Search

Pluto energies of a few kiloelectron volts and suprathermal ions with tens of kiloelectron volts and above. We measure this population using the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument on board the New Horizons spacecraft that flew by Pluto in 2015. Even though the measured ions have gyroradii larger than the size of Pluto and the cross section of its magnetosphere, we find that the boundary of the magnetosphere is depleting the energetic ion intensities by about an order of magnitude close to Pluto. The intensity is increasing exponentially with distance to Pluto and reaches nominal levels of the interplanetary medium at about 190

The two main ultraviolet-signatures resulting from the Io-magnetosphere interaction are the local auroras on Io's atmosphere, and the Io footprints on Jupiter. We study here how Io's daily eclipses ...

We thank Marissa Vogt for assistance with magnetic field code. The work by FB at the University of Colorado was supported as a part NASA's Juno mission supported by NASA through contract 699050X with the Southwest Research Institute. The work by PP was supported in part by NASA Award 80NSSC19K0818.

International audience; Jupiter's ultraviolet (UV) aurorae, the most powerful and intense in the solar system, are caused by energetic electrons precipitating from the magnetosphere into the atmosphere where they excite the molecular hydrogen. Previous studies focused on case analyses and/or greater than 30-keV energy electrons. Here for the first time we provide a comprehensive evaluation of Jovian auroral electron characteristics over the entire relevant range of energies (~100 eV to ~1 MeV). The focus is on the first eight perijoves providing a coarse but complete System III view of the northern and southern auroral regions with corresponding UV observations. The latest magnetic field model JRM09 with a current sheet model is used to map Juno's magnetic foot point onto the UV images and relate the electron measurements to the UV features. We find a recurring pattern where the 3- to 30-keV electron energy flux peaks in a region just equatorward of the main emission. The region corresponds to a minimum of the electron characteristic energy (J. Outside that region, the >100-keV electrons contribute to most (>~70-80%) of the total downward energy flux and the characteristic energy is usually around 100 keV or higher. We examine the UV brightness per incident energy flux as a function of characteristic energy and compare it to expectations from a model.

The Voyagers 1 and 2 spacecraft flew by Jupiter in March and July of 1979, respectively. The Plasma Science instrument (PLS) acquired detailed measurements of the plasma environment in the equatorial region of the magnetosphere between 4.9 and 4 RJ. While bulk plasma properties such as charge density, ion temperature, and bulk flow were reasonably well determined, the ion composition was only well constrained in occasional regions of cold plasma. The ion data obtained by the PLS instrument have been reanalyzed using physical chemistry models to constrain the composition and reduce the number of free parameters, particularly in regions of hotter plasma. This paper describes the method used for fitting the plasma data and presents the results versus time. Two companion papers describe the composition of heavy ions and present analysis of protons plus other minor ions.

We take the plasma parameters derived by Bagenal et al. [2017] from Voyager PLS data in the jovian magnetosphere and examine the radial profiles of density, temperature, composition and azimuthal flow. The plasma sheet shows a relatively uniform structure of decreasing electron density (Ne) and increasing temperature out to about 20 RJ but beyond about 15 RJ there is increasing disorder with sporadic blobs of cold plasma. These small (~0.5 RJ) blobs of cold (~20 eV) plasma make a minor contribution to the net outward flux of iogenic plasma. The ion composition in the cold blobs is consistent with the ion abundances derived from physical chemistry models extending from 6 to ~9 RJ whereupon the collisional reactions slow down and radial transport speeds up, effectively freezing in the ion composition to the following abundances: O+/Ne=15-22%, S++/Ne =10-19%, O++/Ne =4-8%, S+++/Ne =4-6%, S+/Ne=1-5%. Beyond about 7 RJ the component of hot (supra-thermal, ~100s eV) ions becomes a significant fraction of the total density. The radial profile of the plasma's azimuthal flow speed shows that corotation begins to breakdown at about 9 RJ, dipping down to about 20% below corotation before increasing back up to corotation briefly (~17-20 RJ), reaching an asymptotic value of about 225 km/s (corresponding to rigid corotation at ~18 RJ). We present a 2-D model of the plasma sheet beyond 6 RJ based on simple functions for the equatorial profiles of plasma properties and steady-state diffusive equilibrium along magnetic flux tubes.

The Cassini Plasma Spectrometer instrument gathered thermal ion data at Saturn from 2004 to 2012, predominantly observing water group ions and protons. Plasma parameters, with uncertainties, for those two ion species are derived using a forward model of anisotropic convected Maxwellians moving at a shared velocity. The resulting dataset is filtered by various selection criteria to produce a survey of plasma parameters derived within 10° of the equator at radial distances of 5.5 to 30RS(1RS = SaturnOs radius). The previous 2008 work used a simpler method and had just 150 records over 5 orbits, this comprehensive survey has 9736 records over all 9 years. We present the results of this survey and compare them with a previous survey derived from numerical moments, highlighting the differences between the reported densities and temperatures from the two methods. Radial profiles of the plasma parameters in the inner and middle magnetospheres out to ≈22RS are stable year by year, but variable at distances larger than 23RS near the magnetopause. New results include: proton densities increasing in the near magnetopause region, suggestive of plasma mixing, evidence for the global electric field in Saturns' inner magnetosphere extends out to ≈15RS, no evidence for super-corotating plasma nor the middle magnetosphere ‘plasma cam’ feature is present, the thermal plasma β is found to exceed unity at equatorial distances greater than 15RS.

著者人数: 12名, Accepted: 2017-02-16, 資料番号: SA1160335000

We analyze plasma ion observations from the Solar Wind Around Pluto instrument on New Horizons as it traveled back through the dusk flank of the Jovian magnetotail from ~600 to more than 2500 Jovian radii behind the planet. We find that at all distances, light ions (mostly protons) dominate the heavy ions (S++ and O+) that are far more abundant in the near Jupiter plasma disk and that were expected to be the primary ions filling the Jovian magnetotail. This key new observation might indicate that heavy ions are confined closer to the equator than the spacecraft trajectory or a substantial addition of light ions via reconnection and/or mixing along the magnetopause boundary. However, because we find no evidence for acceleration of the tail plasma with distance, a more likely explanation seems to be that the heavy ions are preferentially released down the dawn flank of the magnetotail. Perhaps, this occurs as a part of the process where flux tubes, after expanding as they rotate across the near‐tail region, need to pull back inward in order to fit within the dawnside of the magnetopause. A second major finding of this study is that there are two dominant periods of the plasma structures in the Jovian magnetotail: 3.53 (0.18 full width at half maximum (FWHM)) and 5.35 (0.38 FWHM) days. Remarkably, the first of these is identical within the errors to Europa's orbital period (3.55 days). Both of these results should provide important new fodder for Jovian magnetospheric theories and lead to a better understanding of Jupiter's magnetosphere.

The Io torus produces ultraviolet emissions diagnostic of plasma conditions. We revisit datasets obtained by the Voyager 1, Galileo and Cassini missions at Jupiter. With the latest version (8.0) of the CHIANTI atomic database we analyze UV spectra to determine ion composition. We compare ion composition obtained from observations from these three missions with a theoretical model of the physical chemistry of the torus by Delamere et al. (2005). We find ion abundances from the Voyager data similar to the Cassini epoch, consistent with the dissociation and ionization of SO2, but with a slightly higher average ionization state for sulfur, consistent with the higher electron temperature measured by Voyager. This re-analysis of the Voyager data produces a much lower oxygen:sulfur ratio than earlier analysis by Shemansky (1988), that was also reported by Bagenal (1994). We derive fractional ion compositions in the center of the torus to be: S+/Ne ~ 5%, S++/Ne ~ 20%, S+++/Ne ~ 5%, O+/Ne ~ 20%, O++/Ne ~ 3%, Σ(On+)/Σ(Sn+) ~ 0.8, leaving about 10-15% of the charge as protons. The radial profile of ion composition indicates a slightly higher average ionization state, a modest loss of sulfur relative to oxygen, and Σ(On+)/Σ(Sn+) ~ 1.2 at about 8 RJ, beyond which the composition is basically frozen-in. The Galileo observations of UV emissions from the torus suggest that the composition in June 1996 may have comprised a lower abundance of oxygen than usual, consistent with observations made at the same time by the EUVE satellite (Herbert et al. 2001).

This study provides the first observations of Plutogenic ions and their unique interaction with the solar wind. We find ~20% solar wind slowing that maps to a point only ~4.5 RP upstream of Pluto and a bow shock most likely produced by comet-like mass loading. The Pluto obstacle is a region of dense heavy ions bounded by a “Plutopause” where the solar wind is largely excluded and which extends back >100 RP into a heavy ion tail. The upstream standoff distance is at only ~2.5 RP. The heavy ion tail contains considerable structure, may still be partially threaded by the interplanetary magnetic field (IMF), and is surrounded by a light ion sheath. The heavy ions (presumably CH4+) have average speed, density, and temperature of ~90 km s−1, ~0.009 cm−3, and ~7 × 105 K, with significant variability, slightly increasing speed/temperature with distance, and are N-S asymmetric. Density and temperature are roughly anticorrelated yielding a pressure ~2 × 10−2 pPa, roughly in balance with the interstellar pickup ions at ~33 AU. We set an upper bound of

Juno's Jupiter Energetic particle Detector Instrument (JEDI) often detects energetic electron beams over Jupiter's polar regions. In this paper, we document a subset of intense magnetic field-aligned beams of energetic electrons moving away from Jupiter at high magnetic latitudes both north and south of the planet. The number fluxes of these beams are often dominated by electrons with energies above about 1 MeV. These very narrow beams can create broad angular responses in JEDI with unique signatures in the detector count rates, probably because of >10 MeV electrons. We use these signatures to identify the most intense beams. These beams occur primarily above the swirl region of the polar cap aurora. This polar region is described as being of low brightness and high absorption and the most magnetically "open" at Jupiter.

We present a technique to gather ion composition information in the form of relative abundances of the water group ion species in Saturn's inner magnetosphere, utilizing the Cassini Plasma Spectrometer's Straight-Through Time-of-Flight data from two orbits in 2011. We show that between 4.75 and 8 Saturn radii H2O+ ions dominate the water group species, and from 8 to 10 Saturn radii it is OH+ ions that dominate. Our results show that the relative proportion of H3O+ falls fastest with increasing distance, while the proportion of H2O+ decreases slowly. However, O+ and OH+ increase with distance, and O+ is the least dominant ion species out to eight Saturn radii outside of which it is comparable to H3O+. The relative abundance of H2O+ found here matches theoretical work based on Herschel telescopic data very well. These results are compared with other published work, and further improvements to the technique are discussed.

The giant-planet magnetodiscs are shaped by the radial transport of plasma originating in the inner magnetosphere. Magnetic flux transport is a key aspect of the stretched magnetic field configuration of the magnetodisc. While net mass transport is outward (ultimately lost to the solar wind), magnetic flux conservation requires a balanced two-way transport process. Magnetic reconnection is a critical aspect of the balanced flux transport. We present a comprehensive analysis of current sheet crossings in Saturn's magnetosphere using Cassini magnetometer data from 2004 to 2012 in an attempt to quantify the circulation of magnetic flux, emphasizing local time dependence. A key property of flux transport is the azimuthal bend forward or bend back of the magnetic field. The bend back configuration is an expected property of the magnetodisc with net mass outflow, but the bend forward configuration can be achieved with the rapid inward motion of mostly empty flux tubes following reconnection. We find a strong local time dependence for the bend forward cases, localized mostly in the postnoon sector, indicating that much of the flux-conserving reconnection occurs in the subsolar and dusk sector. We suggest that the reconnection occur in a complex and patchy network of reconnection sites, supporting the idea that plasma can be lost on small scales through a “drizzle”-like process. Auroral implications for the observed flux circulation will also be presented.

Jupiter's nightside magnetosphere stretches out into an extensive magnetotail. New Horizons observed continuously over 1 AU down Jupiter's tail, far more than any other spacecraft. Previously, Voyager 2 showed signatures of the tail as far as 4 AU distance from Jupiter. We combine data from New Horizons' charged particle instruments: Solar Wind Around Pluto, measuring plasma ions (21–7800 eV/Q), and Pluto Energetic Particle Spectrometer Science Investigation, measuring energetic ions and electrons (0.03–1.6 MeV). The magnetosheath is clearly distinguished from the magnetotail, owing to more plasma ions. They are often separated by a boundary layer with intermediate properties at plasma energies but resemblance to the magnetotail at higher energies. Compared to the tail, the sheath contains on average more energetic protons and helium ions (potential solar wind origin) and fewer energetic electrons, oxygen, and sulfur ions (latter two of magnetospheric origin). The difference between tail and sheath for energetic ions and electrons is less pronounced than for plasma ions, which may result from particle exchange. We have ruled out that this is due to gyroradius. Energetic ion count rate enhancements with velocity dispersion cross between tail, boundary layer, and sheath. This indicates occasional magnetic connections since the enhancements are interpreted as particles following magnetic flux tubes. The deep tail magnetic field is therefore not entirely separated from the solar wind. Brief magnetic connection will mostly allow the fastest particles to cross, in agreement with the observations. Most dispersed enhancements include sulfur ions consistent with an origin near Jupiter's X line. We found one event of mostly protons, indicating an origin at larger distances.

The Solar Wind Around Pluto (SWAP) instrument on New Horizons (NH) made in situ observations of ions in Jupiter's distant magnetotail and magnetosheath during its 2007 flyby. NH observed 16 magnetopause crossings between 1654 and 2429 RJ antisunward from Jupiter. We have developed a method to calculate the bulk properties of the plasma ions (density, velocity, and temperature) based on a forward model of the SWAP instrument response. We fit the observations using both Maxwell and kappa distributions. In this paper we describe our technique, which includes accounting for the detailed and asymmetric response of the SWAP electrostatic analyzer, and present the results for the distant Jovian magnetosheath. Finally, we discuss the characteristics of the derived bulk properties and compare these results to previously developed gas dynamic models for magnetospheres of giant planets.

Observations of Jovian magnetopause crossing by a number of different spacecraft have established that Jupiter's magnetosphere has a generally bimodal size distribution, with typical standoff distances at the nose of ~63 and ~92 RJ. Here we examine both the external solar wind structure and time constants and the internal magnetospheric time constants for shedding and refilling material in the Jovian plasma disk. We show that these latter time constants are ~ hours to ~10 h, comparable to the compression time of the magnetopause, but shorter than the typically several day expansion time when the solar wind dynamic pressure decreases. Together, we show that it is the well-developed compressions and rarefactions in the solar wind at ~5 AU that produced the generally bimodally structured solar wind dynamic pressure and hence Jovian magnetospheric size.

[1] The internal sources of plasma in the giant magnetospheres of Jupiter and Saturn affect magnetospheric dynamics in terms of magnetosphere-ionosphere coupling and auroral current systems, as well as the interaction with the solar wind. The radial transport of plasma at Jupiter is well constrained to vary between 300 kg/s and 1200 kg/s over timescales of 20–60 days. Saturn's neutral-dominated inner magnetosphere has presented a challenge for determining the radial mass transport rates with values ranging between 20 kg/s and 280 kg/s over timescales of 20–80 days. We present an estimate of the radial mass transport rates associated with magnetospheric plasma production for Jupiter and Saturn, assuming that magnetospheric plasma loss (equal to the plasma source) can be treated as mass pickup by the solar wind. That is, the magnetospheric plasma is assumed to be at rest with respect to the solar wind, analogous to cometary mass loading of the solar wind flow. This Alfvenic coupling between the solar wind and the magnetosphere suggests mass loading rates of 500–1500 kg/s for Jupiter and 20–50 kg/s for Saturn, consistent with observations and physical chemistry models. We discuss implications for magnetotail structure by considering the contribution of the viscous interaction of the solar wind with the giant magnetospheres. We suggest that a significant portion of the magnetotail structure can be attributed to this viscous interaction.

[1] Using idealized models of the magnetosheath and magnetospheric magnetic fields, plasma densities, and plasma flow, we test for the steady state viability of processes mediating the interaction between the solar wind and the magnetosphere of Saturn. The magnetopause is modeled as an asymmetric paraboloid with a standoff distance of ∼25 RS. We test where on the magnetopause surface large‒scale reconnection may be affected by either a shear flow or diamagnetic drift due to a pressure gradient across the magnetopause boundary. We also test for the onset of the Kelvin‒Helmholtz instability. We find that, for the solar wind and magnetosphere states considered, reconnection is inhibited on the dawn flank due to the large shear flows in this region. Additionally, most of the dawn and dusk equatorial region of the magnetopause is Kelvin‒Helmholtz unstable, due to the presence of the dense magnetospheric plasma sheet and weak magnetic fields on either side of the magnetopause. This study is a follow‒up to a previously published study of the solar wind interaction with Jupiter's magnetosphere.

[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.

[1] The presence of a convection pattern in the Saturnian inner magnetosphere was first observed in moon signatures that were displaced from their expected locations. A noon to midnight directed electric field can account for these positional offsets, and also can explain asymmetries found in plasma temperatures between the day and night sides. By assuming magnetic field models and drift times, estimates of the electric field strength have been made. However the origin of the electric field remains a mystery. This paper, which presents new evidence for the convection pattern, uses measured in situ velocities with measured magnetic field values, at a range of radial distances without the restriction of requiring the presence of a moon. Advances in extracting plasma parameters with reduced uncertainties clearly show the convection pattern that was previously hidden when uncertainties were large. We confirm the earlier findings that there is a distinct drift velocity superposed on the corotating plasma that pushes the plasma toward ∼dawn that dominates over the general plasma outflow. We find that an electric field directed toward 01–02 h Saturnian local time best describes this drift, and that the value of the electric field strength is not constant with radial distance.

[1] Saturn's rapid rotation combined with relatively weak magnetic fields in the outer magnetosphere and sheath lead to conditions that are favorable for the Kelvin-Helmholtz (KH) instability. A Kelvin-Helmholtz unstable magnetopause boundary has important consequences for Saturn's interaction with the solar wind due to mass, momentum, and energy transport that can occur at the magnetopause boundary. Previous attempts to identify vortices have been hampered by limited plasma data to unambiguously reveal vortical flow. The magnetic field data, on the other hand, may be able to identify the KH instability due to intense magnetic fluctuations that are associated with KH vortices. We have conducted two-dimensional hybrid code simulations of Saturn's magnetopause boundary to illustrate the expected magnetic field signatures of KH. Specifically, our simulations show strong field-aligned current sheet filaments or strong bipolar fluctuations of the in-plane magnetic field components, bounding the KH vortices. A global search for these characteristic magnetic field signatures near the magnetopause boundary was made of the Cassini mission data from 2004 to 2009. We find that most of the potential KH activity is found on the dusk flank, contrary to expectations. We suggest that KH growth is supported in the prenoon and subsolar regions and that these vortices are transported through coupling to the rotating planet, past noon and tailward on the dusk flank. In addition, we find many instances in the subsolar magnetosphere of possible plasmoid formation (Bz northward) in conjunction with these intense magnetic field fluctuations.

During Galileo's December 1995 encounter with Io, the magnetometer observed very strong electromagnetic waves near the SO2+ gyrofrequency. The amplitude of these waves previously has been used to determine the density of SO2+, as well as the production rate near Io. In this paper we examine the details of the interaction between pickup ions and ion cyclotron weaves and use these results to constrain the production rate of O+ and lo's exospheric loss rate. In a sub-Alfvenic flow we show that ion cyclotron waves scatter pickup ions into a filled, asymmetric distribution, rather than a shell distribution. In addition, the observed waves are strong enough to cause non-resonant scattering of S+ ions and may result in an asymmetric core population of S+. Because of the absence of waves near the O+ gyrofrequency and the criteria for marginal stability, we find that the O+ production rate was probably under a few times 1026 ions s−1. The amplitude of the observed waves is consistent with an exospheric escape rate of 1–3.5×1027 SO2+ ions s−1, depending on the asymmetry of the loss process.

We model electron densities of the simplest, most symmetric solar minimum streamer structure observed during the Whole Sun Month (WSM) campaign, using coronal observations of both visible white light and extreme ultraviolet (EUV) emission. Using white light data from the SOHO/LASCO/C2 and HAO/Mauna Loa Mark 3 coronagraphs, we determine electron densities by way of a Van de Hulst inversion. We compare the white light densities to those determined from the density sensitive EUV line ratios of Si IX 350/342 A observed by the SOHO/coronal diagnostic spectrometer (CDS). Moreover, from the white light density profiles we calculate hydrostatic temperature profiles and compare to temperatures derived from the Si XII/Mg X line ratio. We find the white light and spectral analysis produce consistent density and temperature information.

On December 7, 1995, the Galileo spacecraft passed through the Io plasma torus, and made observations of this dense, heavy ion plasma. We report an analysis of the data from the spacecraft's plasma spectrometer which measures the angular distribution of the ions. This enables us to determine the composition of the plasma. We compare our results to the models based on the Voyager 1 encounter of 1979, Galileo ultraviolet observations, and Earth-based observations. At the time and longitude of the December 7 Galileo encounter the abundance of oxygen was higher than observed by Voyager, and the O++:O+ ratio was greater. The ion temperatures were not in thermal equilibrium and were generally cooler than Voyager values.

We analytically combine stress-free current sheets with a coronal magnetostatic bulk current model. We begin by imposing a current sheet at the equator as an upper boundary condition on the modeled coronal field. We find that in order to reproduce the sharp gradients across the boundaries of helmet streamers, we also have to add current sheets along the interface between open and closed field lines. We find a description of coronal magnetic field and density in the presence of both bulk and sheet currents that matches both white light and photospheric magnetic flux observations.

During the Voyager 1 encounter with Jupiter, a large number of whistler waves were observed. Previous studies have examined the dispersion of these waves and made estimates of the electron and light ion (i.e., proton) densities. The current paper reexamines this data, taking into account the revised temperatures of the torus species the additional data on ion composition from the Voyager UVS instrument and the role of thermal anisotropy on the plasma densities. These refinements in the density model drastically alter the implications of the whistler wave data. Both the thermal and the nonthermal species must be anisotropic to fit the whistler dispersions. The thermal component must have T⊥/T‖ > 1.75 and the nonthermal component 3 < T⊥/T‖ < 10, The equatorial proton density is low, under 60 cm−3 in all cases. This results in a proton abundance (L shell proton content relative to the total ion content) of no more than 10%, approximately a factor of two lower than the conclusions of previous whistler analysis. At the high latitudes, the implied electron density results in a plasma frequency of under 20 kHz. Finally, it is evident from this analysis that not all of the whistler waves were propagating along the magnetic field lines, as was commonly assumed in previous work.

[1] Using idealized models of the magnetosheath and magnetosphere magnetic fields, plasma densities, and plasma flow, we test for the steady state viability of processes mediating the interaction between the solar wind and the Jovian magnetosphere. The magnetopause is modeled as an asymmetric paraboloid with variable asymmetry. The subsolar standoff of the magnetopause has been shown to exhibit a bimodal probability distribution (Joy et al., 2002). Only the expanded magnetopause is considered, with a standoff of ∼90 RJ. We test where on the magnetopause surface large-scale reconnection may be affected by either a shear flow or diamagnetic drift due to a pressure gradient across the magnetopause boundary. We also test for the onset of the Kelvin-Helmholtz instability. We find that reconnection is inhibited on the dawn flank due to the large shear flows in this region, regardless of magnetopause shape or interplanetary magnetic field orientation. The presence of a high energy plasma population in the magnetosphere may inhibit reconnection over much of the magnetopause area, except when the fields are antiparallel. Additionally, most of the dawn flank of the magnetopause is Kelvin-Helmholtz unstable, regardless of magnetopause asymmetry; and the dusk flank tailward of the planet is Kelvin-Helmholtz unstable when the magnetopause is highly oblate.

Jupiter's main auroral emission is a signature of the current system that transfers angular momentum from the planet to radially outward moving Iogenic plasma. Ray et al. (2010) developed a steady state model of this current system which self-consistently included the effects of a field-aligned potential, and an ionospheric conductance modified by precipitating electrons. The presented parameter space study extends their model to explore how variations in the auroral cavity density and temperature, magnetospheric mass loading rate, and background ionospheric Pedersen conductance affect the current system and resulting auroral emission. We show that while the solutions found by Ray et al. (2010) vary with changes in the system parameters, the gross general trends remain similar to the original solutions. We find that, for an outer constraint of I100 = 86 MA, the high-latitude electron temperature and density have a lower limit of ∼1.5 keV and an upper limit of ∼0.01 cm -3, respectively, in order for solutions to be consistent with observations of Jupiter's auroral emission. For increases in the radial mass transport rate and an outer constraint of Max = 75 kV the auroral emission brightness increases.

[1] The longitudinal modulation in the Io torus has been an open question for decades. A major clue was provided by the discovery of the key modulation of the hot electron population, at both the System III and System IV periods. However, very little progress has been made in explaining the origin of these hot electron modulations. We propose that the hot electrons population is powered by the inward motion of empty flux tubes (i.e. related to the outward transport of the Iogenic plasma), which has been observed in the torus. We propose that the System IV and System III modulation of the hot electron population corresponds to modulation of the intensity of the current system and of the efficiency of the electron acceleration, respectively. We build on the latest models of the Io current system to describe the current system associated with the motion of the empty flux tubes, and the associated electron acceleration. The System III modulation of the hot electron population, due to the modulation of the efficiency of the electron acceleration, can then be related to the topology of the magnetic field. We show through calculation and simulation that the electron acceleration related to the inward motion of the empty flux tube may explain the observations. We discuss the energy budget and show that it is in favor of our hypothesis.

[1] We present simple models of the plasma disks surrounding Jupiter and Saturn based on published measurements of plasma properties. We calculate radial profiles of the distribution of plasma mass, pressure, thermal energy density, kinetic energy density, and energy density of the suprathermal ion populations. We estimate the mass outflow rate as well as the net sources and sinks of plasma. We also calculate the total energy budget of the system, estimating the total amount of energy that must be added to the systems at Jupiter and Saturn, though the causal processes are not understood. We find that the more extensive, massive disk of sulfur- and oxygen-dominated plasma requires a total input of 3–16 TW to account for the observed energy density at Jupiter. At Saturn, neutral atoms dominate over the plasma population in the inner magnetosphere, and local source/loss process dominate over radial transport out to 8 RS, but beyond 8–10 RS about 75–630 GW needs to be added to the system to heat the plasma.

The Jovian moon Io resides in one of the harshest space environments yet visited by spacecraft. Indeed, the energetic particles that make Jupiter's inner magnetosphere interesting to the space physicist give space engineers nightmares. After the initial orbit-insertion passage in December 1995, the Galileo spacecraft was deliberately kept well outside the orbit of Io and the plasma torus. After 20 plus successful orbits exploring the outer 3 Galilean moons, the spacecraft's perijove was reduced to allow 3 close flybys of Io in late 1999 to early 2000.