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Funding Information: C. G. is supported by an ESA Research Fellowship. H. G. acknowledges support by the Swedish National Space Agency grant 108/18. B. S.-C. acknowledges support through UK-STFC grant ST/S000429/1. French co-authors acknowledge the support of CNES for the Rosetta and Comet Interceptor missions. I. M. acknowledges support through grants of Research Council of Norway (262941 and 275503). Publisher Copyright: © 2021, The Author(s), under exclusive licence to Springer Nature B.V. Comets hold the key to the understanding of our Solar System, its formation and its evolution, and to the fundamental plasma processes at work both in it and beyond it. A comet nucleus emits gas as it is heated by the sunlight. The gas forms the coma, where it is ionised, becomes a plasma, and eventually interacts with the solar wind. Besides these neutral and ionised gases, the coma also contains dust grains, released from the comet nucleus. As a cometary atmosphere develops when the comet travels through the Solar System, large-scale structures, such as the plasma boundaries, develop and disappear, while at planets such large-scale structures are only accessible in their fully grown, quasi-steady state. In situ measurements at comets enable us to learn both how such large-scale structures are formed or reformed and how small-scale processes in the plasma affect the formation and properties of these large scale structures. Furthermore, a comet goes through a wide range of parameter regimes during its life cycle, where either collisional processes, involving neutrals and charged particles, or collisionless processes are at play, and might even compete in complicated transitional regimes. Thus a comet presents a unique opportunity to study this parameter space, from an asteroid-like to a Mars- and Venus-like interaction. The Rosetta mission and previous fast flybys of comets have together made many new discoveries, but the most important breakthroughs in the understanding of cometary plasmas are yet to come. The Comet Interceptor mission will provide a sample of multi-point measurements at a comet, setting the stage for a multi-spacecraft mission to accompany a comet on its journey through the Solar System. This White Paper, submitted in response to the European Space Agency’s Voyage 2050 call, reviews the present-day knowledge of cometary plasmas, discusses the many questions that remain unanswered, and outlines a multi-spacecraft European Space Agency mission to accompany a comet that will answer these questionsby combining both multi-spacecraft observations and a rendezvous mission, and at the same time advance our understanding of fundamental plasma physics and its role in planetary systems.

It has been confirmed that dipolarization fronts (DFs) are the result of the interchange instability in the Earth’s magnetotail. In this paper, we use a Hall MHD model to simulate the evolution of the interchange instability that produces DFs along the leading edge. A test particle simulation is performed to study the physical phenomenon of ion acceleration at the DF. The numerical simulation indicates that almost all particles move earthward and dawnward and then drift to the tail. The DF-reflected ion population at the duskside appears earlier as a consequence of the asymmetric Hall electric field. Ions that are distributed in a dawn-dusk asymmetric semicircle behind the DF tend to be accelerated to higher energies (>13.5 keV). These high-energy particles eventually concentrate in the dawnside. Ions experience effective acceleration by the dawnward electric field, while they drift through the dawn flank at the front, toward the tail.

How ion escape from the near-Mars space is one of the biggest puzzles for understanding the atmospheric evolution of Mars. Ions in the plasma wake region continuously escape from the unmagnetized planet. Although the average ion escape rate in the wake region is relatively low, observations also have revealed the presence of events that contribute bursty and enhanced ion escape fluxes. Boundary instabilities and magnetic reconnection are suggested to be the candidate mechanisms. However, there is a lack of evaluation of ion escape caused by reconnection and comparison of the two mechanisms under a similar plasma environment. Here, we show an exciting reconnection event in the Martian wake. Two types of flux ropes are observed during the event. One was generated by reconnection, while others were produced by dayside boundary instability and convected to tail. The escape rate of oxygen ions in the reconnection region was estimated to be about 53–72% of the total tailward escape. Furthermore, the escape flux in the flux rope produced by reconnection was over twice that caused by dayside instabilities.

A single satellite hardly measures the overall ion escape rate from a planet. Therefore, the question concerning the long-term atmospheric evolution of whether a planetary magnetic field protects its atmosphere or aggravates atmospheric loss remains unresolved. Here, combined data from multiple platforms including Cluster, DMSP, IMAGE, and Polar satellites and ground stations show that, when an interplanetary coronal mass ejection (ICME) arrived, an ion transport “highway” formed in the inner magnetosphere on the strongly magnetized Earth, along which the magnetospheric processes promoted quick and significant atmospheric ion escape. The escape rate merely through this highway probably reached 1027 s−1, far exceeding the current overall rates on those nonmagnetized planets, e.g., Mars and Venus. The relevant interplanetary conditions induced by the ICME were extreme by today’s standards but may have been quite common in the ancient solar system. These present-day observations thus allow us to depict a fast but common ion escape scenario on the strongly magnetized planets in ancient times when the Sun was extremely active.

The Mars Rover Magnetometers (RoMAG) will implement the first mobile magnetic field measurements on the surface of Mars. Two identical tri-axial fluxgate magnetometer sensors are mounted at the top and bottom of the mast of rover, respectively. The technology of Helmholtz compensation coil probe and digital closed-loop feedback circuit is utilized to realize high precision measurement. Each magnetometer measures the vector magnetic field with a resolution of 0.01 nT in the range of $\pm65000~\text{nT}$ . The sample rate is up to 32 Hz and the noise is $0.01~\text{nT}/\surd\text{Hz}$ @1 Hz. A rover magnetic compensation procedure was conducted to remove the most important magnetic disturbances. Mobile magnetic field measurements on the Martian surface would obtain fine-scale crust field and provide information about its remnant magnetization and any possible intrinsic magnetic field. It would help us to further understand Martian internal structure, coupling processes of solar wind-magnetosphere-ionosphere.

Due to the sparsity of space probes, it is still not clear on how the magnetic structure of the magnetotail looks like and how it evolves when the Interplanetary Magnetic Field (IMF) directs northw...

Collisionless plasma shocks (CPSs), forming when supersonic plasma streams encounter a magnetized obstacle, are invoked to explain the acceleration of ubiquitously energetic cosmic rays. It has long been theorized from magnetohydrodynamics, but not directly observed that the CPSs develop from the growth of small-amplitude, low-frequency plasma waves which excited by reflected ion beams from the obstacle. We present in situ observations of an entire formation sequence of the periodic plasma shocks by the MAVEN spacecraft’s magnetic field and particle instruments. The magnetometer first detected small-amplitude circularly polarized magnetosonic waves that further steepened and eventually evolved into periodic shocks. Moreover, differing from the traditional understanding, characterizations of the fast mode waves show that the free energy of the wave/shock generation is provided by newborn protons, and the increasing sunward proton fluxes provided persistent energy for wave steepening. The unusual evidence presents itself from the combination of two circumstances: radial-aligned (Sun-Mars) magnetic fields and Martian atmospheric atom (hydrogen) photoionization and solar wind pickup. These observations lead to the conclusion that newborn ions play a crucial role in the formation process of some CPSs in the astrophysical and space plasma.

International audience; It has been long theorized, but not directly observed, that low-frequency magnetosonic plasma waves can steepen and form shocks. We show an example of small-amplitude, sinusoidal magnetosonic waves at the proton gyrofrequency upstream of the Martian bow shock. We hypothesize that these waves are produced by an ion beam instability associated with the ionization of hydrogen atoms by charge exchange with solar wind protons, solar photoionization, and/or electron impact ionization. As the waves are convected toward the planet by solar wind flow, the wave amplitude grows due to additional free energy put into the system by further ion beam particles. Finally, the steepened waves form shocks. Because of their development, the shocks are periodic with the separation at the proton gyroperiod. These observations lead to the conclusion that newborn ions may play a crucial role in the formation process of some collisionless plasma shocks in astrophysical and space plasmas.

It is found that the coupling between the solar wind and polar geomagnetic activity cause the annual and semiannual variation of auroral electrojet. The wavelet spectrum of the SuperMAG index, SML and SMU shows that both of the semiannual and annual variations are permanent features, and spectral power of the annual period of SML and SMU are stronger than that of the semiannual period. Due to coupling of the annual variation and the semiannual variation of SML, the diurnal variation of SML in summer months is not well correlated with results anticipated from the previous principle hypotheses [McIntosh, 1959; Russell and McPherron, 1973; Svalgaard, 1977; Cliver et al., 2000; Newell et al., 2013]. The annual/semiannual variation of SML and SMU is typically coupled together and is dependent on the local time and latitudinal variations of ionospheric conductivity and the electric field. A new index, the annual-corrected SMO index (SMO*), was suggested to indicate the semiannual variation of the westward auroral electrojet (SML). The seasonal-UT distribution of SMO* has good correlation with that predicted by the equinoctial hypothesis [McIntosh, 1959; Svalgaard, 2011].

Spring-Autumnal asymmetry (SA asymmetry) from years 1873–2010 in the geomagnetic activity is investigated based on the geomagnetic indices (aa, Ap and Kp). Results have shown that in general the SA asymmetry of the geomagnetic activity appears to be alternation, except for the interval during which the change of dominant solar dipole magnetic field polarity occurs. Our study implies that both the polarity of the main solar dipole field and the north-south asymmetry of the sunspot area in the two solar hemispheres control the strength of the south component (Bs) of the interplanetary magnetic field (IMF) near the Earth and the resulting SA asymmetry of the geomagnetic activity in the corresponding seasons. A simple sector structure model is devised to establish the relationship between the polarity of solar main dipole magnetic field, the IMF Bs component and the corresponding SA asymmetry in the geomagnetic activity. This is confirmed by examining the solar wind parameters from the spacecraft observations during the recent decades.

Whistler-mode chorus waves are regarded as an important acceleration mechanism contributing to the formation of relativistic and ultra-relativistic electrons in the Jovian radiation belts. Quantitative determination of the chorus wave driven electron scattering effect in the Jovian magnetosphere requires detailed information of both ambient magnetic field and plasma density and wave spectral property, which however cannot be always readily acquired from observations of existed missions to Jupiter. We therefore perform a comprehensive analysis of the sensitivity of chorus induced electron scattering rates to ambient magnetospheric and wave parameters in the Jovian radiation belts to elaborate to which extent the diffusion coefficients depend on a number of key input parameters. It is found that quasi-linear electron scattering rates by chorus can be strongly affected by the ambient magnetic field intensity, the wave latitudinal coverage, and the peak frequency and bandwidth of the wave spectral distribution in the Jovian magnetosphere, while they only rely slightly on the background plasma density profile and the peak wave normal angle, especially when the wave emissions are confined at lower latitudes. Given the chorus wave amplitude, chorus induced electron scattering rates strongly depend on Jovian L-shell to exhibit a tendency approximately proportional to LJ3. Our comprehensive analysis explicitly demonstrates the importance of reliable information of both the ambient magnetospheric state and wave distribution property to understanding the dynamic electron evolution in the Jovian radiation belts and therefore has implications for future mission planning to explore the extreme particle radiation environment of Jupiter and its satellites.

International audience; The location of ultra-low frequency (ULF) quasi-monochromatic wave onset upstream of Venus bow shock is explored using Venus Express magnetic field data. We report the existence of a spatial foreshock boundary behind which ULF waves are present. We have found that the ULF wave boundary at Venus is sensitive to the interplanetary magnetic field (IMF) direction like the terrestrial one and appears well defined for a cone angle larger than 30o. In the Venusian foreshock, the inclination angle of the wave boundary with respect to the Sun-Venus direction increases with the IMF cone angle. We also found that for the IMF nominal direction (θBX = 36°) at Venus’ orbit, the value of this inclination angle is 70o. Moreover, we have found that the inferred velocity of an ion traveling along the ULF boundary is in a qualitative agreement with a quasi-adiabatic reflection of a portion of the solar wind at the bow shock. For an IMF nominal direction at Venus, the inferred bulk speed of ions traveling along this boundary is 1.07 VSW, sufficiently enough to overcome the solar wind convection. This strongly suggests that the backstreaming ions upstream of the Venusian bow shock provide the main energy source for the ULF waves.

The variations in radiation belt boundaries reflect competition between acceleration and loss physical processes of energetic electrons, which is an important issue for radiation belts of planets with an internal magnetic field (e.g., Earth, Jupiter, and Saturn). Based on high-quality measurements from Van Allen Probes spanning the years 2014–2018, we develop an empirical model of the energy-dependent boundaries of Earth's electron radiation belt slot region, showing that the lower boundary follows a logarithmic function of the electron energy while the upper boundary is controlled by two competing energy-dependent processes, namely compression and recovery. The compression process relates linearly to a 15 hr averaged Kp index, while the recovery process is found to be approximately proportional to time. Detailed data-model comparisons demonstrate that our model, using only the Kp index and time epoch as inputs, reconstructs the slot region boundaries in real time for 200 keV to 2 MeV electrons under varying geomagnetic conditions. Such a data-driven empirical model is prerequisite to understanding the dynamic changes of the slot region in response to both solar and geomagnetic activities. The model can be readily incorporated into future global simulations of radiation belt electron dynamics in Earth's inner magnetosphere and provide new insights into the study of Saturn's and Jupiter's radiation belt variability.

Ion escape to space through the interaction of solar wind and Mars is an important factor influencing the evolution of the Martian atmosphere. The plasma clouds (explosive bulk plasma escape), considered an important ion escaping channel, have been recently identified by spacecraft observations. However, our knowledge about Martian plasma clouds is lacking. Based on the observations of the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we study a sequence of periodic plasma clouds that occurred at low altitudes (∼600 km) on Mars. We find that the heavy ions in these clouds are energy-dispersed and have the same velocity, regardless of species. By tracing such energy-dispersed ions, we find the source of these clouds is located in a low-altitude ionosphere (∼120 km). The average tailward moving flux of ionospheric plasma carried by clouds is on the order of 107 cm−2 s−1, which is one order higher than the average escaping flux for the magnetotail, suggesting explosive ion escape via clouds. Based on the characteristics of clouds, we suggest, similar to the outflow of Earth’s cusp, these clouds might be the product of heating due to solar wind precipitation along the open field lines, which were generated by magnetic reconnection between the interplanetary magnetic field and crustal fields that occurred above the source.

We investigate the occurrence rate of dipolarization fronts (DFs) in the plasma sheet by taking full advantage of all four Cluster satellites (C1–4) from years 2001 to 2009. In total, we select 466 joint-observation DF events, in which 318, 282, 254, and 236 DFs are observed by C1, C2, C3, and C4, respectively. Our findings are as follows: (1) the maximum occurrence rate is ∼ 15.3 events per day at X ∼ 15 RE in the XY plane, and the average occurrence rate is ∼ 5.4 events per day over the whole observation period; (2) the occurrence rate on the dusk side of the plasma sheet is larger and decreases with increasing BXY∕BLobe; (3) the occurrence rate within |Y| RE increases gradually from X ≈ −19 to −15 RE and then decreases from X ≈ −15 to −10 RE; (4) the occurrence rate when AE > 200 nT is much larger than that when AE X ≈ −19 to −15 RE. We suggest that both geomagnetic activity and multiple DFs contribute to the high occurrence rate of the DFs. In addition, the finite length of the DF in the dawn–dusk direction can affect the chance that a satellite observes the DF.

Energetic ion distributions in the near-Earth plasma sheet can provide important information for understanding the entry of ions into the magnetosphere and their transportation, acceleration, and losses in the near-Earth region. In this study, 11 years of energetic proton and oxygen observations (> similar to 274 keV) from Cluster/Research with Adaptive Particle Imaging Detectors were used to statistically study the energetic ion distributions in the near-Earth region. The dawn-dusk asymmetries of the distributions in three different regions (dayside magnetosphere, near-Earth nightside plasma sheet, and tail plasma sheet) are examined in Northern and Southern Hemispheres. The results show that the energetic ion distributions are influenced by the dawn-dusk interplanetary magnetic field (IMF) direction. The enhancement of ion intensity largely correlates with the location of the magnetic reconnection at the magnetopause. The results imply that substorm-related acceleration processes in the magnetotail are not the only source of energetic ions in the dayside and the near-Earth magnetosphere. Energetic ions delivered through reconnection at the magnetopause significantly affect the energetic ion population in the magnetosphere. We also believe that the influence of the dawn-dusk IMF direction should not be neglected in models of the particle population in the magnetosphere.

Bursty Bulk Flows (BBFs) have been correlated with Pi2 pulsations and damping oscillations of plasma velocity in many investigations. But the oscillation time scales in BBFs is still an open question. The purpose of this study is to statistically study the oscillated frequency distribution of magnetic field and plasma parameters inside BBFs. The data are obtained by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes during the period of 2008 to 2011. For 424 selected BBF events, we use the wavelet spectrum analysis to select the main, second and third period components of the magnetic field and plasma parameters according to their largest, second and third-largest values of the wavelet power spectral density, respectively. The power spectra show repeated information in the form of multiple peaks or oscillations. The quasi-Gaussian distribution is a good model for the occurrences of the main and secondary periods. The most probable main periods of the magnetic field and plasma parameters are between 143 s and 160 s, which located in the frequency band of Pi2 and Pi3 pulsations. The second and third period ranges from 63 s to 70 s and from 34 s to 37 s, respectively. Main periods of these parameters change little with the radial distance. We conclude that periods of these parameters are formed at the beginning of BBF history. Although the distribution model cannot give the dynamic processes, it identifies the intrinsic frequencies in oscillations of magnetic field and ion velocity inside of BBFs.

We show observational evidence for a new form of collisionless shock in interplanetary space near Mars, small-scale shocks with periodic spacings. Pickup of new ionized hydrogen atoms in a magnetic field aligned with the solar wind direction causes the generation of a magnetosonic wave train through an ion beam instability. The waves have a frequency close to the local proton gyrofrequency. This is a similar physical process as for the formation of cometary plasma waves/turbulence. However, for the case of proton pickup near Mars, each individual magnetosonic wave cycle develops into a small-scale shock. So there is a string of fast mode shocks formed with proton gyroperiod spacings. These small-scale shocks display dissipation in the ions and dispersive whistlers. A fraction of ions trapped/reflected at the small-scale shocks are accelerated by the motional electric field. Observational results demonstrate that periodic shocks can perform the same functions as a single supercritical shock in a high-speed flow.

The “Nose-like” ion spectral structures are characterized by the deep inward penetration of ions from the near-Earth plasma sheet and can extend to low L shells in the inner magnetosphere. The stationary “Nose-like” ion structures are believed to be driven by the stationary electric and magnetic field during quiet geomagnetic times. However, the global distribution picture, especially for each MLT, for both single and double stationary nose structures is still not reached. In this study, we statistically investigate the stationary “Nose-like” ion spectral structure based on 2 years of Van Allen probes observations. We find a clear MLT dependence of the number of stationary noses, with a higher occurrence of single-nose structures from 13 to 03 MLT, while a higher occurrence of double-nose structures from 03 to 13MLT. This is confirmed by calculating the backward drift time using backward tracing method based on the dipole magnetic field and Weimer 96 electric field model for each MLT. Simulation results indicate that the ion spectral gap, which is formed due to long drift time or existence of the ion forbidden region, controls the nose structure at different MLT during geomagnetic quiet times. A possible physical explanation of this MLT dependence can be a combination of the ion spectral gaps created by the region of long drift time in the dayside within the nose energy range and the persistent dawn-dusk asymmetries in the convection electric field.

International audience; The statistical properties of ULF waves observed upstream of Venus foreshock are investigated. The study is restricted to waves which are observed well below the local proton cyclotron frequency. Using the magnetic field observations from Venus Express between May 2006 and February 2012, 115 quasi-monochromatic ULF wave trains have been identified. Statistical results show that the wave periods are mainly from 20 to 30 s in the spacecraft frame, which is about 2-3 times of the local proton cyclotron period. The transverse power dominates the power spectrum, and most of the waves display nearly circular or slightly elliptical polarization in the spacecraft frame. Moreover, these ULF waves mainly have small relative amplitudes with respect to the ambient field magnitude B0 for parallel component (δB||/B0 less than 0.3), while the range of relative amplitudes for perpendicular component δB⊥/B0 is from ~0.1 to ~1.0. Wave propagation angles are mainly less than 30° with respect to the mean magnetic field direction. The obtained results are very similar to the wave properties seen for ULF waves present in the terrestrial foreshock, which suggests that backstreaming ions in the Venusian foreshock form an important energy source for the generation of the waves.

The solar wind-magnetosphere interaction is still an open problem under the influences of the Bx component of the interplanetary magnetic field (IMF). We have performed a series of 3D global magnetohydrodynamic simulations and demonstrated the quantitative effects of the IMF Bx component on the locations and shapes of the bow shock and magnetopause during northward IMF. The IMF Bx can greatly change the configurations of the bow shock and the magnetosphere, and bring north-south asymmetries in the noon-midnight meridian plane. The larger magnetosonic Mach number in the northern hemisphere makes the northern part of the bow shock closer to the Earth, while the situation is opposite in the south. In the downstream of the southern bow shock, the magnetic fields get greatly compressed in the southern magnetosheath, leading to a drastic increased magnetic pressure above the magnetopause. As a result, the southern part of the magnetopause becomes closer to the earth while its northern part has a little change. Moreover, it is found that the subsolar points of the bow shock and magnetopause are not sensitive to the IMF Bx during northward IMF.

It has been confirmed that dipolarization fronts (DFs) can be a result from the existence of interchange instability in the magnetotail. In this paper, we used a Hall MHD model to simulate the evolution of the interchange instability, which produces DFs on the leading edge. A test particle simulation was performed to study the physical phenomenon of ion acceleration on DF. Numerical simulation indicates that almost all particles move towards the earthward and dawnward and then drift to the tail. The DF-reflected ion population on the duskside appears earlier as a consequence of the asymmetric Hall electric field. Ions, with dawn-dusk asymmetric semicircle behind the DF, may tend to be accelerated to a higher energy (> 13.5 keV). These high-energy particles are eventually concentrated in the dawnside. Ions experience effective acceleration by the dawnward electric field Ey while they drift through the dawn flank of the front towards the tail.

Ultralow frequency (ULF) fluctuations are ubiquitous in the magnetosphere and have significant influence on the energetic particle transport. We use Time History of Events and Macroscale Interactions during Substorms (THEMIS) data to give the spatial distribution of the Pi2/Pc4 and Pc5 band magnetic fluctuation amplitude near the magnetic equator in the magnetosphere. Statistical results can be summarized as follows: (1) strong ULF fluctuations are common in the magnetotail plasma sheet; the amplitude of all three components of magnetic fluctuations decreases with decreasing radial distance; (2) during periods of high AE index, fluctuations can propagate toward the Earth as far as the data cutoff in the nightside of the magnetosphere, and the amplitude of magnetic fluctuations is clearly stronger near the dusk sector of the synchronous orbit than that near the dawn sector, suggesting that the substorm particle injection has significant contribution to these fluctuations; (3) intense compressional Pc5 band magnetic fluctuations are a persistent feature near two flanks of the magnetosphere. Clear peaks of the compressional Pi2/Pc4 band magnetic fluctuation power near two flanks can be found during periods of fast solar wind, while the power of compressional Pi2/Pc4 band fluctuations is weak when the solar wind is slow. (4) Solar wind dynamic pressure and its variations can globally affect the ULF fluctuation power in the magnetosphere. Magnetic fluctuations near the noonside can penetrate from the magnetopause to the synchronous orbit or inner when solar wind pressure variations are large.

We use the Cluster data from 2001 to 2009 to investigate the occurrence of Pi2-band waves in the plasma sheet. To study the generation mechanisms of these waves, we examine the association between Pi2-band waves and dynamic processes in the plasma sheet (fast flows and substorm activity) and the direction of the solar wind velocity. For a total of 80 large-amplitude Pi2-band waves in the plasma sheet, we find that Cluster records fast flows during 62 events, 11 waves without fast flows occur during substorm time, 3 events occur when the solar wind velocity significantly changes its direction, and 4 events are not associated with any of the above activities. Most of the observed Pi2-band waves are predominantly compressional, while 2 events are transverse. Based on this statistical study, we suggest that fast flows maybe the main driver of Pi2-band waves/oscillations in the plasma sheet, especially considering that most of these waves are compressional. The relatively small number of other events indicates that other mechanisms also play a role in creating Pi2-band waves/oscillations in the plasma sheet but are relatively rare. In all wave events of this study, the plasma pressure and magnetic pressure vary in antiphase, suggesting that these waves have the slow-mode feature.

We present a newly developed global magnetohydrodynamic (MHD) model to study the responses of the Earth’s magnetosphere to the solar wind. The model is established by using the space-time conservation element and solution element (CESE) method in general curvilinear coordinates on a six-component grid system. As a preliminary study, this paper is to present the model’s numerical results of the quasi-steady state and the dynamics of the Earth’s magnetosphere under steady solar wind flow with due northward interplanetary magnetic field (IMF). The model results are found to be in good agreement with those published by other numerical magnetospheric models.

In this paper, the space-time conservation element and solution element (CESE) method in general curvilinear coordinates is successfully applied to the three-dimensional magnetohydrodynamic (MHD) simulations of the interaction between the solar wind and Saturn's magnetosphere on a six-component grid system. As a new numerical model modified for the study of the interaction between the solar wind and Saturn's magnetosphere, we obtain the large-scale configurations of Saturn's magnetosphere under the steady solar wind with due southward interplanetary magnetic field (IMF) conditions. The numerical results clearly indicate that the global structure of Saturn's magnetosphere is strongly affected by the rotation of Saturn as well as by the solar wind. The subsolar standoff distances of the magnetopause and the bow shock in our model are consistent with those predicted by the data-based empirical models. Our MHD results also show that a plasmoid forms in the magnetotail under the effect of the fast planetary rotation. However, somewhat differently from the previous models, we find that there are two flow vortices generated on the duskside under due southward IMF at Saturn. On the duskside, the clockwise one closer to the planet is excited by the velocity shear between the rotational flows and the sunward flows, while the anticlockwise one is generated from the velocity shear between the tailward flows along the magnetopause and the sunward flows.