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See detailEnergy-flux relationship in the FUV Jovian aurora deduced from HST-STIS spectral observations
Gustin, Jacques ULg; Gérard, Jean-Claude ULg; Grodent, Denis ULg et al

in Journal of Geophysical Research. Space Physics (2004), 109(A10),

[1] Far ultraviolet spectral observations of the Jovian aurora have been made since 1997 with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope at low spectral resolution ... [more ▼]

[1] Far ultraviolet spectral observations of the Jovian aurora have been made since 1997 with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope at low spectral resolution. The combination of the spectral resolution with the intensity variation along the STIS slit provides information on the latitudinal variation of the precipitating auroral electron energy flux and the mean electron energy, from which the electron current density at the top of the atmosphere can also be deduced. It is found that the mean electron energies associated with the main oval lie in the range 30 - 200 keV and show a tendency to increase with the precipitating energy flux. The current densities lie in the range similar to 0.04 - 0.4 muA m(-2), consistent with previous estimates, and are also positively correlated with the energy flux. The observed relationship between the auroral time-integrated energy fluxes and the electron energies in the main oval is compatible with that expected from Knight's theory of field-aligned currents. The best agreement between the observed data and the Knight curves is obtained for an electron temperature of T-e = 2.5 keV and a source density N = 0.003 cm(-3), that is within the range of values observed in the equatorial plane during the Voyager flybys. No systematic dependence of the electron energy with magnetic local time is found, but the morning sector around 0800 MLT shows greater variability than other regions of the oval. Analysis of time-tagged data shows that the main oval energy flux usually varies steadily over the several minute intervals of observation and that the mean electron energy usually undergoes correlated variations such that the current density remains relatively constant. It is shown that these overall properties are also consistent with Knight's theory of auroral electron acceleration associated with field-aligned current flow, from which it is inferred that the temporal variations observed are often due to slow changes in the magnetospheric "source'' electron parameters in the presence of near-steady magnetosphere-ionosphere coupling currents. By contrast, time-integrated emissions in the polar region are found to be associated with similar mean electron energies to the main oval but with typically smaller energy fluxes and current densities. Pressure balance arguments are advanced, which indicate that the brighter of these emissions must be associated with an auroral acceleration mechanism perhaps similar to that operative in the main oval, while it remains possible that the weaker emissions could result from precipitation from a quasi-isotropic hot magnetospheric electron source. [less ▲]

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See detailJovian auroral spectroscopy with FUSE: analysis of self-absorption and implications for electron precipitation
Gustin, Jacques ULg; Feldman, Paul D.; Gérard, Jean-Claude ULg et al

in Icarus: International Journal of Solar System Studies (2004), 171(2), 336-355

High-resolution (similar to 0.22 Angstrom) spectra of the north jovian aurora were obtained in the 905-1180 Angstrom window with the Far Ultraviolet Spectroscopic Explorer (FUSE) on October 28, 2000. The ... [more ▼]

High-resolution (similar to 0.22 Angstrom) spectra of the north jovian aurora were obtained in the 905-1180 Angstrom window with the Far Ultraviolet Spectroscopic Explorer (FUSE) on October 28, 2000. The FUSE instrument resolves the rotational structure of the H-2 spectra and the spectral range allows the study of self-absorption. Below 1100 Angstrom, transitions connecting to the upsilon" less than or equal to 2 levels of the H-2 ground state are partially or totally absorbed by the overlying H2 molecules. The FUSE spectra provide information on the overlying H2 column and on the vibrational distribution of H-2. Transitions from high-energy H-2 Rydberg states and treatment of self-absorption are considered in our synthetic spectral generator. We show comparisons between synthetic and observed spectra in the 920-970, 1030-1080, and 1090-1180 Angstrom spectral windows. In a first approach (single-layer model), the synthetic spectra are venerated in a thin emitting layer and the emerging photons are absorbed by a layer located above the source. It is found that the parameters of the single-layer model best fitting the three spectral windows are 850, 800, and 800 K respectively for the H-2 gas temperature and 1.3 x 10(18), 1.5 x 10(20), and 1.3 x 10(20) cm(-2) for the H-2 self-absorbing vertical column respectively. Comparison between the H-2 column and a 1-D atmospheric model indicates that the short-wavelength FUV auroral emission originates from just above the homopause. This is confirmed by the high H-2 rovibrational temperatures, close to those deduced from spectral analyses of H-3(+) auroral emission. In a second approach, the synthetic spectral generator is coupled with a vertically distributed 3 energy degradation model, where the only input is the energy distribution of incoming electrons (multi-layer model). The model that best fits globally the three FUSE spectra is a sum of Maxwellian functions, with characteristic energies ranging from 1 to 100 keV, giving rise to an emission peak located at 5 mubar, that is similar to 100 km below the methane homopause. This multi-layer model is also applied to a re-analysis of the Hopkins Ultraviolet Telescope (HUT) auroral spectrum and accounts for the H2 self-absorption as well as the methane absorption. It is found that no additional discrete soft electron precipitation is necessary to fit either the FUSE or the HUT observations. (C) 2004 Elsevier Inc. All rights reserved. [less ▲]

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See detailCharacteristics of Saturn's FUV aurora observed with the Space Telescope Imaging Spectrograph
Gérard, Jean-Claude ULg; Grodent, Denis ULg; Gustin, Jacques ULg et al

in Journal of Geophysical Research. Space Physics (2004), 109

We analyze a set of 15 FUV images obtained between October 1997 and January 2001 with the Hubble Space Telescope Imaging Spectrograph (STIS), providing a good view of Saturn's south auroral oval. It is ... [more ▼]

We analyze a set of 15 FUV images obtained between October 1997 and January 2001 with the Hubble Space Telescope Imaging Spectrograph (STIS), providing a good view of Saturn's south auroral oval. It is found that the morphology and brightness distribution of the aurora are dynamical with variations occurring on time scales of hours or less. The dayside main oval lies between 70° and 80° and is generally brighter and thinner in the morning than in the afternoon sector. The afternoon sector is characterized by more diffuse emission at higher latitudes. Weak emission is also observed poleward of the main oval up to the pole. A spot of enhanced auroral precipitation, tentatively identified as the optical signature of the dayside cusp, is sometimes observed poleward of the main oval in the noon sector, especially during periods when the morning arc is not fully developed. A spiral structure of the main oval with arcs at two latitudes in the same sector is occasionally observed. The brightness of the main oval ranges from below the STIS threshold of 1 kR of H[SUB]2[/SUB] emission up to ~75 kR. The total electron precipitated power varies between 20 and 140 GW, that is, comparable to the Earth's active aurora but about two orders of magnitude less than on Jupiter. An increasing trend of the precipitated power between the 1997 and the 2000-2001 observations may be related to the rising solar activity. Six spectra of the aurora in the noon sector covering the 1200-1700 Å range are dominated by emissions of the Lyman-alpha line and H[SUB]2[/SUB] Werner and Lyman bands. Their comparison with a synthetic model of electron excited H[SUB]2[/SUB] emissions indicates the presence of a weak absorption below 1400 Å by a column of methane ranging between 7 × 10[SUP]15[/SUP] and 2 × 10[SUP]16[/SUP] cm[SUP]-2[/SUP]. The corresponding energy of the primary auroral electrons is estimated 12 +/- 3 keV, using a low-latitude model atmosphere based on Voyager occultation measurements. The main oval brightness and the characteristic electron energy are generally consistent with recent models of Saturn's aurora, which colocate the main oval with the narrow upward field-aligned current system associated with departure from plasma corotation near the open-closed field line boundary. The latitude of the bright morning arc is somewhat lower than model predictions based on the plasma flow velocity measured by Voyager in the middle magnetosphere. [less ▲]

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See detailStudy of the vertical structure of Saturn's atmosphere using HST/WFPC2 images
Munoz, O.; Moreno, F.; Molina, A. et al

in Icarus: International Journal of Solar System Studies (2004), 169(2), 413-428

We have studied the vertical structure of hazes at six different latitudes (-60degrees, -50degrees, -30degrees, -10degrees, +30degrees, and +50degrees) on Saturn's atmosphere. For that purpose we have ... [more ▼]

We have studied the vertical structure of hazes at six different latitudes (-60degrees, -50degrees, -30degrees, -10degrees, +30degrees, and +50degrees) on Saturn's atmosphere. For that purpose we have compared the results of our forward radiative transfer model to limb-to-limb retlectivity scans at four different wavelengths (230, 275, 673.2, and 893 nm). The images were obtained with the Hubble Space Telescope Wide Field Planetary Camera 2 in September 1997, during fall on Saturn's northern hemisphere. The spatial distribution of particles appears to be very variable with latitude both in the stratosphere and troposphere. For the latitude range +50degrees to -50degrees, an atmospheric structure consisting of a stratospheric haze and a tropospheric haze interspersed by clear gas regions has been found adequate to explain the center to limb reflectivities at the different wavelengths. This atmospheric structure has been previously used by Ortiz et al. (1996, Icarus 119, 53-66) and Stam et al. (2001, Icarus 152, 407-422). In this work the top of the tropospheric haze is found to be higher at the southern latitudes than at northern latitudes. This hemispherical asymmetry seems to be related to seasonal effects. Different latitudes experience different amount of solar insolation that can affect the atmospheric structure as the season varies with time. The haze optical thickness is largest (about 30 at 673.2 nm) at latitudes +/-50 and -10 degrees, and smallest (about 18) at 30 degrees. The stratospheric haze is found to be optically thin at all studied latitudes from -50 to +50 degrees being maximum at -10degrees (r = 0.033). At -60degrees latitude, where the UV images show a strong darkening compared to other regions on the planet, the cloud structure is remarkably different when compared to the other latitudes. Here, aerosol and gas are found to be uniformly mixed down to the 400 mbar level. (C) 2004 Elsevier Inc. All rights reserved. [less ▲]

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See detailThe Solar Wind Upstream of Saturn: Cassini Plasma measurements and Saturn's Aurora
Crary, F. J.; Young, D. T.; Barraclough, B. et al

Conference (2004, May 17)

For a full solar rotation in January and early February, 2004, the Cassini spacecraft and Hubble and Chandra Space Telescopes were used to make simultaneous observations of the solar wind and Saturn's ... [more ▼]

For a full solar rotation in January and early February, 2004, the Cassini spacecraft and Hubble and Chandra Space Telescopes were used to make simultaneous observations of the solar wind and Saturn's aurora. We report here on initial results from data taken with the Cassini Plasma Spectrometer's electron and high-resolution ion sensors in the solar wind upstream of Saturn. These measurements, combined with those of other particles and fields instruments on Cassini show two shock and corotating interaction regions, which reached Saturn approximately twelve hours later. An auroral response to each of these events was observed by the Hubble Space Telescope. [less ▲]

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See detailHST STIS Observations of Saturn's Auroral Variations Concurrent with the Cassini Solar Wind Campaign in Jan. 2004
Clarke, J. T.; Gérard, Jean-Claude ULg; Grodent, Denis ULg et al

Conference (2004, May 17)

Saturn's magnetosphere is often referred to as "intermediate between the cases of the Earth and Jupiter". Due to very limited measurements of Saturn's magnetosphere and auroral activity, however, it has ... [more ▼]

Saturn's magnetosphere is often referred to as "intermediate between the cases of the Earth and Jupiter". Due to very limited measurements of Saturn's magnetosphere and auroral activity, however, it has never been clear in detail what this statement means. A recent campaign of HST STIS UV imaging of Saturn's aurora has been carried out over 8-30 Jan. 2004 concurrent with measurements of the approaching solar wind by Cassini. This imaging set is much more comprehensive than any earlier observations of Saturn's aurora, obtained at a time when Saturn's southern auroral oval is completely visible due to the large apparent tilt of Saturn. The data provide the opportunity to determine the mean distribution of the auroral emissions, the degree of corotation of any bright regions, any variations with local time of the emissions, the latitudinal motions of the main oval with time and location, and other parameters. In addition, each of these can be compared with the approaching solar wind conditions and Saturn's kilometric radiation (SKR) intensity from Cassini measurements. Quick looks at the data from HST and Cassini demonstrate that the measurements have been made successfully, and the coverage includes dramatic variations in Saturn's auroral activity as well as at least two solar wind shocks passing Cassini. This presentation will concentrate on the measured properties of Saturn's aurora in the context of comparisons with the magnetospheres of the Earth and Jupiter. [less ▲]

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See detailComparisons of Saturn Kilometric Radiation and Saturn's UV Aurora
Kurth, William; Gurnett, D. A.; Clarke, J. T. et al

Conference (2004, May 17)

During the period 8 to 30 January 2004, a campaign to study the correlation between the solar wind and the response of Saturn's aurora was carried out using Cassini and the Hubble Space Telescope. In ... [more ▼]

During the period 8 to 30 January 2004, a campaign to study the correlation between the solar wind and the response of Saturn's aurora was carried out using Cassini and the Hubble Space Telescope. In particular, fields and particles instruments on Cassini were used to monitor the solar wind near Saturn and Saturn kilometric radio emissions nearly continuously. STIS images from Hubble were obtained approximately every other day to record Saturn's UV auroral morphology and intensity. In this paper we focus particularly on the relationship between the Saturn kilometric emissions and the auroral brightness and morphology. The radio emissions are generally believed to be generated by the cyclotron maser instability on auroral field lines similar to the situation for auroral kilometric radiation at Earth. A number of studies have shown a direct relationship between the radio emissions and discrete auroral arcs at Earth. Hence, one expects a relationship between the radio emissions and the aurora at Saturn. During the campaign, two corotating interaction regions (CIRs) swept past Cassini and Saturn (which were of order 0.5 AU apart at the time). Accompanying the CIRs were high solar wind densities. As expected from Voyager studies, the higher solar wind density resulted in generally more intense radio emissions. The UV images show that Saturn's UV aurora brighten considerably in response to the CIRs, as well. Furthermore, the brightest aurora usually appear in the local morning, consistent with the Voyager-determined SKR source region on field lines connecting to the magnetopause and the Kelvin-Helmholtz hypothesis for the origin of accelerated electrons. A more detailed examination of the auroral phenomena show much more complex variations, however. The radio emission frequency extent and peak frequency vary remarkably from one Saturn rotation to the next. Similarly, the auroral morphology changes dramatically. For example, it appears the evolution of the auroral oval to higher latitudes (higher L-shells) is correlated with a shift in the frequency of peak radio emissions to lower frequencies. This can be explained through an analysis of the cyclotron maser beaming geometry. We examine this and other aspects of the correlations between the radio emissions and the aurora. [less ▲]

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See detailAuroral Processes at Earth, Jupiter and Saturn.
Grodent, Denis ULg

Conference (2004, May 17)

We review the main characteristics of the auroral ultraviolet emissions at Earth, Jupiter and Saturn. Based on auroral morphology considerations, we discuss and compare the different solar wind ... [more ▼]

We review the main characteristics of the auroral ultraviolet emissions at Earth, Jupiter and Saturn. Based on auroral morphology considerations, we discuss and compare the different solar wind - magnetosphere - ionosphere coupling processes giving rise to these emissions. Earth's magnetosphere is usually described as 'open', meaning that its field reconnects with the interplanetary magnetic field (IMF) frozen in the solar wind. This reconnection process allows solar-wind plasma and energy to be transferred to the magnetosphere and to provide the main driving force for the auroral emissions. Different cases of solar-wind plasma conditions have been recognized to give rise to different types of auroral features. Jupiter is opposed to Earth, with a 'closed' magnetosphere. Its larger distance to the Sun and its enormous magnetic field make it difficult for the reconnection process with the IMF to occur efficiently. Io's volcanism is considered to be the prime (internal) plasma source for the magnetosphere, and corotation enforcement of this outward moving plasma is the likely process generating field aligned currents, responsible for the main auroral emissions. Saturn's aurora has not been as extensively studied as Earth's and Jupiter's. Owing to fainter magnetic field and internal plasma source than Jupiter, it has been expected to be intermediate between the cases of Earth and Jupiter. Recent detailed analysis of the Terrestrial, Jovian and Saturnian auroral morphology and dynamics suggests that the simple open/closed/open-closed magnetosphere picture is somewhat oversimplified. They show a much more complex situation with, for example, auroral activity without solar-wind reconnection at Earth, Earth-like reconnection signatures at Jupiter, or extreme auroral variability at Saturn. [less ▲]

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See detailA possible auroral signature of a magnetotail reconnection process on Jupiter
Grodent, Denis ULg; Gérard, Jean-Claude ULg; Clarke, J. T. et al

in Journal of Geophysical Research. Space Physics (2004), 109(A5),

[1] Several theoretical models and in situ observations consistently suggest that the process of nightside reconnection associated with Earth's magnetospheric substorms is also taking place in the ... [more ▼]

[1] Several theoretical models and in situ observations consistently suggest that the process of nightside reconnection associated with Earth's magnetospheric substorms is also taking place in the midnight tail region of Jupiter. We report the observation of a new auroral feature which takes the form of isolated spots appearing near the northern dusk-midnight limb, poleward of the main auroral oval. This feature was clearly detected in three HST-STIS data sets obtained on 14, 16, and 18 December 2000. Its position poleward of the main auroral oval, and lagging corotation, shows that it is magnetically connected to a distant region of the nightside magnetosphere. It is therefore suggested that these transient spots are the auroral signatures of reconnection processes occurring in the nightside tail of the distant Jovian magnetosphere. The cause of this reconnection is somewhat analogous to the case of a solar coronal mass ejection, so one may refer to it as a "Jovian mass ejection.'' [less ▲]

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See detailThe main characteristics of Saturn's aurora
Grodent, Denis ULg; Gérard, Jean-Claude ULg; Saglam, A. et al

Conference (2004, April 25)

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See detailAuroral and Non-auroral X-ray Emissions from Jupiter: A Comparative View
Bhardwaj, A.; Elsner, R.; Gladstone, R. et al

Poster (2004)

Jovian X-rays can be broadly classified into two categories: (1) "auroral" emission, which is confined to high-latitudes ( ˜>60° ) at both polar regions, and (2) "dayglow" emission, which originates from ... [more ▼]

Jovian X-rays can be broadly classified into two categories: (1) "auroral" emission, which is confined to high-latitudes ( ˜>60° ) at both polar regions, and (2) "dayglow" emission, which originates from the sunlit low-latitude ( ˜<50° ) regions of the disk (hereafter called "disk" emissions). Recent X-ray observations of Jupiter by Chandra and XMM-Newton have shown that these two types of X-ray emission from Jupiter have different morphological, temporal, and spectral characteristics. In particular: 1) contrary to the auroral X-rays, which are concentrated in a spot in the north and in a band that runs half-way across the planet in the south, the low-latitude X-ray disk is almost uniform; 2) unlike the ˜40±20-min periodic oscillations seen in the auroral X-ray emissions, the disk emissions do not show any periodic oscillations; 3) the disk emission is harder and extends to higher energies than the auroral spectrum; and 4) the disk X-ray emission show time variability similar to that seen in solar X-rays. These differences and features imply that the processes producing X-rays are different at these two latitude regions on Jupiter. We will present the details of these and other features that suggest the differences between these two classes of X-ray emissions from Jupiter, and discuss the current scenario of the production mechanism of them. [less ▲]

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See detailIdentification of a magnetic anomaly at Jupiter from satellite footprints
Grodent, Denis ULg

E-print/Working paper (2004)

Repeated imaging of Jupiter's aurora has shown that the northern main oval has a distorted 'kidney bean' shape in the general range of 90-140? System III longitude, which appears unchanged since 1994 ... [more ▼]

Repeated imaging of Jupiter's aurora has shown that the northern main oval has a distorted 'kidney bean' shape in the general range of 90-140? System III longitude, which appears unchanged since 1994. While it is more difficult to observe the conjugate regions in the southern aurora, no corresponding distortion appears in the south. Recent improved accuracy in locating the satellite footprint auroral emissions has provided new information about the geometry of Jupiter's magnetic field in this and other areas. The study of the magnetic field provides us with insight into the state of matter and the dynamics deep down Jupiter. There is currently no other way to do this from orbit. The persistent pattern of the main oval implies a disturbance of the local magnetic field, and the increased latitudinal separation of the locus of satellite footprints from each other and from the main oval implies a locally weaker field strength. It is possible that these phenomena result from a magnetic anomaly in Jupiter's intrinsic magnetic field, as was proposed by A. Dessler in the 1970's. There is presently only limited evidence from the scarcity of auroral footprints observed in this longitude range. We propose to obtain HST UV images with specific observing geometries of Jupiter to determine the locations of the auroral footprints of Io, Europa, and Ganymede in cycle 13 to accurately determine the magnetic field geometry in the suggested anomaly region, and to either confirm or refute the suggestion of a local magnetic anomaly. [less ▲]

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See detailSimultaneous Chandra X-ray, HST UV, and Ulysses Radio Observations of Jupiter's Aurora
Elsner, R. F.; Bhardwaj, A.; Waite, J. H. et al

Poster (2004)

Observations of Jupiter carried out by the Chandra ACIS-S instrument over 24-26 February, 2003, show that the auroral X-ray spectrum consists of line emission consistent with high-charge states of ... [more ▼]

Observations of Jupiter carried out by the Chandra ACIS-S instrument over 24-26 February, 2003, show that the auroral X-ray spectrum consists of line emission consistent with high-charge states of precipitating ions, and not a continuum as might be expected from bremsstrahlung. The part of the spectrum due to oxygen peaks around 650 eV, which indicates a high fraction of fully-stripped oxygen in the precipitating ion flux. The OVIII emission lines at 653 eV and 774 eV, as well as the OVII emission lines at 561 eV and 666 eV, are clearly identified. There is also line emission at lower energies in the spectral region extending from 250 to 350 eV for which sulfur and carbon lines are possible candidates. The Jovian auroral spectra differ significantly from measured cometary X-ray spectra. The charge state distribution of the oxygen ion emission evident in the measured auroral spectra strongly suggests that, independent of the source of the energetic ions (magnetospheric or solar wind) the ions have undergone additional acceleration. For the magnetospheric case, acceleration to energies exceeding 10 MeV is apparently required. The ion acceleration also helps to explain the high intensities of the X-rays observed. The phase space densities of unaccelerated source populations of either solar wind or magnetospheric ions are orders of magnitude too small to explain the observed emissions. The Chandra X-ray observations were executed simultaneously with observations at ultraviolet wavelengths by the Hubble Space Telescope and at radio wavelengths by the Ulysses spacecraft. These additional data sets provide interesting hints as to the location of the source region and the acceleration characteristics of the generation mechanism. The combined observations suggest that the source of the X rays is magnetospheric in origin, and that strong field-aligned electric fields are present which simultaneously create both the several-MeV energetic ion population and the relativistic electrons believed to be responsible for the generation of 40 minute quasi-periodic radio outbursts. [less ▲]

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See detailJupiter's Aurora
Clarke, John T.; Grodent, Denis ULg; Cowley, Stan S. W. et al

in Bagenal, Fran; Dowling, Timothy; McKinnon, William (Eds.) Jupiter : The Planet, Satellites and Magnetosphere (2004)

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See detailThe morphology of Saturn's ultraviolet auroral oval and its time variations
Gérard, Jean-Claude ULg; Grodent, Denis ULg; Clarke, J. T.

Conference (2004)

Global images of the FUV auroral emission surrounding Saturn's south pole have been obtained with Space Telescope Imaging Spectrograph (STIS) since 1998. During this period, the planet's tilt offered a ... [more ▼]

Global images of the FUV auroral emission surrounding Saturn's south pole have been obtained with Space Telescope Imaging Spectrograph (STIS) since 1998. During this period, the planet's tilt offered a nearly complete view of the south auroral oval. Several tens of images were obtained in January 2004, concurrent with in situ measurements of the solar wind parameters made with instruments on board the Cassini probe. This unique set of auroral images includes time-tagged exposures providing information on fast time variations. A subset of collected images is used to define a quiet reference oval. This oval serves a framework to investigate local time and longitudinal variations of brightness and latitudinal motions of the oval, including variation of its radius. Occasionally, the oval is not closed, or reduces to a bright spot apparently as a consequence of enhanced solar wind pressure pulses reaching the front of the magnetosphere. Comparisons with terrestrial counterpart indicate that auroral substorms are not observed on the nightside oval. Instead, some features are seen to rotate with the planet as in the Jovian aurora. These characteristics will be compared with predictions of recent models describing the global current system coupling the magnetosphere and the ionosphere. In particular, the latitude of the oval, departure from corotation and the longitudinal brightness distribution can be used to test the paradigm that the main auroral oval maps to a region of enhanced field-aligned current and to improve current models. [less ▲]

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See detailJupiter's main auroral oval observed with HST-STIS
Grodent, Denis ULg; Clarke, J. T.; Kim, J. et al

in Journal of Geophysical Research. Space Physics (2003), 108(A11),

An extended series of FUV images obtained on 7 days during winter 2000-2001, with fixed pointing, yielded highly accurate tracking of emisson features as Jupiter rotated. They provided newly detailed ... [more ▼]

An extended series of FUV images obtained on 7 days during winter 2000-2001, with fixed pointing, yielded highly accurate tracking of emisson features as Jupiter rotated. They provided newly detailed measurements of the degree of corotation of auroral emissions and their variations with changing central meridian longitude. This 2-month data set provides a statistical average location of the auroral emission and leads to the definition of new "reference ovals.'' The overall auroral morphology pattern is shown to be fixed in System-III longitude and unchanged over a 5-year period. When arranged in central meridian longitude ranges, the images show a significant contraction of the northern main oval as the central meridian longitude increases from 115 to 255degrees. The main auroral oval brightness is globally very stable in comparison with its terrestrial counterpart. It is shown to vary with magnetic local time, increasing from noon to dusk and then decreasing again in the magnetic evening. Hectometric emissions observed simultaneously with Galileo and Cassini reveal interplanetary shocks propagating outward from the Sun which may be related to the contraction of the main auroral oval observed in the HST images taken on 14 December 2000. In addition, we find that a brightening and a significant contraction of the main oval observed on 13 January 2001 corresponded to a time of increased solar wind dynamic pressure. [less ▲]

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See detailJupiter's polar auroral emissions
Grodent, Denis ULg; Clarke, J. T.; Waite, J. H. et al

in Journal of Geophysical Research. Space Physics (2003), 108(A10),

[1] This paper reports a study of Jupiter's polar auroral emissions observed in an extended series of FUV images. They were obtained on seven days, during winter 2000-2001, with the STIS camera on board ... [more ▼]

[1] This paper reports a study of Jupiter's polar auroral emissions observed in an extended series of FUV images. They were obtained on seven days, during winter 2000-2001, with the STIS camera on board the Hubble Space Telescope. The fixed pointing yielded highly accurate and consistent tracking of emisson features as Jupiter rotated, allowing the analysis of the auroral morphology and brightness on timescales ranging from seconds to days. In the Northern Hemisphere, the polar emissions, located poleward of the main oval, usually represent about 30% of the total auroral FUV emitted power. They show emission bursts lasting similar to100 s, while the main oval remains stable. The polar region may be divided into three regions apparently fixed in magnetic local time: the dawnside dark region, the poleward swirl region, and the duskside active region in which flares and arc-like features are observed. Each of these UV emission regions can be identified with its infrared counterpart and probably relates to a different sector of the Dungey cycle or Vasyliunas cycle plasma flows. [less ▲]

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See detailSpectral observations of transient features in the FUV Jovian polar aurora
Gérard, Jean-Claude ULg; Gustin, Jacques ULg; Grodent, Denis ULg et al

in Journal of Geophysical Research. Space Physics (2003), 108

Images of the Jovian FUV aurora show several morphologically and dynamically different regions. The main oval is usually fairly steady while the high-latitude emission inside the oval can vary over ... [more ▼]

Images of the Jovian FUV aurora show several morphologically and dynamically different regions. The main oval is usually fairly steady while the high-latitude emission inside the oval can vary over timescales of <1 min. We describe time and spatially resolved spectral observations of the Jovian aurora made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. Time-tagged spectra have been used to determine variations of the amount of hydrocarbon absorption above the auroral source layer. Rapid brightenings are observed in the high-latitude emission with typical lengths of 50-100 s. They generally are not correlated with enhancements of the main oval in the same longitude sector, indicating decoupled source regions in the magnetosphere. The location of the high-latitude spots and their variability suggest that, unlike the main auroral oval, the polar cap brightenings are connected to short-term variations of the solar wind parameters. Results from a two-stream electron transport model are used to convert observed spectral color ratios into mean electron energies <E>. Electron energies during these brightenings typically range from 40 to 120 keV, close to the values found in the main oval. The time evolution of <E> generally shows little correlation or anticorrelation with the energy flux precipitated during these transient events. This feature suggests that the mechanism responsible for rapid transient brightenings does not increase the energy of the precipitated electrons, but it enhances their number flux. Pitch angle scattering into the loss cone by magnetic pumping of energy perpendicular to the field lines following magnetic field compression is a possible process. [less ▲]

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