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We present an analysis of Europa's far-UV spectral albedo using observations during the 1999-2015 time period made by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Disk-in ...

In December 2001, the Space Telescope Imaging Spectrograph of the Hubble Space Telescope obtained far-ultraviolet spectral images of Jupiter's moon Callisto. The leading and trailing hemispheres we ...

We study the morphology of Ganymede's FUV aurora by analyzing spectral images obtained over the past two decades by the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. The ...

Far ultraviolet spectral observations have been made with the Hubble Space Telescope in the time-tag mode using the Space Telescope Imaging Spectrograph (STIS) long slit. The telescope was slewed in such a way that the slit projection scanned from above the polar limb down to midlatitudes, allowing us to build up the first spectral maps of the FUV Jovian aurora. The shorter wavelengths are partly absorbed by the methane layer overlying part of the auroral emission layer. The long-wavelength intensity directly reflects the precipitated energy flux carried by the auroral electrons. Maps of the intensity ratio of the two spectral regions have been obtained by combining spectral emissions in two wavelength ranges. They show that the amount of absorption by methane varies significantly between the different components of the aurora and inside the main emission region. Some of the polar emissions are associated with the hardest precipitation, although the auroral regions of strong electron precipitation do not necessarily coincide with the highest electron energies. Outputs from an electron transport model are used to create maps of the distribution of the characteristic electron energies. Using model atmospheres adapted to auroral conditions, we conclude that electron energies range between a few tens to several hundred keV. Comparisons of derived energies are in general agreement with those calculated from magnetosphere-ionosphere coupling models, with values locally exceeding the standard model predictions. These results will provide useful input for three-dimensional modeling of the distribution of particle heat sources into the high-latitude Jovian upper atmosphere.

[1] The auroral emission close to the foot of Io's magnetic flux tube has been known for over a decade to be one of the key parameters characterizing the electrodynamic interaction between the satellite and Jupiter's magnetosphere. Ten years of observations of Io's magnetic footprint brightness have been conducted via far ultraviolet imaging by two instruments, the Space Telescope Imaging Spectrograph and the Advanced Camera for Surveys, on board the Hubble Space Telescope. The variation of Io's magnetic footprint brightness was found to have a strong correlation with the satellite's location in system III longitude. The persistent pattern of the variation of the brightness of the auroral emission at Io's magnetic footprint with longitude over 10 years of observations implies that the footprint emissions are primarily determined by processes other than temporal variations of the plasma torus. The changing location of Io in the plasma torus with longitude corresponds to changes in centrifugal latitude and to the plasma density in the vicinity of Io, likely affecting the electrodynamic interaction at the satellite. To test this, quantitative models of electron density and the generated power near Io are applied to simulate the observed footprint brightness variation pattern. We find, however, that the longitudinal variations in plasma conditions needed to produce changes in the electrodynamic interaction comparable to the observed modulation of the footprint emissions would require an unrealistically colder plasma torus than previously measured. We quantify the additional energy needed to produce the asymmetric emission peaks at 110° and 270° longitudes.

We observed Titan with the Hubble Space Telescope in November 2000 using the Wide-Field Plane- tary Camera (WFPC2) and the Space Telescope Imaging Spectrograph (STIS). Considerable change is apparent be- tween these and observations in 1994-1997 : in particular the North-South asymmetry at visible wavelengths has re- versed, with the southern hemisphere brighter, as during the Voyager epoch. The asymmetry shows considerable varia- tion with wavelength in the near-infrared : in the 889nm methane band the asymmetry (which formerly resembled a 'smile') has reversed in the last 3 years, while at 953nm the southern limb is still bright. A south polar collar, dark at ultraviolet wavelengths, is apparent some five years after equinox.

Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) observations of Io acquired in 1997 [Roesler et al., 1999] provided the first simultaneous spatially resolved measurements of emission from neutral sulfur and oxygen, the dominant atomic species in Io's exospheric corona. Previous measurements of Io's corona relied primarily on sunlight resonantly scattered from sodium, a trace element in Io's atmosphere, and required measurement during mutual satellite eclipses to obtain the necessary spatial resolution. We present here spatial profiles of Io's extended emissions derived from observations spanning the time period from October 1997 to February 2000. The STIS Far Ultraviolet Multi-Anode Microchannel Array (FUV-MAMA) detector permits measurement of the emissions with a spatial resolution of ∼0.05 Io radii out to distances of ∼20 Io radii. Useful measurements are limited to ∼10 Io radii owing to the low signal-to-noise ratio of the extended emission features. The coronal emission profiles vary considerably in slope and intensity and are generally brighter for Io west (duskside) of Jupiter. Emission profiles obtained near western elongation are relatively symmetric about Io; profiles obtained in other orbital positions display varying degrees of asymmetry, with enhanced emissions and generally steeper slopes in the downstream direction relative to the plasma flow. The downstream-upstream profile asymmetry is thought to be caused by higher electron densities in Io's plasma wake. While the coverage of the data is limited in both Jovian System III coordinates and geocentric phase, the intensities of emission from regions both near Io and in the extended corona vary with System III longitude in a near-simultaneous fashion, suggesting local torus electron density as the probable source of this modulation. The observed ratio of oxygen to sulfur emission is relatively constant in time, perhaps reflecting the stoichiometric ratio of the SO 2 source molecules. Eclipse and posteclipse observations on February 25, 2000, show a dramatic increase in profile emission brightness and slope, suggesting a dynamic response by a sublimation-supported component of Io's SO 2 atmosphere and associated atomic species.

The first observations of Io with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) showed that the brightest ultraviolet emissions come from localized regions near Io's equator, designated “equatorial spots.” This paper presents a detailed study of the location, shape, and brightness of the equatorial spots in near-monochromatic images obtained using STIS in the first-order long-slit spectroscopy mode. This study provides evidence that the equatorial emissions are linked to the interaction between the Jovian magnetosphere and Io's atmosphere. The morphology of the equatorial spots reported here provides additional information on the nature of this complex electrodynamic interaction. We find the following principal results: the locations of the equatorial spots are correlated with the Jovian magnetic field orientation at Io, but with a relation that is not 1:1; the equatorial spots are centered 10°–30° longitude downstream from Io's sub-Jovian longitude; the brightness of the emissions in this data set is correlated with Io's distance from the plasma torus centrifugal equator; and the anti-Jovian equatorial spots are ∼20% brighter than the sub-Jovian equatorial spots.

Imaging spectroscopy of Io in the ultraviolet (1160–1720 A) was carried out with the Space Telescope Imaging Spectrograph on HST on three dates in October 1997 and August 1998. Among the initial results was the observation of concentrated regions of Hi Lyman-α flux near the poles of Io that exhibited a morphology and temporal variability different from those of the atomic oxygen and sulfur emission regions seen near the equatorial limbs. We examine the suggestion that the primary source of Lyman-α emission is surface reflected solar radiation that penetrates the thin polar atmosphere, but is strongly absorbed by the thicker SO2 atmosphere near Io's equator. Spectral and spatial analyses lead to derived SO2 column densities that are in good agreement with those derived from earlier HST observations of Io's albedo in the 2000–2300 A wavelength range. The Lyman-α images clearly illustrate features of Io's atmosphere that have been deduced from previous observations and theoretical modeling: a non-uniformity with respect to the sub-solar point dominated by a freezing out of the SO2 near the poles and variation with both longitude and time due to the variability of the sources of the atmospheric gas. Lyman-α imaging is demonstrated to be an extremely powerful and direct way to globally map the dynamic atmosphere of Io.

[1] The most dynamic part of the Jovian UV aurora is located inside the main auroral oval. This region is known to regularly show localized but dramatic enhancements on timescales of several tens of seconds, called polar flares. They have often been associated with the polar cusp, based on their location in the polar cap. The present study is based on the longest high-time resolution image sequences ever acquired by the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. We report the first observations of a regularity in the occurrence of these flares, with a timescale of 2–3 minutes. We use a magnetic flux mapping model to identify the region corresponding to these emissions in the equatorial plane: the radial distance ranges from 55 to 120 Jovian radii and the local times are between 10:00 and 18:00. The analogy with similar phenomena observed at Earth suggests that these quasi-periodic auroral flares could be related to pulsed reconnections at the dayside magnetopause. Indeed, the flares' projected location in the equatorial plane and their rate of re-occurrence show some similarities with the properties of the flux transfer events observed by the Pioneer and Voyager probes.

[1] During the past decade, FUV imaging of Jupiter's auroral region by the Hubble Space Telescope (HST) using two instruments, the Space Telescope Imaging Spectrograph (STIS) and the Advanced Camera for Surveys (ACS), has provided detailed information on the electrodynamic interaction between Io's, Ganymede's, and Europa's atmospheres and plasma in Jupiter's magnetosphere. This interaction is responsible for the satellites' auroral footprints in Jupiter's atmosphere connected via magnetic flux tubes to the satellites' interaction regions. The observed brightness of each auroral footprint is considered to be one main observable quantity to characterize the interaction environment at the satellites. Previous observations of Io's magnetic footprints using HST STIS images showed that the footprint emission appears brightest when Io is centered in the plasma torus. With the much larger data set obtained from the 2007 HST campaigns, we find the same variation observed by Serio and Clarke (2008), but with significantly better statistics over a time period of 10 years. These results confirm that Io's footprint brightness varies mainly with the satellite's location in Jupiter's plasma torus over a long time scale. Additional observations of the downstream emissions and their variations were presented by Bonfond et al. (2007). In Ganymede's case, the relation between the footprint brightness and the satellite's position in Jupiter's magnetosphere shows some evidence for the same general trend, although the data are noisier than the data for Io. Ganymede's footprint brightness appears to be less consistent over time than Io's. The variation of Ganymede's footprints over short time periods was studied by Grodent et al. (2009). Europa's fainter footprint brightness makes it difficult to see any systematic trend.

[1] Saturn’s ultraviolet (UV) auroras are highly variable, with much of the activity believed to be controlled by conditions in the solar wind. Like Jupiter, Saturn is also expected to have an electrodynamic interaction between the planet’s ionosphere and its satellites’ atmospheres, a process that is responsible for the moon’s auroral footprint components at Jupiter. Recent observations from several instruments on the Cassini spacecraft, e.g., the UltraViolet Imaging Spectrograph (UVIS) and the Cassini Plasma Spectrometer (CAPS), show strong evidence that Enceladus makes significant plasma contributions to Saturn’s magnetosphere. Modeling by Pontius and Hill (2006) suggests that Enceladus’ mass loading region may be comparable in extent to Io’s, whose auroral footprint emissions are clearly detected. However, the upper limits of Enceladus’ magnetic footprint emissions are predicted to be only a few tenths of kilo-Rayleighs (kR). We present results of a search for Enceladus’ auroral footprint using series of UV images of Saturn’s aurora taken by the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) in January 2004 and the Advanced Camera for Surveys (ACS) between February 2005 and January 2007. We detected no auroral evidence of the electromagnetic connection between Enceladus’ atmosphere and Saturn’s ionosphere, in agreement with modeling predictions. As our study could detect no signature of currents linking Saturn’s ionosphere and Enceladus’ interaction region, the implication is that the field-aligned currents associated with mass loading at Enceladus are too weak to cause auroral emission.

[1] The electromagnetic interaction between Io and Jupiter's magnetic field leads to single or multiple ultraviolet spots near the feet of the Io flux tube. Variations of spot numbers and brightness and of inter-spot distances have been observed to be linked to Io's position in its plasma torus. We have studied the evolution of the Io UV footprints with a time resolution of a few tens of seconds using the Space Telescope Imaging Spectrograph (STIS) in time-tag mode. We present evidence of systematic strong brightness variations of the main spots (up to 50%) with a typical growth time of 1 minute. Additionally, unanticipated simultaneous fluctuations of both primary and secondary spots have also been found in the southern hemisphere. Our findings suggest that the footprint brightness is not only actively controlled by the plasma directly interacting with Io but also by the poorly constrained electron acceleration region between Io and Jupiter.

[1] Observation of an exceptionally bright (peaking at ∼1.8 MR) Jovian auroral morning arc was obtained with the Space Telescope Imaging Spectrograph (STIS) on 21 September 1999, both in the imaging and spectral modes. The images of the HST orbit are used to describe the variation of the position of the bright arc, while the time-tagged spectra are examined to derive the properties of the precipitating auroral electrons, such as their mean energy and the electron current density at the top of the atmosphere. The first and the last images of the HST orbit, separated by 37 min, show that the bright morning emission is situated on the reference oval, with a “leading” edge fixed in λIII longitudes (i.e., rotating with the planet), and a “trailing” edge that extends into the nightside. The auroral arc is divided in two branches, as was also observed in some previous analyses. An isolated bright spot is also observed at λIII ∼184°. Its brightness reaches 500 kR and it also approximately corotates with Jupiter. Four regions of the auroral morning arc captured by the STIS aperture were extracted from the spectral observation. The four associated low-resolution spectra (∼4.8 A) show very different characteristics. In particular, two spectra reveal unusually high color ratios (18.5 and 45.5), with corresponding mean electron energies of ∼280 and ∼460 keV, respectively. The current densities associated with three of the spectra lie in the range 0.09–0.2 μA m−2, consistent with previous estimates, while the fourth spectrum is characterized by a mean current density of 0.54 μA m−2, outside the range ∼0.04–0.4 μA m−2 obtained in a previous study of G140L spectra of the Jovian main oval. Assuming that main oval aurorae are caused by field-aligned electric fields, the relationship between the energy flux and the current density derived from the spectra has been compared to the Knight's theory of field-aligned currents. Because of the very high acceleration potential derived from two of the extracted spectra, a relativistic treatment of the Knight theory was used. Assuming an electron temperature Te = 2.5 keV, it is seen that the two regions corresponding to earlier local times (higher λIII longitudes) reveal an electron source density lower than the values observed in the equatorial plane during the Voyager flybys. On the other hand, the equatorward region (lowest latitude) exhibits an electron source density in the upper range of usual values. Analysis of time-tag spectra reveals that the variations of the energy flux and the color ratios are large but continuous and generally covary.

[1] The Space Telescope Imaging Spectrograph camera on board Hubble Space Telescope obtained 68 FUV images of Saturn's southern auroral emission between 8 and 30 January 2004, during Cassini's approach to Saturn's magnetosphere. The HST observations took place in four different solar wind regimes with a low-field rarefaction region from 8 to 16 January, a minor compression event on 17 January, a rarefaction region with intermediate field strengths from 19 to 25 January, and a major compression region from 26 to 30 January. The images have been projected onto polar maps in order to characterize and compare the general morphology of the auroral emission. The first 20 images were obtained during a period covering about 70% of one full rotation of Saturn. They show that the bright ring of auroral emission actually consists of several arcs of different width and brightness and forming along different parallels. Overall, the auroral region is shown to rotate at ∼65% of the full planetary rotation, although the angular velocity of some isolated auroral structures constantly decrease with time, down to 20%. The strongest auroral precipitations are observed in the morning sector. The polar projections of the 48 remaining images confirm dramatic changes in morphology, characterized by different average zones. During the campaign, short intervals during which the auroral region is significantly contracted and clearly forms a spiral were followed by intervals of reinflation of the auroral region. It is suggested that the two major auroral contraction events corresponded to the arrival of the solar wind shocks observed by Cassini on 17 January and 25 January. The present analysis indicates that Saturn's auroral morphology responds to the solar wind conditions at Saturn.

[1] Images from the Hubble Space Telescope (HST) document the seasonal migration of haze in Titan's atmosphere. Image profiles show darkening of the north relative to the south at 439 nm (blue) but no change at 619 nm. The limb profile at 889 nm has inverted, becoming north-bright, a variation consistent with haze transport towards the winter hemisphere by winds. The complex altitude-time variation of the north-south haze differences are indicated in resolved spectra acquired with the Space Telescope Imaging Spectrograph (STIS): the continuum slope of a north/south ratio spectrum changes sign, becoming red, between 2000 and 2002, although the 889 nm band had already reversed by 2000, suggesting the haze distribution changes most rapidly at high altitudes.

The high resolution UV capabilities (lamda/delta lambda = 10(sup 5)) of the Hubble Space Telscope (HST) equipped with the Space Telescope Imaging Spectrograph (STIS) reflects a need for high resolution laboratory UV spectral data base for comparison with observation.

[1] 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 H2 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 A range are dominated by emissions of the Lyman-α line and H2 Werner and Lyman bands. Their comparison with a synthetic model of electron excited H2 emissions indicates the presence of a weak absorption below 1400 A by a column of methane ranging between 7 × 1015 and 2 × 1016 cm−2. 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.

[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 ∼0.04–0.4 μA 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 Te = 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.

[1] During the Io plasma torus crossings by the Galileo spacecraft in late 1999 and early 2000 we observed the dawn ansa of the torus using the Hubble Space Telescope (HST). Here we analyze 74 FUV HST/Space Telescope Imaging Spectrograph observations (1150 ≤ λ ≤ 1720 A) of the dawn ansa of the Io plasma torus made during Galileo's C23, I24, and I27 torus passages. These data include spectral scans at various System III longitudes plus two undispersed images of S++ 1720 A emission. Because of simultaneous observations by other instruments, the full value of these data will be developed by joint analysis, but here we present our results for their independent value. The S++ images show that the “ribbon” feature seen at visible wavelengths in ground-based observations is also clearly present in the FUV. The spectral scans were fitted using a simple plasma emission model integrated along the line of sight in order to invert the line-of-sight density superposition under the assumption of local azimuthal symmetry. Fits to the S+ 1256 A, S++ 1199 A, and S+++ 1410 A emissions reveal that the S++ and S+++ densities peak ∼0.2 RJ farther from Jupiter than does the S+ density. Moreover, the S++ vertical distribution is 25% more extended than that of S+, indicating that the field-aligned S++ kinetic temperature T∥ is ∼50% higher. It also appears that the S+ abundance dropped by at least a factor of 2 between the September–October 1999 and February 2000 epochs, while the other two sulfur ion abundances remained unchanged.

[1] 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] Spectral observation of both polar regions of Jupiter in the far ultraviolet (FUV) obtained with the Space Telescope Imaging Spectrograph (STIS), on board the Hubble Space Telescope from July 1997 to January 2001 have been combined with FUV images to map the FUV color ratio along the STIS slit. Spatially resolved spectra of the aurora carried at ∼12 A resolution have been used to determine the amount of methane absorption as measured by the FUV color ratio of the Io magnetic footprint and its trailing tail. It is found that the absorption is systematically less than in the main polar aurora, indicating a higher altitude source region. The color ratio of the north tail is shown to slowly decrease downstream from the footprint. The combination of these spectral data with a two-stream model of the interaction of energetic electrons with the Jovian thermosphere indicates that the mean energy of the electrons creating the north FUV emission ranges from ∼55 keV at the Io footprint to ∼40 keV, 20 degrees downstream in the tail. In parallel, the incident electron energy flux drops by a factor ∼6 over the same angular distance. These observations are consistent with the steady state slippage picture where the subcorotating flux tube is accelerated very slowly up to corotation owing to the nonideal coupling. It is argued that small deviations from corotation can supply sufficient energy to fuel the observed auroral emissions. It is suggested that the parallel electric field accelerating electrons out of the flux tube only moderately depends on the time elapsed since the contact with Io, although the mapping between a point in the tail and Io is very uncertain in the presence of magnetic field line slippage.