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Waves on the surface of a planetary magnetopause promote energy transport into the magnetosphere, representing an important aspect of solar wind–magnetosphere coupling. At Saturn's magnetopause it has been proposed that growth of the Kelvin–Helmholtz (K–H) instability produces greater wave activity on the dawn side of the surface than on the dusk side. We test this hypothesis using data taken by the Cassini spacecraft during crossings of Saturn's magnetopause. Surface orientation perturbations are primarily controlled by the local magnetospheric magnetic field orientation, and are generally greater at dusk than at dawn. 53% of all crossings were part of a sequence of regular oscillations arising in consecutive surface normals that is strong evidence for tailward propagating surface waves, with no detectable local time asymmetry in this phenomenon. We estimate the dominant wave period to be ∼5h at dawn and ∼3h at dusk. The role played by the magnetospheric magnetic field, tailward wave propagation, and the dawn–dusk difference in wave period suggests that K–H instability is a major wave driving mechanism. Using linear K–H theory we estimate the dominant wavelength to be ∼10 Saturn radii (RS) and amplitude to be ∼1 RS at both dawn and dusk, giving propagation speeds of ∼30 and ∼50kms−1 at dawn and dusk, respectively. The lack of the hypothesized dawn–dusk asymmetry in wave activity demonstrates that we need to revise our understanding of the growth of the K–H instability at Saturn's magnetopause, which will have implications for the study of other planetary magnetospheres.

The source of the various planetary-period signals in Saturn’s magnetosphere is at present unknown. We investigate the possibility that the source of these signals is an axially asymmetric wind system in the thermosphere. We describe a feedback mechanism that has the potential to drive such axially asymmetric wind systems. The proposed mechanism relates thermospheric winds to heating from particle precipitation, via the generation of horizontal and field-aligned currents. The relevant physical processes are investigated using a highly simplified general circulation model of Saturn’s thermosphere and ionosphere. Our principal result is that the feedback mechanism is effective in permanently breaking the axial symmetry of the thermosphere, generating a drifting vortex-like structure at high latitudes. However, the precipitating electron energies required to power this structure are of the order of 5 MeV, 2–3 orders of magnitude greater than the observed auroral electron energies, and the highly axially asymmetric distribution of precipitation required across the polar regions of the planet is also inconsistent with observations. Despite these flaws, the model qualitatively explains several features of the observed variation in the pulsing of SKR emissions; in particular, the seasonal variation and the faster rotation rate in the winter hemisphere. We cannot reproduce the apparent 7 month lag in the response of the Saturn Kilometric Radiation (SKR) rotation rate to seasonal variation, but instead suggest the possibility that this effect may have its origin in long chemical time-scales in the upper atmosphere. We also predict the possible existence of secondary periodic features in the SKR emissions with periods of ∼15 planetary rotations, driven by complex wave behaviour in the thermosphere.

The moon Enceladus, embedded in Saturn’s radiation belts, is the main internal source of neutral and charged particles in the Kronian magnetosphere. A plume of water ice molecules and dust released through geysers on the south polar region provides enough material to feed the E-ring and also the neutral torus of Saturn and the entire magnetosphere. In the time period 2005–2010 the Cassini spacecraft flew close by the moon 14 times, sometimes as low as 25km above the surface and directly through the plume. For the very first time measurements of plasma and energetic particles inside the plume and its immediate vicinity could be obtained. In this work we summarize the results of energetic electron measurements in the energy range 27keV to 21MeV taken by the Low Energy Magnetospheric Measurement System (LEMMS), part of the Magnetospheric Imaging Instrument (MIMI) onboard Cassini in the vicinity of the moon in combination with measurements of the magnetometer instrument MAG and the Electron Spectrometer ELS of the plasma instrument CAPS onboard the spacecraft. Features in the data can be interpreted as that the spacecraft was connected to the plume material along field lines well before entering the high density region of the plume. Sharp absorption signatures as the result of losses of energetic electrons bouncing along those field lines, through the emitted gas and dust clouds, clearly depend on flyby geometry as well as on measured pitch angle/look direction of the instrument. We found that the depletion signatures during some of the flybys show “ramp-like” features where only a partial depletion has been observed further away from the moon followed by nearly full absorption of electrons closer in. We interpret this as partially/fully connected to the flux tube connecting the moon with Cassini. During at least two of the flybys (with some evidence of one additional encounter) MIMI/LEMMS data are consistent with the presence of dust in energetic electron data when Cassini flew directly through the south polar plume. In addition we found gradients in the magnetic field components which are frequently found to be associated with changes in the MIMI/LEMMS particles intensities. This indicates that complex electron drifts in the vicinity of Enceladus could form forbidden regions for electrons which may appear as intensity drop-outs.

Saturn’s rich magnetospheric environment is unique in the solar system, with a large number of active magnetospheric processes and phenomena. Observations of this environment from the Cassini spacecraft has enabled the study of a magnetospheric system which strongly interacts with other components of the saturnian system: the planet, its rings, numerous satellites (icy moons and Titan) and various dust, neutral and plasma populations. Understanding these regions, their dynamics and equilibria, and how they interact with the rest of the system via the exchange of mass, momentum and energy is important in understanding the system as a whole. Such an understanding represents a challenge to theorists, modellers and observers. Studies of Saturn’s magnetosphere based on Cassini data have revealed a system which is highly variable which has made understanding the physics of Saturn’s magnetosphere all the more difficult. Cassini’s combination of a comprehensive suite of magnetospheric fields and particles instruments with excellent orbital coverage of the saturnian system offers a unique opportunity for an in-depth study of the saturnian plasma and fields environment. In this paper knowledge of Saturn’s equatorial magnetosphere will be presented and synthesised into a global picture. Data from the Cassini magnetometer, low-energy plasma spectrometers, energetic particle detectors, radio and plasma wave instrumentation, cosmic dust detectors, and the results of theory and modelling are combined to provide a multi-instrumental identification and characterisation of equatorial magnetospheric regions at Saturn. This work emphasises the physical processes at work in each region and at their boundaries. The result of this study is a map of Saturn’s near equatorial magnetosphere, which represents a synthesis of our current understanding at the end of the Cassini Prime Mission of the global configuration of the equatorial magnetosphere.

We employ Mariner 10 measurements of the interplanetary magnetic field in the vicinity of Mercury to estimate the rate of magnetic reconnection between the interplanetary magnetic field and the Hermean magnetosphere. We derive a time-series of the open magnetic flux in Mercury's magnetosphere. from which we can deduce the length of the magnetotail The length of the magnetotail is shown to be highly variable. with open field lines stretching between 15R(sub H) and 8S0R(sub H) downstream of the planet (median 150R(sub H)). Scaling laws allow the tail length at perihelion to be deduced from the aphelion Mariner 10 observations.

In June 2006 Venus Express crossed several times the outer boundary of Venus induced magnetosphere, its magnetosheath and its bow shock. During the same interval the Cluster spacecraft surveyed the dawn flank of the terrestrial magnetosphere, intersected the Earth's magnetopause and spent long time intervals in the magnetosheath. This configuration offers the opportunity to perform a joint investigation of the interface between Venus and Earth's outer plasma layers and the shocked solar wind. We discuss the kinetic structure of the magnetopause of both planets, its global characteristics and the effects on the interaction between the planetary plasma and the solar wind. A Vlasov equilibrium model is constructed for both planetary magnetopauses and provides good estimates of the magnetic field profile across the interface. The model is also in agreement with plasma data and evidence the role of planetary and solar wind ions on the spatial scale of the equilibrium magnetopause of the two planets. The main characteristics of the two magnetopauses are discussed and compared.

Observations of unusually large magnetic fields in the ionosphere indicate periods of maximum stress on Titan’s ionosphere and potentially of the strongest loss rates of ionospheric plasma. During Titan flyby T42, the observed magnetic field attained a maximum value of 37 nT between an altitude of 1200 and 1600 km, about 20 nT stronger than on any other Titan pass and close to five times greater in magnetic pressure. The strong fields occurred near the corotation-flow terminator rather than at the sub-flow point, suggesting that the flow which magnetized the ionosphere was from a direction far from corotation and possibly towards Saturn. Extrapolation of solar wind plasma conditions from Earth to Saturn using the University of Michigan MHD code predicts an enhanced solar wind dynamic pressure at Saturn close to this time. Cassini’s earlier exits from Saturn’s magnetosphere support this prediction because the Cassini Plasma Spectrometer instrument saw a magnetopause crossing three hours before the strong field observation. Thus it appears that Titan’s ionosphere was magnetized when the enhanced solar wind dynamic pressure compressed the Saturnian magnetosphere, and perhaps the magnetosheath magnetic field, against Titan. The solar wind pressure then decreased, leaving a strong fossil field in the ionosphere. When observed, this strong magnetic flux tube had begun to twist, further enhancing its strength.

We determine the field-aligned mapping of Saturn's auroras into the magnetosphere by combining UV images of the southern dayside oval obtained by the Hubble Space Telescope (HST) with a global model of the magnetospheric magnetic field. The model is tailored to simulate prevailing conditions in the interplanetary medium, corresponding to high solar wind dynamic pressure and variable interplanetary magnetic field (IMF) strength and direction determined from suitably lagged field data observed just upstream of Saturn's dayside bow shock by the Cassini spacecraft. Two out of four images obtained in February 2008 when such simultaneous data are available are examined in detail, exemplifying conditions for northward and southward IMF. The model field structure in the outer magnetosphere and tail is found to be very different in these cases. Nevertheless, the dayside UV oval is found to have a consistent location relative to the field structure in each case. The poleward boundary of the oval is located close to the open-closed field boundary and thus maps to the vicinity of the magnetopause, consistent with previous results. The equatorward boundary of the oval then maps typically near the outer boundary of the equatorial ring current appropriate to the compressed conditions prevailing. Similar results are also found for related images from the January 2004 HST data set. These new results thus show that the mapped dayside UV oval typically spans the outer magnetosphere between the outer part of the ring current and the magnetopause. It does not encompass the region of primary corotation flow breakdown within the inner Enceladus torus.

The giant planetary magnetospheres surrounding Jupiter and Saturn respond in quite different ways, compared to Earth, to changes in upstream solar wind conditions. Spacecraft have visited Jupiter and Saturn during both solar cycle minima and maxima. In this paper we explore the large-scale structure of the interplanetary magnetic field (IMF) upstream of Saturn and Jupiter as a function of solar cycle, deduced from solar wind observations by spacecraft and from models. We show the distributions of solar wind dynamic pressure and IMF azimuthal and meridional angles over the changing solar cycle conditions, detailing how they compare to Parker predictions and to our general understanding of expected heliospheric structure at 5 and 9 AU. We explore how Jupiter’s and Saturn’s magnetospheric dynamics respond to varying solar wind driving over a solar cycle under varying Mach number regimes, and consider how changing dayside coupling can have a direct effect on the nightside magnetospheric response. We also address how solar UV flux variability over a solar cycle influences the plasma and neutral tori in the inner magnetospheres of Jupiter and Saturn, and estimate the solar cycle effects on internally driven magnetospheric dynamics. We conclude by commenting on the effects of the solar cycle in the release of heavy ion plasma into the heliosphere, ultimately derived from the moons of Jupiter and Saturn.

The existence of an oxygen exosphere and ionosphere in Saturn’s main ring region has been confirmed by the Saturn Orbital Insertion (SOI) observations of the Cassini spacecraft. Through the ion–molecule collisions, the ring atmosphere could serve as a source of O 2 + ions throughout Saturn’s magnetosphere. If photolysis of ice in the main rings is the dominant source of O2, then the complex structure of the ring atmosphere/ionosphere and the injection rate of neutral O2 will be subject to modulation by the seasonal variation of Saturn along its orbit (Tseng, Wei-Ling, Ip, W.-H., Johnson, R.E., Cassidy, T.A., Erlod, M.K. [2010]. Icarus 206, 382–389). In addition, the radio and plasma wave science (RPWS) instrument onboard Cassini found that a large amount of the Enceladus-originated water-group plasma would be deposited on the outer edge of the A ring (Farrell, W.M., Kaiser, M.L., Gurnett, D.A., Kurth, W.S., Persoon, A.M., Wahlund, J.E., Canu, P. [2008]. Geophys. Res. Lett. 35, L02203). A large amount of Enceladus’ plume neutrals (water-group neutrals) would collide with the main rings through collisional interaction with the ambient neutrals and plasma ions (Jurac, S., Richardson, J.D. [2007]. Geophys. Res. Lett. 34, L08102; Cassidy, T.A., Johnson, R.E. [2010]. Icarus, in press). These absorbed ions and neutrals could be recycled to neutral oxygen molecules via grain-surface chemistry to contribute the ring oxygen atmosphere. In this work, we have examined the mass budget of the ring oxygen atmosphere of Saturn taking into account such an “exogenic” source. The maximum O2 source rate from recycling of Enceladus-originated plasma and neutrals is probably comparable or higher to the one from photolytic decomposition of ices. In the above case, the neutral O2 source rate would be independent of the solar insolation angle. Therefore, even at Saturn’s Equinox, the extended oxygen atmosphere still could be an important supplier of oxygen ions in the saturnian magnetosphere. We have performed several studies for different recycling source rates from Enceladus. These predictions need further the Cassini Plasma Spectrometer (CAPS) and the Magnetospheric Imaging Instrument (MIMI) observations to be verified in future.

著者人数: 20名, Accepted: 2011-08-30, 資料番号: SA1002971000

The event of March 12–19, 2009, when a moderately high-speed solar wind stream flew around the Earth’s magnetosphere and carried millihertz ultralow-frequency (ULF) waves, has been analyzed. The stream caused a weak magnetic storm (Dst min = −28 nT). Since March 13, fluxes of energetic (up to relativistic) electrons started increasing in the magnetosphere. Comparison of the spectra of ULF oscillations observed in the solar wind and magnetosphere and on the Earth’s surface indicated that a stable common spectral peak was present at frequencies of 3–4 mHz. This fact is interpreted as evidence that waves penetrated directly from the solar wind into the magnetosphere. Possible scenarios describing the participation of oscillations in the acceleration of medium-energy (E > 0.6 MeV) and high-energy (E > 2.0 MeV) electrons in the radiation belt are discussed. Based on comparing the event with the moderate magnetic storm of January 21–22, 2005, we concluded that favorable conditions for analyzing the interaction between the solar wind and the magnetosphere are formed during a deep minimum of solar activity.

Brief information is given on the magnetospheres of the planets in the solar system that have intrinsic magnetic fields: Mercury, Earth, Jupiter, and Saturn. A universal model is constructed for the magnetosphere of a planet. Modifications of this model that are applied to individual planets are considered. The proposed models describe the basic physical processes that are responsible for the structure and dynamics of the magnetosphere. The numerical results of the simulations are compared with the direct measurements of magnetic fields and charged particle fluxes in the vicinity of the planets obtained in spacecraft (SC) missions.

To gain better understanding of auroral processes in Saturn's magnetosphere, we compare ultraviolet (UV) auroral images obtained by the Hubble Space Telescope (HST) with the position of the open-closed field line boundary in the ionosphere calculated using a magnetic field model that employs Cassini measurements of the interplanetary magnetic field (IMF) as input. Following earlier related studies of pre-orbit insertion data from January 2004 when Cassini was located ~ 1300 Saturn radii away from the planet, here we investigate the interval 12–15 February 2008, when UV images of Saturn's southern dayside aurora were obtained by the HST while the Cassini spacecraft measured the IMF in the solar wind just upstream of the dayside bow shock. This configuration thus provides an opportunity, unique to date, to determine the IMF impinging on Saturn's magnetosphere during imaging observations, without the need to take account of extended and uncertain interplanetary propagation delays. The paraboloid model of Saturn's magnetosphere is then employed to calculate the magnetospheric magnetic field structure and ionospheric open-closed field line boundary for averaged IMF vectors that correspond, with appropriate response delays, to four HST images. We show that the IMF-dependent open field region calculated from the model agrees reasonably well with the area lying poleward of the UV emissions, thus supporting the view that the poleward boundary of Saturn's auroral oval in the dayside ionosphere lies adjacent to the open-closed field line boundary.

Saturn's Secrets Probed The Cassini spacecraft was launched on 15 October 1997. It took it almost 7 years to reach Saturn, the second-largest planet in the solar system. After almost 6 years of observations of the series of interacting moons, rings, and magnetospheric plasmas, known as the Kronian system, Cuzzi et al. (p. 1470 ) review our current understanding of Saturn's rings—the most extensive and complex in the solar system—and draw parallels with circumstellar disks. Gombosi and Ingersoll (p. 1476 ; see the cover) review what is known about Saturn's atmosphere, ionosphere, and magnetosphere.

Previous studies have shown that the occurrence probability of Saturn Kilometric Radiation (SKR) appears to be influenced by the local time of Titan. Using a more extensive set of data than the original study, we confirm the correlation of higher occurrence probability of SKR when Titan is located near local midnight. In addition, the direction finding capability of the Cassini Radio Plasma Wave instrument (RPWS) is used to determine if this radio emission emanates from particular source regions. We find that most source regions of SKR are located in the mid-morning sector of local time even when Titan is located near midnight. However, some emission does appear to have a source in the Saturnian nightside, consistent with electron precipitation from field lines that have recently mapped to near Titan.

Driven magnetic reconnection in the magnetospheric plasma, where collisions are extremely rare, is studied using a two-fluid collisionless plasma model. A local approximation is used for the dayside magnetosphere in which the magnetic field at the magnetopause has a null X-point with a guide field. Numerical computations are made for two situations that can occur: namely, a continuous solar wind drive and a bursty drive. They are compared under different circumstances focussing on the way the reconnection rate changes in time and on the resulting magnetic configurations. It is shown that the case of sequential impulsive events is more efficient than the continuous reconnection case.

A 3D, multi-species hybrid model (kinetic ions, fluid electrons) has been applied to the interaction between Saturn's largest satellite Titan and the plasma in the giant planet's outer magnetosphere. In contrast to the idealized picture deduced from Voyager 1 data, recent observations made by the Cassini magnetometer instrument suggest that the ambient magnetic field is not directed perpendicular to Titan's orbital plane. Therefore, our purpose is to investigate systematically how Titan's induced magnetosphere is affected by a tilt of the upstream magnetic field. In the first part of our study, the structure of Titan's induced magnetosphere is analyzed as a function of the angle between the ambient magnetic field B 0 and the bulk velocity u 0 of the corotating plasma flow. Our simulations show that introducing a flow-aligned magnetic field component goes along with an asymmetrization of Titan's magnetotail, in addition to the asymmetry that already arises from the large gyroradii of the ion species involved in the interaction. In the vicinity of Titan, the field lines become strongly twisted, permitting the wakeside magnetic lobe structure to even penetrate into the satellite's geometric plasma shadow. However, despite the increased complexity of Titan's magnetic environment, the overall characteristics of the pick-up tail remain practically the same as in the case of “ideal” magnetic field orientation ( B 0 ⊥ u 0 ) . In the second part of our study, we investigate in real-time the transition that Titan's plasma interaction undergoes during a change of the ambient magnetic field direction. In contrast to earlier analyses of Titan's plasma environment under non-stationary upstream conditions, the tilt of the ambient magnetic field is again taken into account. While in the case of B 0 being perpendicular to u 0 , the reconfiguration of Titan's induced magnetosphere is mainly governed by reconnection, our simulations suggest that when a flow-aligned field component is included, convection of the field lines around the obstacle's ionosphere plays the key role for the reconfiguration process.

During its tour Cassini has observed numerous plasma injection events in Saturn's inner magnetosphere. Here, we present a case study of one “young” plasma bubble observed when Cassini was in the equatorial plane. The bubble was observed in the equatorial plane at ∼7 Saturn radii from Saturn and had a maximum azimuthal extent of ∼0.25 Rs (Rs=Saturn radius ∼60330 km). We show that the electron density inside the event is lower by a factor ∼3 and the electron temperature higher by over an order of magnitude compared to its surroundings. The injection contains slightly increased magnetic field magnitude of 49 nT compared with a background field of 46 nT. Modelling of pitch angle distributions inside the plasma bubble and measurements of plasma drift provide a novel way to estimate that the bubble originated between 9

We demonstrate that under some magnetospheric conditions protons and oxygen ions are accelerated once per Saturn magnetosphere rotation, at a preferred local time between midnight and dawn. Although enhancements in energetic neutral atom (ENA) emission may in general occur at any local time and at any time in a Saturn rotation, those enhancements that exhibit a recurrence at a period very close to Saturn's rotation period usually recur in the same magnetospheric location. We suggest that these events result from current sheet acceleration in the 15–20 Rs range, probably associated with reconnection and plasmoid formation in Saturn's magnetotail. Simultaneous auroral observations by the Hubble Space Telescope (HST) and the Cassini Ultraviolet Imaging Spectrometer (UVIS) suggest a close correlation between these dynamical magnetospheric events and dawn-side transient auroral brightenings. Likewise, many of the recurrent ENA enhancements coincide closely with bursts of Saturn kilometric radiation, again pointing to possible linkage with high latitude auroral processes. We argue that the rotating azimuthal asymmetry of the ring current pressure revealed in the ENA images creates an associated rotating field aligned current system linking to the ionosphere and driving the correlated auroral processes.

We have derived a model of Saturn's internal planetary magnetic field from data obtained during the first three years of the Cassini Mission. This model is based on the most complete set of observations yet obtained in Saturn's magnetosphere and includes data from forty-five periapsis passes at a wide variety of geometries. Due to uncertainties in the rotation rate of the planet the model is constrained to be axisymmetric. To derive the model, the external currents are modeled explicitly as an equatorial ring current centered on Saturn's equator and the internal planetary magnetic field is derived using standard inversion techniques. The field is adequately described by a model of degree 3 and the spherical harmonic coefficients are g 10 = 21 , 162 , g 20 = 1514 , g 30 = 2283 . Units are nanoTeslas (nT) and are based on a planetary radius of 60,268 km. The model is consistent with a northward offset of the magnetic equator from the rotational equator of 0.036 Saturn radii. Reanalysis and comparison with data obtained by Pioneer-11 and Voyager-1 and -2 shows little evidence for secular variation in the field in the almost thirty years since those data were obtained.

Anderson & Schubert (2007, Science,317,1384) proposed that Saturn's rotation period can be ascertained by minimizing the dynamic heights of the 100 mbar isosurface with respect to the geoid; they derived a rotation period of 10h 32m 35s. We investigate the same approach for Jupiter to see if the Jovian rotation period is predicted by minimizing the dynamical heights of its isobaric (1 bar pressure level) surface using zonal wind data. A rotation period of 9h 54m 29s is found. Further, we investigate the minimization method by fitting Pioneer and Voyager occultation radii for both Jupiter and Saturn. Rotation periods of 9h 55m 30s and 10h 32m 35s are found to minimize the dynamical heights for Jupiter and Saturn, respectively. Though there is no dynamical principle requiring the minimization of the dynamical heights of an isobaric surface, the successful application of the method to Jupiter lends support to its relevance for Saturn. We derive Jupiter and Saturn rotation periods using equilibrium theory in which the solid-body rotation period (no winds) that gives the observed equatorial and polar radii at the 100 mbar level is found. Rotation periods of 9h 55m 20s and 10h 31m 49s are found for Jupiter and Saturn, respectively. We show that both Jupiter's and Saturn's shapes can be derived using solid-body rotation, suggesting that zonal winds have a minor effect on the planetary shape for both planets. The agreement in the values of Saturn's rotation period predicted by the different approaches supports the conclusion that the planet's period of rotation is about 10h 32m., 15 pages, accepted for publication in Planetary and Space Science

The plasma environment of Saturn's largest satellite Titan is known to be highly variable. Since Titan's orbit is located within the outer magnetosphere of Saturn, the moon can leave the region dominated by the magnetic field of its parent body in times of high solar wind dynamic pressure and interact with the thermalized magnetosheath plasma or even with the unshocked solar wind. By applying a three-dimensional hybrid simulation code (kinetic description of ions, fluid electrons), we study in real-time the transition that Titan's plasma environment undergoes when the moon leaves Saturn's magnetosphere and enters the supermagnetosonic solar wind. In the simulation, the transition between both plasma regimes is mimicked by a reversal of the magnetic field direction as well as a change in the composition and temperature of the impinging plasma flow. When the satellite enters the solar wind, the magnetic draping pattern in its vicinity is reconfigured due to reconnection, with the characteristic time scale of this process being determined by the convection of the field lines in the undisturbed plasma flow at the flanks of the interaction region. The build-up of a bow shock ahead of Titan takes place on a typical time scale of a few minutes as well. We also analyze the erosion of the newly formed shock front upstream of Titan that commences when the moon re-enters the submagnetosonic plasma regime of Saturn's magnetosphere. Although the model presented here is far from governing the full complexity of Titan's plasma interaction during a solar wind excursion, the simulation provides important insights into general plasma-physical processes associated with such a disruptive change of the upstream flow conditions.

This review presents modern views on the physics of magnetospheres of Solar System planets having an intrinsic magnetic field, and on the structure of magnetospheric magnetic fields. Magnetic fields are generated in the interiors of Mercury, Earth, Jupiter, Saturn, Uranus, and Neptune via the dynamo mechanism. These fields are so strong that they serve as obstacles for the plasma stream of the solar wind. A magnetosphere surrounding a planet forms as the result of interaction between the solar wind and the planetary magnetic field. The dynamics of magnetospheres are primary enforced by solar wind variations. Each magnetosphere is unique. The review considers common and individual sources of magnetic fields and the properties of planetary magnetospheres.

We have investigated statistically the electron density below 5 cm−3 in the magnetosphere of Saturn (7–80 RS, Saturn radii) using 44 orbits of the floating potential data from the RPWS Langmuir probe (LP) onboard Cassini. The density distribution shows a clear dependence on the distance from the Saturnian rotation axis (√X2+Y2) as well as on the distance from the equatorial plane (|Z|), indicating a disc-like structure. From the characteristics of the density distribution, we have identified three regions: the extension of the plasma disc, the magnetodisc region, and the lobe regions. The plasma disc region is at LL is the radial distance to the equatorial crossing of the dipole magnetic field line, and confined to |Z|RS. The magnetodisc is located beyond L=15, and its density has a large variability. The variability has quasi-periodic characteristics with a periodicity corresponding to the planetary rotation. For Z>15 RS, the magnetospheric density distribution becomes constant in Z. However, the density still varies quasi-periodically with the planetary rotation also in this region. In fact, the quasi-periodic variation has been observed all over the magnetosphere beyond L=15. The region above Z=15 RS is identified as the lobe region. We also found that the magnetosphere can occasionally move latitudinally under the control of the density in the magnetosphere and the solar wind. From the empirical distributions of the electron densities obtained in this study, we have constructed an electron density model of the Saturnian nightside magnetosphere beyond 7 RS. The obtained model can well reproduce the observed density distribution, and can thus be useful for magnetospheric modelling studies.

By analyzing hot ion and electron parameters together with magnetic field measurements from Cluster, an event of magnetopause crossing of the spacecraft has been investigated. At the latitude of about 40° and magnetic local time (MLT) of 13:20 during the southward interplanetary magnetic field (IMF), a transition layer was observed, with the magnetospheric field configuration and cold dense plasma features of the magnetosheath. The particle energy-time spectrograms inside the layer were similar to but still a little different from those in the magnetosheath, obviously indicating the solar wind entry into the magnetosphere. The direction and magnitude of the accelerated ion flow implied that reconnection might possibly cause such a solar wind entry phenomenon. The bipolar signature of the normal magnetic component BN in magnetopause coordinates further supported happening of reconnection there. The solar wind plasma flowed toward the magnetopause and entered the magnetosphere along the reconnected flux tube. The magnetospheric branch of the reconnected flux tube was still inside the magnetosphere after reconnection and supplied the path for the solar wind entry into the dayside magnetosphere. The case analysis gives observational evidence and more details of how the reconnection process at the dayside low latitude magnetopause caused the solar wind entry into the magnetosphere.

With a Saturnian magnetopause average stand-off distance of about 21 planetary radii, Titan spends most of its time inside the rotating magnetosphere of its parent planet. However, when Saturn's magnetosphere is compressed due to high solar wind dynamic pressure, Titan can cross Saturn's magnetopause in the subsolar region of its orbit and therefore to interact with the shocked solar wind plasma in Saturn's magnetosheath. This situation has been observed during the T32 flyby of the Cassini spacecraft on 13 June 2007. Until a few minutes before closest approach, Titan had been located inside the Saturnian magnetosphere. During the flyby, Titan encountered a sudden change in the direction and magnitude of the ambient magnetic field. The density of the ambient plasma also increased dramatically during the pass. Thus, the moon's exosphere and ionosphere were exposed to a sudden change in the upstream plasma conditions. The resulting reconfiguration of Titan's plasma tail has been studied in real-time by using a three-dimensional, multi-species hybrid simulation model. The hybrid approximation treats the electrons of the plasma as a massless, charge-neutralizing fluid, while ion dynamics are described by a kinetic approach. In the simulations, the magnetopause crossing is modeled by a sudden change of the upstream magnetic field vector as well as a modification of the upstream plasma composition. We present real-time simulation results, illustrating how Titan's induced magnetotail is reconfigured due to magnetic reconnection. The simulations allow to determine a characteristic time scale for the erosion of the original magnetic draping pattern that commences after Titan has crossed Saturn's magnetopause. Besides, the influence of the plasma composition in the magnetosheath on the reconfiguration process is discussed in detail. The question of whether the magnetopause crossing is likely to yield a detachment of Titan's exospheric tail from the satellite is investigated as well.

The four identical Cluster spacecraft, launched in 2000, orbit the Earth in a tetrahedral configuration and on a highly eccentric polar orbit (4–19.6 R E ). This allows the crossing of critical layers that develop as a result of the interaction between the solar wind and the Earth’s magnetosphere. Since 2004 the Chinese Double Star TC-1 and TC-2 spacecraft, whose payload comprise also backup models of instruments developed by European scientists for Cluster, provided two additional points of measurement, on a larger scale: the Cluster and Double Star orbits are such that the spacecraft are almost in the same meridian, allowing conjugate studies. The Cluster and Double Star observations during the 2005 and 2006 extreme solar events are presented, showing uncommon plasma parameters values in the near-Earth solar wind and in the magnetosheath. These include solar wind velocities up to ∼900 km s −1 during an ICME shock arrival, accompanied by a sudden increase in the density by a factor of ∼5 and followed by an enrichment in He ++ in the secondary front of the ICME. In the magnetosheath ion density values as high as 130 cm −3 were observed, and the plasma flow velocity there reached values even higher than the typical solar wind velocity. These resulted in unusual dayside magnetosphere compression, detection of penetrating high-energy particles in the magnetotail, and ring current development following several successive injections of energetic particles in the inner magnetosphere, which “washed out” the previously formed nose-like ion structures.

Planetary aurorae are generally produced by currents flowing between the planet's ionosphere and magnetosphere, which accelerate energetic charged particles that then hit the upper atmosphere. Recent models of Saturn's aurora predict only weak emission away from the main auroral oval. Stallard et al. now present Cassini infrared images taken from a novel angle, providing the first nightside auroral view. They reveal emissions both poleward and equatorward of the main oval. The polar emissions vary with time, and seem not to be linked with strong magnetospheric compressions. This aurora appears to be unique to Saturn and cannot be explained by current models of Saturn's magnetosphere. The majority of planetary aurorae are produced by electrical currents flowing between the ionosphere and the magnetosphere which accelerate energetic charged particles that hit the upper atmosphere. At Saturn, these processes collisionally excite hydrogen, causing ultraviolet emission1,2,3,4,5,6,7,8, and ionize the hydrogen, leading to H3+ infrared emission9,10,11,12,13,14,15. Although the morphology of these aurorae is affected by changes in the solar wind6,11, the source of the currents which produce them is a matter of debate16,17. Recent models predict only weak emission away from the main auroral oval18. Here we report images that show emission both poleward and equatorward of the main oval (separated by a region of low emission). The extensive polar emission is highly variable with time, and disappears when the main oval has a spiral morphology; this suggests that although the polar emission may be associated with minor increases in the dynamic pressure from the solar wind, it is not directly linked to strong magnetospheric compressions. This aurora appears to be unique to Saturn and cannot be explained using our current understanding of Saturn’s magnetosphere. The equatorward arc of emission exists only on the nightside of the planet, and arises from internal magnetospheric processes that are currently unknown.

The equatorial and polar satellites of the Double Star Project (DSP) were launched successfully on December 29, 2003 and July 25, 2004, respectively, and both of them are operating smoothly. The DSP provides a good opportunity for investigating the structure of the magnetosphere. Based on the DSP data collected during 2004, we have surveyed the distribution of the magnetic fields and plasmas in the magnetosphere. It is found that: (1) Near the Earth’s equatorial plane within geocentric distances of less than 7 RE, the Earth’s magnetic field is dipolar. In the vicinity of the magnetopause, the magnetic field is enhanced by a factor of about 1.5, and on the nightside, the magnetic field can vary significantly from the Earth’s dipole field, likely caused by the presence of the near-Earth tail current sheet. (2) In the day-side magnetosheath, the electron and ion densities are usually both in the range of 10–30 cm−3; the ion and electron temperatures are usually about 200 and 50 eV, respectively. The flow pattern is usually smooth, with a low velocity in the subsolar region and with significantly higher velocities in the dawn and dusk flanks. (3) In the region between the magnetopause and plasmasphere the density is low, approximately 0.5–5 cm−3, and the temperature is high, about 1–10 keV for ions and 0.1–5 keV for electrons. The ion temperature has an apparent anisotropy, with the ratio of the perpendicular and parallel temperatures being about 1.0–1.3 for the night-and dusk-side magnetosphere and about 1.3–2.0 for the day-and dawn-side magnetosphere. There is an evident sunward convection of about 50 km/s in the magnetosphere. On the dawn side, the flow becomes somewhat turbulent, and in the vicinity of the night-noon meridian plane, the convection is rather slow. (4) The high-energy electrons with energies higher than 2 MeV are mainly located in the regions with 3 < L < 4.5; the size of the high-energy electrons area varies with time, it may expand and shrink occasionally according to different solar wind conditions and magnetic activities.

During the interval 06:14–07:30 UT on August 24, 2005, since the Earth’s magnetopause was suddenly compressed by the persistent high-speed solar wind stream with the southward component of the interplanetary magnetic field (IMF), the magnetopause moved inward for about 3.1 RE. Meanwhile, TC-1 satellite shifted from northern plasma sheet to the northern lobe/mantle region, although it kept inward flying during the interval 06:00–07:30UT. The shift of TC-1 from the plasma sheet to the lobe/mantle is caused by the simultaneous inward displacements of the plasma sheet and near-Earth lobe/mantle region, and their inward movement velocity is larger than the inward motion velocity of TC-1. The joint inward displacements of the magnetopause, the lobe/mantle region and the plasma sheet indicate that the whole magnetosphere shrinks inward due to the magnetospheric compression by the high-speed solar wind stream, and the magnetospheric ions are attached to the magnetic field lines (i.e. ‘frozen’ in magnetic field) and move inward in the shrinking process of magnetosphere. The large shrinkage of magnetosphere indicates that the near-Earth magnetotail compression caused by the strong solar wind dynamic pressure is much larger than its thickening caused by the southward component of the IMF, and the locations of magnetospheric regions with different plasmas vary remarkably with the variation of the solar wind dynamic pressure.

Observations by MESSENGER show that Mercury's magnetosphere is immersed in a comet-like cloud of planetary ions. The most abundant, Na + , is broadly distributed but exhibits flux maxima in the magnetosheath, where the local plasma flow speed is high, and near the spacecraft's closest approach, where atmospheric density should peak. The magnetic field showed reconnection signatures in the form of flux transfer events, azimuthal rotations consistent with Kelvin-Helmholtz waves along the magnetopause, and extensive ultralow-frequency wave activity. Two outbound current sheet boundaries were observed, across which the magnetic field decreased in a manner suggestive of a double magnetopause. The separation of these current layers, comparable to the gyro-radius of a Na + pickup ion entering the magnetosphere after being accelerated in the magnetosheath, may indicate a planetary ion boundary layer.

Since the Voyager mission it is known that Saturn Kilometric Radiation (SKR) is strongly influenced by external forces, i.e., the solar wind and in particular the solar wind ram pressure. Recent studies using Cassini data essentially confirmed these findings for particular periods during the first Cassini orbit of Saturn. The data coverage of SKR by the Cassini/RPWS experiment for the period of six months prior to Saturn Orbit Insertion (July 1, 2004) is rather continuous, whereas there are gaps in the solar wind plasma data. The strong correlation of SKR with the solar wind may provide an indication on the variations of the solar wind plasma, specifically during the gap periods. These periods lacking solar wind data are substituted by Ulysses solar wind data which have been propagated over ∼4 AU, applying magnetohydrodynamic propagation models. Cross correlation studies showed that Ulysses solar wind data can be taken as a substitute for missing Cassini data. The use of SKR as monitor for solar wind variations is discussed. With the present set of observations the SKR proxy lacks significant reliability.

Icy grains and satellites orbiting in Saturn's magnetosphere are immersed in a plasma that sputters their surfaces. This limits the lifetime of the E-ring grains and ejects neutrals that orbit Saturn until they are ionized and populate its magnetosphere. Here we re-evaluate the sputtering rate of ice in Saturn's inner magnetosphere using the recent Cassini data on the plasma ion density, temperature and composition [Sittler Jr., E.C., et al., 2007a. Ion and neutral sources and sinks within Saturn's inner magnetosphere: Cassini results. Planet. Space Sci. 56, 3–18.] and a recent summary of the relevant sputtering data for ice [Fama, M., Shi, J., Baragiola, R.A., 2008. Sputtering of ice by low-energy ions. Surf. Sci. 602, 156–161.]. Although the energetic (>10 keV) ion component at Saturn is much smaller than was assumed to be the case after Voyager [Jurac, S., Johnson, R.E., Richardson, J.D., Paranicas, C., 2001a. Satellite sputtering in Saturn's magnetosphere. Planet. Space Sci. 49, 319–326; Jurac, S., Johnson, R.E., Richardson, J.D., 2001b. Saturn's E ring and production of the neutral torus. Icarus 149, 384–396.], we show that the sputtering rates are sensitive to the temperature of the thermal plasma and are still robust, so that sputtering likely determines the lifetime of the grains in Saturn's tenuous E-ring.

This paper constructs a self-consistent stationary model of the Saturn's magnetosphere, in which the centrifugal force is included. Based on the thin-filament model, numerical simulation of radial plasma transport in the Saturn's magnetosphere is taken. The results show that the plasma near Dione may be transported to the farther place because of the centrifugal interchange instability. This type of transportation is dominated by the centrifugal force, the viscidity and energy transfer do little on the transportation (almost negligible) but the ionospheric conductance has important influence. Lower value of conductance causes faster transportation while higher value of conductance will block transportation to some extent. The plasma near Dione (6.3RS from the Saturn) has been transported to 10RS away from the Saturn in 5.52 hours when the ionospheric conductance is set to 2S. Our simulations also show that the plasma is unstable if the density is disturbed, when the Lorentz force and the gradient of plasma thermal pressure can not balance with the centrifugal force, radial transportation will happen. The radial plasma transport in the Saturn's magnetosphere is that the denser plasma becomes cooler because of adiabatic expansion in the process of outward transport while the injected lighter plasma becomes hotter due to the adiabatic compression.

Using a global numerical model, we have studied how the present Martian magnetosphere may have looked in the past when the planet had a global intrinsic magnetic field. A Mars version (HYB-Mars) of the self-consistent quasi-neutral hybrid model was used which treats the ions as particles and the electrons as a massless charge-neutralizing fluid. We compare four cases where an intrinsic dipole magnetic field was 0 nT (the present situation), 10, 30, and 60 nT at the surface of Mars along the magnetic equator. We find that the 10 nT dipolar magnetic field already results in a magnetosphere which in many respects is more Earth-like than, a non-magnetized, “induced” magnetosphere. However, the 10 nT dipole magnetosphere is still relatively strongly connected to the interplanetary magnetic field, while the 30 nT dipole case, and especially the 60 nT dipole case, results in a magnetosphere whose morphology is determined predominantly by the Martian intrinsic magnetic field. A change of the magnetosphere due to a decreasing dipole magnetic field strength from 60 to 0 nT could have happened during the history of Mars when a globally magnetized Mars turned into the present, globally non-magnetized, planet.

Context. As the largest in our solar system the Jovian magnetosphere is treated in the literature as the dominant source of a fewMeV electrons, which have been measured by various spacecraft. In particular, observations obtained with Ulysses have significantly broadened the available data base. Aims. We simulated the transports of MeV electrons in the heliosphere on the basis of a time-dependent, three-dimensional modulation model. For this purpose the cosmic rays, the Jovian and the Saturnian electron sources have been, for the first time, considered together in the simulation of electron fluxes. The simulated electron intensities are discussed along the Ulysses and Cassini trajectories. The Ulysses spacecraft already passed by the planet Jupiter twice (1992, 2004), and Cassini passed by Jupiter in 2001 and reached Saturn in the year 2004. The strength of the electron source at Jupiter is relatively well known and well modelled. To determine the source strength of Saturn in comparison to that of Jupiter, we compared all available spacecraft measurements at Jupiter/Saturn in the overlapping energy range. We study the general distribution of Kronian electrons by successively using three different strengths of the Saturn source in our simulations. In addition, the effects of the solar activity are taken into account by varying the velocity field of the solar wind and the anisotropic diffusion tensor. Methods. Studying the particle diffusion is particularly relevant, because the really unknown function in the used transport equation is the diffusion tensor. The Jovian and/or Kronian electrons are suitable for studying the transport of energetic particles because the source locations are well known. Results. At 1 MeV the intensities along the Ulysses and the Cassini trajectories are clearly influenced by the presence of Kronian electrons. Conclusions. Our results reveal that the electrons from the Kronian magnetosphere, as the second largest, cannot be neglected in the very-low MeV energy range.

We model the open magnetic field region in Saturn's southern polar ionosphere during two compression regions observed by the Cassini spacecraft upstream of Saturn in January 2004, and compare these with the auroral ovals observed simultaneously in ultraviolet images obtained by the Hubble Space Telescope. The modelling employs the paraboloid model of Saturn's magnetospheric magnetic field, whose parameters are varied according to the observed values of both the solar wind dynamic pressure and the interplanetary magnetic field (IMF) vector. It is shown that the open field area responds strongly to the IMF vector for both expanded and compressed magnetic models, corresponding to low and high dynamic pressure, respectively. It is also shown that the computed open field region agrees with the poleward boundary of the auroras as well as or better than those derived previously from a model in which only the variation of the IMF vector was taken into account. The results again support the hypothesis that the auroral oval at Saturn is associated with the open-closed field line boundary and hence with the solar wind interaction.

The study of planetary magnetospheres allows us to understand processes occurring in the Earth’s magnetosphere by showing us how these processes respond under different conditions. We illustrate lessons learned about the control of the size of the magnetosphere by the dynamic pressure of the solar wind; how cold plasma is lost from magnetospheres; how free energy is generated to produce ion cyclotron waves; the role of fast neutrals in a planetary magnetosphere; the interchange instability; and reconnection in a magnetodisk. Not all information flow is from Jupiter and Saturn to Earth; some flows the other way.

Owing to the next space mission Messenger (NASA) and BepiColombo (ESA/JAXA), there is a renewed interest in modelling the Mercury's environment. The geometry of the Mercury's magnetosphere, as well as its response to the solar wind conditions, is one of the major issues. The weak magnetic field of the planet and the increasing weight of the IMF BXBX component at Mercury's orbit, introduce critical differences with respect to the Earth's case, such as a strong north–south asymmetry and a significant solar wind precipitation into the dayside magnetosphere even for non-negative IMF BZBZ. With the aim of analysing the interaction between the solar wind and Mercury's magnetosphere, we have developed an empirical–analytical magnetospheric model starting from the Toffoletto–Hill TH93 code. Our model has been tuned to reproduce the key features of the Mariner 10 magnetic data, and to mimic the magnetic field topology obtained by the self-consistent hybrid simulation developed by Kallio and Janhunen [Solar wind and magnetospheric ion impact on Mercury's magnetosphere. Geophys. Res. Lett. 30, 1877, doi: 10.1029/2003GL017842]. The new model has then been used to study the effect of the magnetic reconnection on the magnetosheath plasma entry through the open areas of the dayside magnetosphere (cusps), which are expected to be one of the main sources of charged particles circulating inside the magnetosphere. We show that, depending on the Alfven speeds on both sides of the magnetopause discontinuity, the reconnection process would be able to accelerate solar wind protons up to few tens of keV: part of these ions can hit the surface and then trigger, via ion-sputtering, the refilling of the planetary exosphere. Finally, we show that non-adiabatic effects are expected to develop in the cusp regions as the energy gained by injected particles increases. The extent of these non-adiabatic regions is shown to be also modulated by upstream IMF condition.

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission to Mercury offers our first opportunity to explore this planet’s miniature magnetosphere since the brief flybys of Mariner 10. Mercury’s magnetosphere is unique in many respects. The magnetosphere of Mercury is among the smallest in the solar system; its magnetic field typically stands off the solar wind only ~1000 to 2000 km above the surface. For this reason there are no closed drift paths for energetic particles and, hence, no radiation belts. Magnetic reconnection at the day side magnetopause may erode the subsolar magnetosphere, allowing solar wind ions to impact directly the regolith. Inductive currents in Mercury’s interior may act to modify the solar wind interaction by resisting changes due to solar wind pressure variations. Indeed, observations of these induction effects may be an important source of information on the state of Mercury’s interior. In addition, Mercury’s magnetosphere is the only one with its defining magnetic flux tubes rooted beneath the solid surface as opposed to an atmosphere with a conductive ionospheric layer. This lack of an ionosphere is probably the underlying reason for the brevity of the very intense, but short-lived, ~l-2 min, substorm-like energetic particle events observed by Mariner 10 during its first traversal of Mercury’s magnetic tail. Because of Mercury’s proximity to the sun, 0.3-0.5 AU, this magnetosphere experiences the most extreme driving forces in the solar system. All of these factors are expected to produce complicated interactions involving the exchange and recycling of neutrals and ions among the solar wind, magnetosphere, and regolith. The electrodynamics of Mercury’s magnetosphere are expected to be equally complex, with strong forcing by the solar wind, magnetic reconnection, and pick-up of planetary ions all playing roles in the generation of field-aligned electric currents. However, these field-aligned currents do not close in an ionosphere, but in some other manner. In addition to the insights into magnetospheric physics offered by study of the solar wind-Mercury system, quantitative specification of the “external” magnetic field generated by magnetospheric currents is necessary for accurate determination of the strength and multipolar decomposition of Mercury’s intrinsic magnetic field. MESSENGER’S highly capable instrumentation and broad orbital coverage will greatly advance our understanding of both the origin of Mercury’s magnetic field and the acceleration of charged particles in small magnetospheres. In this article, we review what is known about Mercury’s magnetosphere and describe the MESSENGER science team’s strategy for obtaining answers to the outstanding science questions surrounding the interaction of the solar wind with Mercury and its small, but dynamic, magnetosphere.

Context. Our knowledge of energetic particles in Saturn’s inner magnetosphere is based on observations made during the flybys of Pioneer 11, Voyager 1, Voyager 2, and recently by Cassini. The most important features of the energetic particle population in the inner Saturnian magnetosphere are: 1) the rings and the many large and small satellites inside this region reduce the population of particles whose energies are higher than 0.5 MeV to values of the order of 10 3 times less than would otherwise be present; 2) the sputtering and outgassing of the surfaces of the satellites injects particles into the system and by some physical process, particles of the resultant plasma are accelerated to energies of the order of tens of keV; 3) the radial distribution of very energetic protons Ep > tens of MeV exhibits three major peaks associated with rings and satellites; 4) a proton population Ep ∼ 1 MeV lies outside the orbit of Enceladus; 5) a proton population Ep < 0.25 MeV has an apparent origin associated with Dione, Tethys, Enceladus, E-ring, Mimas, and G-ring; 6) a population of low-energy electrons is associated with the satellites. Aims. We propose a mechanism to explain the energetic particle population observed in Saturn’s inner magnetosphere based on the stochastic behavior of the electric field. Methods. To simulate the stochastic electric field we employ a Monte Carlo Method taking into account the magnetic field fluctuations obtained from the observations made by Voyager 1 spacecraft. Results. Assuming different initial conditions, like the source of charged particles and the distribution function of their velocities, we find that particles injected with very low energies ranging from 0.104 eV to 0.526 keV can be accelerated to reach much higher energies ranging from 0.944 eV to 0.547 keV after a few seconds.

We study the dependence of Saturn's magnetospheric magnetic field structure on the interplanetary magnetic field (IMF), together with the corresponding variations of the open-closed field line boundary in the ionosphere. Specifically we investigate the interval from 8 to 30 January 2004, when UV images of Saturn's southern aurora were obtained by the Hubble Space Telescope (HST), and simultaneous interplanetary measurements were provided by the Cassini spacecraft located near the ecliptic ~0.2 AU upstream of Saturn and ~0.5 AU off the planet-Sun line towards dawn. Using the paraboloid model of Saturn's magnetosphere, we calculate the magnetospheric magnetic field structure for several values of the IMF vector representative of interplanetary compression regions. Variations in the magnetic structure lead to different shapes and areas of the open field line region in the ionosphere. Comparison with the HST auroral images shows that the area of the computed open flux region is generally comparable to that enclosed by the auroral oval, and sometimes agrees in detail with its poleward boundary, though more typically being displaced by a few degrees in the tailward direction.

We put forward a model and deduce the distribution of Saturn's Kilometric Radiation (SKR) Source. The model is as follows, the Kelvin-Helmholtz Instability (KHI) will generate kinds of waves, in which there is the Alfven wave. It will propagate to the Saturn's polar area by following the bending magnetic field line. It will also cause the Cyclotron Maser Instability (CMI) leading to the generation of extraordinary wave (R-X mode) in the process. In our views, the region of the CMI, near the ionosphere of Saturn (1Rs above the polar area, Rs is Saturn radius), is regarded as the same area as the Saturn's Kilometric Radiation Source. In this paper, we deduce that the KHI will appear before 11 LT (Saturn's Local Time), the Alfven wave will take 38.3 min to reach Saturn, the sunward magnetic field line will bend almost 80°. Finally, we get the distribution of SKR source. It locates from 7~9 LT at a relatively lower latitude to 18 LT at the eveningside.

We consider the contribution of the solar wind-driven Dungey-cycle to flux transport in Jupiter's and Saturn's magnetospheres, the associated voltages being based on estimates of the magnetopause reconnection rates recently derived from observations of the interplanetary medium in the vicinity of the corresponding planetary orbits. At Jupiter, the reconnection voltages are estimated to be ~150 kV during several-day weak-field rarefaction regions, increasing to ~1 MV during few-day strong-field compression regions. The corresponding values at Saturn are ~25 kV for rarefaction regions, increasing to ~150 kV for compressions. These values are compared with the voltages associated with the flows driven by planetary rotation. Estimates of the rotational flux transport in the "middle" and "outer" magnetosphere regions are shown to yield voltages of several MV and several hundred kV at Jupiter and Saturn respectively, thus being of the same order as the estimated peak Dungey-cycle voltages. We conclude that under such circumstances the Dungey-cycle "return" flow will make a significant contribution to the flux transport in the outer magnetospheric regions. The "return" Dungey-cycle flows are then expected to form layers which are a few planetary radii wide inside the dawn and morning magnetopause. In the absence of significant cross-field plasma diffusion, these layers will be characterized by the presence of hot light ions originating from either the planetary ionosphere or the solar wind, while the inner layers associated with the Vasyliunas-cycle and middle magnetosphere transport will be dominated by hot heavy ions originating from internal moon/ring plasma sources. The temperature of these ions is estimated to be of the order of a few keV at Saturn and a few tens of keV at Jupiter, in both layers.

Jupiter is a planet of superlatives: the most massive planet in the solar system, rotates the fastest, has the strongest magnetic field, and has the most massive satellite system of any planet. These unique properties lead to volcanoes on Io and a population of energetic plasma trapped in the magnetic field that provides a physical link between the satellites, particularly Io, and the planet Jupiter. There are strong differences between the magnetospheres of Earth and Jupiter but there are also underlying basic physical principles that all magnetospheres share in common. This paper provides a rough sketch of the magnetosphere of Jupiter, briefly describes the current understanding and lists outstanding issues. As at Earth, a major issue of the jovian system is how the magnetospheric plasma is coupled to the planet's ionosphere.

We use the Cassini Radio and Plasma Wave Science/Langmuir probe measurements of the electron density from the first 110 flybys of Titan to study how Saturn´s magnetosphere influences Titan´s ionosphere. The data is first corrected for biased sampling due to varying solar zenith angle and solar energy flux (solar cycle effects). We then present results showing that the electron density in Titan´s ionosphere, in the altitude range 1600-2400 km, is increased by about a factor of 2.5 when Titan is located on the nightside of Saturn (Saturn local time (SLT) 21-03 h) compared to when on the dayside (SLT 09-15 h). For lower altitudes (1100-1600 km) the main dividing factor for the ionospheric density is the ambient magnetospheric conditions. When Titan is located in the magnetospheric current sheet, the electron density in Titan´s ionosphere is about a factor of 1.4 higher compared to when Titan is located in the magnetospheric lobes. The factor of 1.4 increase in between sheet and lobe flybys is interpreted as an effect of increased particle impact ionization from 200 eV sheet electrons. The factor of 2.5 increase in electron density between flybys on Saturn´s nightside and dayside is suggested to be an effect of the pressure balance between thermal plus magnetic pressure in Titan´s ionosphere against the dynamic pressure and energetic particle pressure in Saturn´s magnetosphere. Fil: Edberg, N. J. T.. University of Iowa; Estados Unidos. Swedish Institute of Space Physics; Suecia Fil: Andrews, D. J.. Swedish Institute of Space Physics; Suecia Fil: Bertucci, Cesar. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina Fil: Gurnett, D. A.. University of Iowa; Estados Unidos Fil: Holmberg, M. K. G.. Swedish Institute of Space Physics; Suecia Fil: Jackman, C. M.. University Of Southampton; Reino Unido Fil: Kurth, W. S.. University of Iowa; Estados Unidos Fil: Menietti, J. D.. University Of Iowa; Estados Unidos Fil: Opgenoorth, H. J.. Swedish Institute of Space Physics; Suecia Fil: Shebanits, O.. Swedish Institute of Space Physics; Suecia Fil: Vigren, E.. Swedish Institute of Space Physics; Suecia Fil: Wahlund, J. E.. Swedish Institute of Space Physics; Suecia