References of "Clarke, J. T"
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See detailThe Degree of Correlation of Jovian and Saturnian Auroral Emissions With Solar Wind Conditions
Clarke, J. T.; Nichols, J.; Gérard, Jean-Claude ULiege et al

Conference (2008, December 01)

While the terrestrial aurorae are known to be driven primarily by the interaction of the Earth's magnetosphere with the solar wind, auroral emissions on Jupiter and Saturn are thought to be driven ... [more ▼]

While the terrestrial aurorae are known to be driven primarily by the interaction of the Earth's magnetosphere with the solar wind, auroral emissions on Jupiter and Saturn are thought to be driven primarily by internal processes, with the main energy source being the planets' rapid rotation. Limited evidence has suggested there might be some influence of the solar wind on Jupiter's aurorae, and indicated that auroral storms on Saturn can occur at times of solar wind pressure increases. To investigate in detail the dependence of auroral processes on solar wind conditions, a large campaign of observations of these planets has been undertaken using the Hubble Space Telescope, in association with measurements from planetary spacecraft and solar wind conditions both propagated from one AU and measured near each planet. The data indicate a consistent brightening of both the auroral emissions and Saturn Kilometric Radiation (SKR) at Saturn close in time to the arrival of solar wind shocks and pressure increases, consistent with a direct physical relationship between Saturnian auroral processes and solar wind conditions. This correlation has been strengthened by the final campaign observations in Feb. 2008. At Jupiter the situation is less clear, with increases in total auroral power seen near the arrival of solar wind forward shocks, while little increase has been observed near reverse shocks. In addition, auroral dawn storms have been observed when there was little change in solar wind conditions. The data are consistent with some solar wind influence on some Jovian auroral processes, while the auroral activity also varies independently of the solar wind. This extensive data set will serve to constrain theoretical models for the interaction of the solar wind with the magnetospheres of Jupiter and Saturn. [less ▲]

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See detailCoordinated measurements of auroral processes at Saturn from the Cassini spacecraft and HST
Mitchell, D. G.; Kurth, William; Hospodarsky, G. B. et al

Conference (2008, December 01)

One of the primary Cassini mission objectives at Saturn is to characterize Saturn's aurora-its spatial morphology, associated particle energization, radio wave generation, and magnetospheric currents ... [more ▼]

One of the primary Cassini mission objectives at Saturn is to characterize Saturn's aurora-its spatial morphology, associated particle energization, radio wave generation, and magnetospheric currents, relationship with solar wind pressure and magnetic field, and its large scale mapping to the magnetosphere. By design, the Cassini orbital tour included high inclination and low periapsis orbits late in the prime mission specifically to address many of these topics. In this presentation, we will provide a snapshot of the current state of our investigation into the relationship between magnetospheric measurements of particles and fields, and the aurora. For in situ data, we will show measurements of upward traveling light ion conics (~30 keV to 200 keV), often accompanied by electron beams (<20 keV to ~1 MeV) and enhanced broadband noise (10 Hz to a few kHz), throughout the outer magnetosphere on field lines that nominally map from well into the polar cap (dipole L > 50) to well into the closed field region (dipole L < 10). Sometimes the particle phenomena and the broadband noise occur in pulses of roughly five-minute duration, separated by tens of minutes. At other times they are relatively steady over an hour or more. Magnetic signatures associated with some of the pulsed events are consistent with field aligned current structures. Correlative observations of solar wind (Cassini) and aurora (HST) have established a strong relationship between solar wind pressure and auroral activity (brightness) (Crary et al., Nature, 2005; Clarke et al., JGR, 2008). A similar correspondence between bright auroral arcs and ring current ion acceleration will be shown here. So while some auroral forms seem to be associated with the open/closed field boundary (i.e. in the cusp-Bunce et al., JGR, 2008), we also demonstrate that under some magnetospheric conditions for which protons and oxygen ions are accelerated once per Saturn magnetosphere rotation at a preferred local time between midnight and dawn, simultaneous auroral observations by the HST reveal a close correlation between these dynamical magnetospheric events and dawn-side transient auroral brightenings. Likewise, many of the recurrent energetic neutral atom enhancements coincide closely with bursts of Saturn kilometric radiation, again suggesting a linkage with high latitude auroral processes. Finally, we will show some intriguing results of auroral movie sequences from the Cassini UVIS instrument with corresponding ring current movies from the Magnetospheric Imaging Instrument Ion and Neutral Camera (MIMI/INCA). [less ▲]

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See detailAuroral polar dawn spots: Signatures of internally driven reconnection processes at Jupiter's magnetotail
Radioti, Aikaterini ULiege; Grodent, Denis ULiege; Gérard, Jean-Claude ULiege et al

in Geophysical Research Letters (2008), 35(3), 03104

We report the presence of polar spots located in the dawn auroral region, based on the HST ACS 2007 campaign. We study the location of these features in the equatorial plane as well as their time scales ... [more ▼]

We report the presence of polar spots located in the dawn auroral region, based on the HST ACS 2007 campaign. We study the location of these features in the equatorial plane as well as their time scales and periodicities, based on a comprehensive series of images taken between February 21 and June 11, 2007. It is shown that the majority of polar dawn spots magnetically map to the dawn sector. Additionally, they occur quasi-periodically every 2-3 days, a periodicity observed for the first time in auroral features. Because of their mapped location and their periodic cycle, we interpret the polar dawn spots as signatures of internally driven magnetic reconnection in the Jovian magnetotail. [less ▲]

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See detailOrigin of Saturn's aurora: Simultaneous observations by Cassini and the Hubble Space Telescope
Bunce, E. J.; Arridge, C. S.; Clarke, J. T. et al

in Journal of Geophysical Research (2008), 113(A9),

Outer planet auroras have been imaged for more than a decade, yet understanding their physical origin requires simultaneous remote and in situ observations. The first such measurements at Saturn were ... [more ▼]

Outer planet auroras have been imaged for more than a decade, yet understanding their physical origin requires simultaneous remote and in situ observations. The first such measurements at Saturn were obtained in January 2007, when the Hubble Space Telescope imaged the ultraviolet aurora, while the Cassini spacecraft crossed field lines connected to the auroral oval in the high-latitude magnetosphere near noon. The Cassini data indicate that the noon aurora lies in the boundary between open- and closed-field lines, where a layer of upward-directed field-aligned current flows whose density requires downward acceleration of magnetospheric electrons sufficient to produce the aurora. These observations indicate that the quasi-continuous main oval is produced by the magnetosphere-solar wind interaction through the shear in rotational flow across the open-closed-field line boundary. [less ▲]

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See detailOscillation of Saturn's southern auroral oval
Nichols, J. D.; Clarke, J. T.; Cowley, S. W. H. et al

in Journal of Geophysical Research (2008), 113(A11),

Near-planetary-period oscillations in the Cassini plasma and magnetic field data have been observed throughout Saturn's magnetosphere despite the fact that Saturn's internal magnetic field is apparently ... [more ▼]

Near-planetary-period oscillations in the Cassini plasma and magnetic field data have been observed throughout Saturn's magnetosphere despite the fact that Saturn's internal magnetic field is apparently highly axisymmetric. In addition, the period of the Saturn kilometric radiation has been shown to vary over time. In this paper we present results from the recent Hubble Space Telescope observations of Saturn's southern ultraviolet auroral emission. We show that the center of the auroral oval oscillates with period 10.76 h +/- 0.15 h for both January 2007 and February 2008, i.e., close to the periods determined for oscillations in other magnetospheric phenomena. The motion of the oval center is described for 2007 by an ellipse with semimajor axis similar to 1.4 degrees +/- 0.3 degrees oriented toward similar to 09-21 h LT, eccentricity similar to 0.93, and center offset from the spin axis by similar to 1.8 degrees toward similar to 04 h LT. For 2008 the oscillation is consistent with an ellipse with semimajor axis similar to 2.2 degrees +/- 0.3 degrees oriented toward similar to 09-21 h LT, eccentricity similar to 0.99, and a center offset from the spin axis by similar to 2.2 degrees toward similar to 03 h LT. The motion of the auroral oval is thus highly elliptical in both cases, and the major oscillation axis is oriented toward prenoon/premidnight. This result places an independent constraint on the magnitude of the planet's dipole tilt and may also indicate the presence of an external current system that imposes an asymmetry in the ionospheric field modulated close to the planetary period. [less ▲]

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See detailMeasured Correlations of Auroral Emissions from Jupiter and Saturn With Solar Wind Variations
Clarke, J. T.; Nichols, J.; Gérard, Jean-Claude ULiege et al

Conference (2007, December 01)

An extended set of Hubble Space Telescope (HST) observations of the UV auroral emissions from Jupiter and Saturn has been carried out in three campaigns over Jan.-June 2007. This is by far the most ... [more ▼]

An extended set of Hubble Space Telescope (HST) observations of the UV auroral emissions from Jupiter and Saturn has been carried out in three campaigns over Jan.-June 2007. This is by far the most extensive series of remote high resolution imaging of planetary aurora to date, and provides new physical insight into the cause and effect relationships governing the controlling processes for the giant planet auroral emissions. Simultaneous in situ measurements of local solar wind and magnetospheric plasma conditions have been made during two of these campaigns by Cassini at Saturn in Jan. 2007 and by the New Horizons mission approaching Jupiter in Feb. 2007. The UV auroral emission brightness and distributions have also been compared with estimates of the solar wind conditions near each planet extrapolated from near-Earth measurements, which can be verified by comparison with Cassini and New Horizons in situ data. It has been found that there is a good correlation at both planets between total auroral power and solar wind dynamic pressure, at least for the major solar wind disturbances arriving at each planet. At the same time, the nature of the auroral brightenings differs between Jupiter and Saturn, and the source regions of auroral activity are quite different in the two magnetospheres. In this presentation, the HST and solar wind data and the nature of the correlations will be presented. The physical significance of the correlations will be discussed, based on the much denser set of measurements now available. [less ▲]

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See detailCassini UVIS Observations of Saturn's Auroras
Pryor, W. J.; Ajello, J. M.; Gustin, Jacques ULiege et al

Conference (2007, June 25)

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See detailSaturn's auroral morphology and activity during quiet magnetospheric conditions
Gérard, Jean-Claude ULiege; Grodent, Denis ULiege; Cowley, S. W. H. et al

in Journal of Geophysical Research. Space Physics (2006), 111

We report the results of a coordinated Hubble Space Telescope-Cassini campaign that took place between 26 October and 2 November 2005. During this period, Saturn's magnetosphere was in an expanded state ... [more ▼]

We report the results of a coordinated Hubble Space Telescope-Cassini campaign that took place between 26 October and 2 November 2005. During this period, Saturn's magnetosphere was in an expanded state and the solar wind was quiet, as indicated by the location of the magnetopause, in situ particle measurements, weak auroral SKR emission, and the generally low brightness of the aurora. We describe the morphology and dynamics of the aurora during this period in parallel with concurrent Cassini measurements. We show that the aurora exhibits considerable longitudinal structure and time variations over intervals of a few hours, in spite of the absence of observable external triggers and generally low intensity. In particular, enhancements of the dawn-morning oval are seen while no apparent indication of solar wind activity is observed. These features rotate at a speed corresponding to about 65% of the planet's angular velocity. We also describe energetic neutral atom measurements indicating that an ENA acceleration event occurred in the magnetotail on 26 October without any measured signature of solar wind activation. These observations suggest an intrinsically dynamical magnetosphere where injection of hot plasma occasionally takes place in the night or dawn sector during quiet magnetospheric conditions, possibly connected with either the Dungey or the Vasyliunas convection cycle. [less ▲]

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See detailComprehensive auroral imaging of Saturn during the International Heliophysical Year
Nichols, J. D.; Clarke, J. T.; Duval, J. et al

Conference (2006, December 01)

As part of the International Heliophysical Year in 2007, a large-scale campaign is planned to observe the UV auroras of Jupiter and Saturn with the Hubble Space Telescope (HST). In this talk we will ... [more ▼]

As part of the International Heliophysical Year in 2007, a large-scale campaign is planned to observe the UV auroras of Jupiter and Saturn with the Hubble Space Telescope (HST). In this talk we will provide an overview of the Saturn campaign. Previous HST observations of Saturn's auroras have greatly extended our knowledge of the processes that occur in the planet's magnetosphere. In particular, Saturn's main oval auroras vary much more than Jupiter's in terms of latitudinal position and extent, and have been shown to be correlated with the conditions in the solar wind. However, these campaigns have generally been limited by their short-term nature, and in order to establish exactly how the auroras depend on the solar wind we must make observations continually over at least one complete solar rotation. This is the goal of the 2007 campaign, in which Saturn will be observed for a period of 30 days in January and February. The timing is fortuitous since not only will the planet be in opposition, allowing near-Earth measurements of the interplanetary medium to be extrapolated to Saturn's orbit, the Cassini spacecraft will be in a position to obtain in-situ plasma and magnetic field measurements from the magnetosphere and solar wind. Cassini will also make observations of Saturn's UV and SKR emissions throughout the campaign, and the planet will be observed by ground-based IR and radio telescopes. Here we provide a brief review of our current understanding of Saturn's auroras, along with an overview of the coordinated observations planned at Saturn and the key science goals we aim to address. [less ▲]

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See detailThe HST UV Auroral Imaging Campaign of Jupiter and Saturn during the International Heliophysical Year
Clarke, J. T.; Gérard, Jean-Claude ULiege; Grodent, Denis ULiege et al

Conference (2006, December 01)

An extended campaign of observations of the UV auroral emissions from Jupiter and Saturn is scheduled for three periods beginning in Jan. 2007 and ending in late June 2008. This will be by far the most ... [more ▼]

An extended campaign of observations of the UV auroral emissions from Jupiter and Saturn is scheduled for three periods beginning in Jan. 2007 and ending in late June 2008. This will be by far the most extensive series of remote high resolution imaging of planetary aurora to date, and should provide new physical insight into the cause and effect relationships governing the controlling processes for the giant planet auroral emissions. These observations will overlap with in situ measurements of local solar wind and magnetospheric plasma conditions by Cassini at Saturn in Jan. 2007 and by the New Horizons mission approaching Jupiter in Feb. 2007. The UV auroral emission brightness and distributions will also be compared with extrapolated estimates of the solar wind conditions near each planet from periods just before planetary opposition in Jan. 2007 (Saturn) and June 2007 (Jupiter). The HST observations will also be coordinated with ground-based observations of near-IR auroral and nonthermal radio emissions. This paper will give an overview of the program, including the schedule of HST observations and the schedule of known coordinated observations. While a preliminary schedule has been submitted for the HST observations, this schedule will be finalized only when the HST orbit is sufficiently well known for the periods of observation for detailed pointing to be specified. By the time of Fall AGU, it should be possible to show the detailed schedule and pointing for the Jan-Feb 2007 observations. The paper will include a presentation of the plans for the rapid reduction and distribution of the HST auroral images to the community. [less ▲]

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See detailCharacteristics of Jovian morning bright FUV aurora from Hubble Space Telescope/Space Telescope Imaging Spectrograph imaging and spectral observations
Gustin, Jacques ULiege; Cowley, S. W. H.; Gérard, Jean-Claude ULiege et al

in Journal of Geophysical Research. Space Physics (2006), 111(A9),

Observation of an exceptionally bright (peaking at similar to 1.8 MR) Jovian auroral morning arc was obtained with the Space Telescope Imaging Spectrograph (STIS) on 21 September 1999, both in the imaging ... [more ▼]

Observation of an exceptionally bright (peaking at similar to 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 lambda(III) 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 lambda(III) similar to 184 degrees. 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 (similar to 4.8 degrees) show very different characteristics. In particular, two spectra reveal unusually high color ratios (18.5 and 45.5), with corresponding mean electron energies of similar to 280 and similar to 460 keV, respectively. The current densities associated with three of the spectra lie in the range 0.09-0.2 mu A m(-2), consistent with previous estimates, while the fourth spectrum is characterized by a mean current density of 0.54 mu A m(-2), outside the range similar to 0.04-0.4 mu A 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 T-e = 2.5 keV, it is seen that the two regions corresponding to earlier local times (higher lambda(III) 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. [less ▲]

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See detailMorphology of the ultraviolet Io footprint emission and its control by Io's location
Gérard, Jean-Claude ULiege; Saglam, Adem ULiege; Grodent, Denis ULiege et al

in Journal of Geophysical Research. Space Physics (2006), 111(A4),

[1] A total of 74 images of the ultraviolet footprint of the Io flux tube (IFT) on Jupiter's upper atmosphere made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope have ... [more ▼]

[1] A total of 74 images of the ultraviolet footprint of the Io flux tube (IFT) on Jupiter's upper atmosphere made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope have been analyzed to characterize their location, morphology, and brightness distribution. The observations cover a wide range of central meridian Jovian longitudes and Io orbital positions and include north and south footprint emissions. Comparing the location of the IFT with that expected from the VIP4 model of the Jovian magnetic field, we find that the lead angle is generally not significantly different from zero in the System III longitude sector 125 degrees - 195 degrees. Instead, the lead angles reach about 8 degrees in the 50 degrees sector, coinciding with a region of possible magnetic anomaly. We observe that the brightness of the main footprint shows intrinsic intensity changes that appear to be controlled by the system III longitude of Io and its position above or below the center of the torus. The size of the primary spot magnetically maps into a region varying from 1 to over 10 Io diameters in Io's orbital plane. Multiple footprints are observed with varying brightness relative to the mean spot. The number of spots is found to increase as Io gets closer to the torus outer edge facing the spots. The separation between the first and second spots is typically 1 degrees-3 degrees of longitude and increases when Io is displaced from the torus center in the direction of the IFT signature. These features confirm that Alfven waves play an important role and generate energization of precipitated electrons. However, the observed variation of the FUV spot structure with Io's position appears inconsistent with models where reflections of Alfven wings occur between the torus boundary and Jupiter's ionosphere. Instead, the multiple spots apparently correspond to electron precipitation generated by Alfven waves reflected inside the plasma torus. [less ▲]

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See detailEuropa's FUV auroral tail on Jupiter
Grodent, Denis ULiege; Gérard, Jean-Claude ULiege; Gustin, Jacques ULiege et al

in Geophysical Research Letters (2006), 33(6),

Ultraviolet images of Jupiter's northern aurora obtained in 2005 confirm the existence of an electromagnetic interaction between Europa and the Jovian ionosphere. The auroral signature shows a two ... [more ▼]

Ultraviolet images of Jupiter's northern aurora obtained in 2005 confirm the existence of an electromagnetic interaction between Europa and the Jovian ionosphere. The auroral signature shows a two-component structure: a quasi-circular Europa spot, followed by a previously undetected faint tail emission trailing in the direction of corotation flow. The characteristic brightness for the auroral spot is similar to 14 +/- 1 kR above background, and approximately 7 +/- 1 kR for the tail. The spot's size is similar to 1100 km, magnetically mapping to an interaction region <= 15 Europa diameters. The auroral tail extends over similar to 5000 km, which maps along a region of at least 70 Europa diameters. The ultraviolet power emitted by both components varies from a fraction to several GW. The present study suggests auroral interaction at Europa similar to that at Io, but scaled-down by an order of magnitude, including a sub-corotating plasma plume in the geometrical wake of Europa. [less ▲]

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See detailCurrent thinking about Jupiter's magnetic anomaly
Grodent, Denis ULiege; Gérard, Jean-Claude ULiege; Gustin, Jacques ULiege et al

Conference (2006)

Repeated imaging of Jupiter's aurora has shown that the northern main oval has a distorted 'kidney bean' shape in the general range of 90-150o 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 90-150o 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 auroral footprint emission of Io has provided new information about the geometry of Jupiter's magnetic field in this and other areas. The persistent pattern of the main oval implies a disturbance of the local magnetic field, and the increased latitudinal separation of the locus of the Io footprint from the main oval implies a locally weaker field strength. The most recent images obtained with the Hubble Space Telescope Advance Camera for Surveys (ACS) allow us to complement previous observations with the location of the auroral footprints of Io, Europa, and Ganymede in the region of interest. Their footpaths vary in parallel and form a kink in the 90-150° S3 sector which strongly suggests the presence of a magnetic anomaly in this region. [less ▲]

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See detailOpen Flux Estimates and Reconnection Rates in Saturn's Magnetosphere, Derived Using HST and Cassini Data
Badman, S. V.; Bunce, E. J.; Clarke, J. T. et al

Conference (2005, December 01)

During January 2004, a sequence of 68 UV images of Saturn's southern aurora was obtained by the Hubble Space Telescope (HST), coordinated with measurements of the upstream interplanetary conditions made ... [more ▼]

During January 2004, a sequence of 68 UV images of Saturn's southern aurora was obtained by the Hubble Space Telescope (HST), coordinated with measurements of the upstream interplanetary conditions made by the Cassini spacecraft. Using the poleward edge of the observed aurora as a proxy for the open-closed field line boundary, the open flux content of the southern polar region has been estimated. It is found to range from 15 to 50 GWb during the interval, such a large variation providing evidence of a significant magnetospheric interaction with the solar wind, in particular with structures associated with corotating interaction regions (CIRs). Cassini measurements of the upstream interplanetary conditions have been used to estimate the rate of open flux production at the magnetopause. By comparison with the open flux values obtained from the images, the rate of open flux closure in Saturn's magnetotail is also estimated. We find that the rate of open flux production is high (200 kV) in the high-field strength intervals following the onset of CIR-related compressions, and elevated (30-40 kV) in an intermediate-field strength rarefaction region. High flux closure rates (100-200 kV) are estimated in association with the onset of solar wind compressions and are intermittently increased (30-60 kV) during a weak-field rarefaction region. The disturbed auroral forms seen in the images are discussed in relation to the estimated reconnection rates. Recent studies have suggested that Saturn Kilometric Radiation emissions are also disturbed by the onset of CIR compressions at Saturn. [less ▲]

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See detailHST/ACS UV Imaging of Saturn's Southern Aurora in a Quiet State
Wannawichian, S.; Clarke, J. T.; Gérard, Jean-Claude ULiege et al

Conference (2005, December 01)

Prior observations of Saturn's aurora have suggested that the aurora are highly variable, with much of the activity controlled by conditions in the solar wind. Observations of Saturn's UV aurora on 17 ... [more ▼]

Prior observations of Saturn's aurora have suggested that the aurora are highly variable, with much of the activity controlled by conditions in the solar wind. Observations of Saturn's UV aurora on 17 February 2005 were performed with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) to further test this. The UV Solar Blind Camera (SBC) imaged the UV emissions from 115.0 to 170.0 nm for a period of 5 HST orbits, or 8 hours, corresponding to 74% of a Saturn rotation. In that observation period, HST imaged the southern auroral region in sunlight, but not the northern auroral region because of the tilt of Saturn rotation axis. Saturn's aurora appeared in its most quiet state, comparable or fainter than those observed by HST's Space Telescope Imaging Spectrograph (STIS) in January 2004. Discrete emissions were detected, with some evidence of latitudinal variations of localized emissions and motions. At the same time, Cassini's Ultraviolet Imaging Spectrograph (UVIS) studied the intensity and spectral distribution of Saturn's northern night side emission region. With the benefit of simultaneous observations, we found that the characteristics of Saturn's emission region in the day side southern aurora appeared correlated with Saturn kilometric radio (SKR) emissions, charged particles and magnetic field measurements in the night side Saturn magnetosphere. The faint UV aurora are consistent with the previously reported correlation between radio and UV emissions, and the low disturbance in Saturn's magnetosphere observed by Cassini. In this quiet state, the auroral oval brightness is a few kilorayleighs (KR). The summed images show evidence of an offset auroral oval toward midnight responding to solar wind pressure and more diffuse features in the dusk side. The specific properties of Saturn's aurora in its minimum state will be presented, and compared with more active periods. [less ▲]

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See detailOpen flux estimates in Saturn's magnetosphere during the January 2004 Cassini-HST campaign, and implications for reconnection rates
Badman, S. V.; Bunce, E. J.; Clarke, J. T. et al

in Journal of Geophysical Research. Space Physics (2005), 110(A11),

During 8-30 January 2004, a sequence of 68 UV images of Saturn's southern aurora was obtained by the Hubble Space Telescope (HST), coordinated for the first time with measurements of the upstream ... [more ▼]

During 8-30 January 2004, a sequence of 68 UV images of Saturn's southern aurora was obtained by the Hubble Space Telescope (HST), coordinated for the first time with measurements of the upstream interplanetary conditions made by the Cassini spacecraft. Using the poleward edge of the observed aurora as a proxy for the open-closed field line boundary, the open flux content of the southern polar region has been estimated. It is found to range from similar to 15 to similar to 50 GWb during the interval, such a large variation providing evidence of a significant magnetospheric interaction with the solar wind, in particular with the interplanetary structures associated with corotating interaction regions (CIRs). The open flux is found to decline slowly during a rarefaction region in which the interplanetary magnetic field remained very weak, while decreasing sharply in association with the onset of CIR-related solar wind compressions. Such decreases are indicative of the dominating role of open flux closure in Saturn's tail during these intervals. Increases in open flux are found to occur in the higher-field compression regions after the onsets, and in a following rarefaction region of intermediate field strength. These increases are indicative of the dominating role of open flux production at Saturn's magnetopause during these intervals. The rate of open flux production has been estimated from the upstream interplanetary data using an empirical formula based on experience at Earth, with typical values varying from similar to 10 kV during the weak-field rarefaction region, to similar to 200 kV during the strong-field compression. These values have been integrated over time between individual HST image sets to estimate the total open flux produced during these intervals. Comparison with the changes in open flux obtained from the auroral images then allows us to estimate the amount of open flux closed during these intervals, and hence the averaged tail reconnection rates. Intermittent intervals of tail reconnection at rates of similar to 30-60 kV are inferred in rarefaction regions, while compression regions are characterised by rates of similar to 100-200 kV, these values representing averages over the similar to 2-day intervals between HST image sequences. [less ▲]

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See detailOpen flux estimates in Saturn's magnetosphere during the January 2004 Cassini-HST campaign, and implications for reconnection rates
Badman, S. V.; Bunce, E. J.; Clarke, J. T. et al

Conference (2005, August 07)

Detailed reference viewed: 23 (6 ULiège)