further con=rmed by Galileo=SSI observations of the visible aurora on the rightside. The VIP4 model results from Voyager and Pioneer magnetometer measurements which constrain the =rst moments of the highly asymmetric planetary magnetic =eld and the current sheet contribution at the time of the Voyager observations (assumed azimuthally symmetric), and from the locus of the IR footprint of Io which adds constraints on the surface =eld at this latitude, i.e. on some of the poorly constrained high multipole terms of the planetary =eld. The deviation of our northern and southern main ovals from the VIP4 model ovals indicates, either that the magnetospheric particle source does not follow a circle in the equatorial plane, or that the magnetic =eld we are using for the magnetic mapping diBers somewhat from the actual =eld. This may happen either near the equator and result from a corotating longitudinal asymmetry of the current sheet aBecting the shape of very stretched magnetic =eld lines beyond ∼ 20RJ , or from the presence of high latitude local surface anomalies of the planetary =eld not accounted for by the model. In the =rst case, since we expect the northern and southern ovals to be magnetically conjugate, the innermost (ri ) and outermost (re ) radial distances of the particle source must be the same and be reached at the same equatorial SIII longitude for both hemispheres. For the northern oval, the magnetic mapping indicates that ◦ rj = 10RJ at III eq = 250 − 260 and re (not derivable with ◦ VIP4, but beyond 40 − 50RJ ) lies at III eq = 50 − 60 . ◦ For the southern oval, ri = 15RJ III eq = 200–210 and ◦ re lies near III eq = 0 . A source whose radial distance would vary with longitude cannot thus account by itself for our observations, a modi=cation of the magnetic =eld model is also required to rotate one hemis◦ phere by ∼ 50 with respect to the other and to adjust the magnitude of the radial distances. For the same reason, a longitudinal anomaly of the current sheet with distortion of the magnetic =eld lines at large distances, although not formally dismissed, seems much less plausible than the existence of local surface magnetic anomalies at high latitudes. This study thus provides additional strong constraints on the magnetic =eld model. We have then extrapolated the shape of our reference oval to higher latitude in order to better characterize structures previously identi=ed inside the northern main oval. These structures, described in Prang*e et al. (1998), include (i) faint ◦ secondary partial ovals often detected at longitudes ¿ 180 (as the main oval), and (ii) what we named the ‘transpolar ◦ emission’, running almost along a 150 –160 meridian across the area delineated by the main oval. The secondary ovals are well-de=ned narrow structures, whereas the ‘transpolar emission’ looks more complex, at times with several parallel branches at the high latitude edge of the duskside diBuse emission region.

To extrapolate the shape of higher latitude reference ovals, we have de=ned the geometrical transformation allowing to derive higher latitude ovals from the 30RJ VIP4 model oval. We have then applied a similar transformation to our northern reference main oval and found that, not only was it possible to =t each of the high latitude arcs visible in our June 1994 data set by a reduced reference oval connected to the outer magnetosphere, but that these ovals also =tted, at a smaller longitude, quasi-linear features at the lower and higher latitude edges of the ‘transpolar emission’. These same reduced reference ovals were also found to =t the inner partial ovals seen on the images of August 1994, September 1996 and February 1992, together with branches on both sides of the ‘transpolar emission’, suggesting in all cases the existence of complete high latitude ovals connected to the outer to the very outer magnetosphere. In all cases, the innermost reference oval is at, or close to, the limit of the north aurorae where the emissions systematically drop down to very low levels at all wavelengths (the ‘Yin region’ in Satoh and Connerney, 1999) and this work strongly suggests that it corresponds to the polar cap boundary. From date to date, we observe some diBerences in the exact shape and size of ◦ this boundary (especially in the longitude range 160 –180 ), which presumably result from diBerent solar wind and=or interplanetary magnetic =eld conditions. Our tentative polar cap boundaries should be compared with the predictions of MHD models of the interaction of the solar wind with the magnetosphere under these varying conditions. The diBerence in brightness between the ‘transpolar emission’ branches and their longer longitude ‘inner arcs’ continuation generally indicates increased particle precipitation on the dusk sector of the outer magnetosphere all the way out to the magnetopause. This includes the duskside diBuse auroral region (the ‘Yang region’ in Satoh and Connerney, 1999) whose dawnside counterpart must be the emission-free region on top of which the high latitude inner arcs are superimposed. This local time asymmetry vanishes in the near-to-middle magnetosphere (inside ∼ 20–30RJ ) since it is not observed on the main oval nor or at a lower latitude. This indicates diBerent precipitating processes as a function of radial distance, and the limit between the two regimes is presumably the limit of penetration of the solar wind control on particle precipitation. We have also revisited the inconclusive Prang*e et al. (1998)’s investigation of the longitude-versus-local-time control of the ‘transpolar emission’. The southern end of this bright structure includes several bright features which can now be organized as a function of their distance to one of our reference ovals. By selecting a series of bright spots at the equatorward edge of our innermost reduced oval, we can con=dently claim that they are not =xed in SIII longitude, but that they remain close to local magnetic noon ◦ ◦ while Jupiter rotates, for all CMLs between 105 and 227 . This =nding, combined with our tentative identi=cation of the innermost reduced oval with the polar cap boundary, suggests that these emissions might be the surface signature

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of the northern polar cusp. This emission is not a permanent one and is not visible in all our datasets. This may indicate some eBect of the solar wind conditions on the precipitating Oux into the polar cusp. An alternative interpretation, could be that we have captured a dayside aurora as those transiently observed at high latitude on Earth according to Zhou and Tsurutani (1999) where their location is also hardly distinguishable from the polar cusp. This bright spot could also be related to the high latitude ‘auroral Oares’ presented by Clarke et al. (1999). Finally, there seems to exist an interesting, but still not understood, diBerence between the high latitude emissions in the north and in the south. We have not detected any secondary high latitude oval nor any ‘transpolar emission’ in the southern hemisphere. A duskside diBuse emission however, is also present inside the southern main oval, and its high latitude limit might visualize the duskside polar cap boundary. But there seems to be no sign of large precipitating Ouxes of any particular high latitude magnetic shell as in the north. In any case, the north–south asymmetry of the interaction of the magnetosphere with the solar wind on open =eld lines, which is implied by this interpretation, has still to be explained, and it would be extremely important to compare our =ndings with the predictions of MHD models for this interaction. Acknowledgements These data were obtained from various Hubble Space Telescope programs. The Hubble Space Telescope is operated by AURA. We wish to thank the HST team, and in particular Alex Storrs and Andrew Lubenow for their help in the scheduling of these complex observations. R.P. also thanks Bruce Tsurutani, Margaret Kivelson, and Krishan Khurana for fruitful discussions. Part of this study has been funded by the French INSU Programme National de Plan*etologie. References Ballester, G.E., Clarke, J.T., Trauger, J.T., Harris, W.M., Stapelfeldt, K.R., Crisp, D., Evans, R.W., Burgh, E.B., Burrows, C.J., Casertano, S., Gallagher, J.S.III, GriLths, R.E., Hester, J.J., Hoessel, J.G., Holtzman, J.A., Krist, J.E., Meadows, V., Mould, J.R., Sahai, R., Scowen, P.A., Watson, A.M., Westphal, J.A., 1996. Time-resolved observations of Jupiter’s far-ultraviolet aurora. Science 274, 409–413. Broadfoot, A.L., et al., 1979. Extreme ultraviolet observations from Voyager 1 encounter with Jupiter. Science 204, 979–982. Clarke, J.T., Moos, H.M., Atreya, S.T., Lane, A.L., 1980. Observations from Earth orbit and variability of the polar aurora of Jupiter. Astrophys. J. Lett. 241, L179–L182. Clarke, J.T, Ballester, G.E., Trauger, J., Evans, R., Connerney, J.E.P., Stapelfeldt, K., Crisp, D., Feldman, P.D., Burrows, C.J., Casertano, S., Gallagher, J.S.III, GriLths, R.E., Hester, J.J., Hoessel, J.G., Holtzman, J.A., Krist, J.E., Meadows, V., Mould, J.R., Scowen, P.A., Watson, A.M., Westphal, J.A., 1996. Far-ultraviolet imaging of Jupiter’s aurora and the Io “footprint”. Science 274, 404–409.

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