Global Hybrid Simulation Research Articles

Mercury’s magnetosphere–solar wind interaction for northward and southward interplanetary magnetic field: Hybrid simulation results

Analysis of global hybrid simulations of Mercury’s magnetosphere–solar wind interaction is presented for northward and southward interplanetary magnetic field (IMF) orientations in the context of MESSENGER’s first two encounters with Mercury. The global kinetic simulations reveal the basic structure of this interaction, including a bow shock, ion foreshock, magnetosheath, cusp regions, magnetopause, and a closed ion ring belt formed around the planet within the magnetosphere. The two different IMF orientations induce different locations of ion foreshock and different magnetospheric properties: the dayside magnetosphere is smaller and cusps are at lower latitudes for southward IMF compared to northward IMF whereas for southward IMF the nightside magnetosphere is larger and exhibits a thin current sheet with signatures of magnetic reconnection and plasmoid formation. For the two IMF orientations the ion foreshock and quasi-parallel magnetosheath manifest ion-beam-driven large-amplitude oscillations, whereas the quasi-perpendicular magnetosheath shows ion-temperature-anisotropy-driven wave activity. The ions in Mercury’s belt remain quasi-trapped for a limited time before they are either absorbed by Mercury’s surface or escape from the magnetosphere. The simulation results are compared with MESSENGER’s observations.

Global hybrid simulation of magnetic reconnection in the magnetosheath

A three-dimensional (3-D) global hybrid simulation is carried out for the generation and structure of magnetic reconnection in the magnetosheath due to interaction of an interplanetary Tangential Discontinuity (TD) with the bow shock and magnetosphere. Runs are performed for solar wind TDs possessing different initial half-widths. As the TD propagates through the bow shock toward the magnetopause, it is greatly narrowed by a two-step compression processes, a "shock compression'' followed by a subsequent ``convective compression''. In cases with a relatively thin solar wind TD, 3-D patchy reconnection is initiated in the transmitted TD, forming magnetosheath flux ropes. Multiple components of ion particles are present in the velocity distribution in the magnetosheath merging, accompanied by ion heating. For cases with a relatively wide initial TD, a dominant single X-line appears in the subsolar magnetosheath after the transmitted TD is narrowed. A shock analysis is performed for the detailed structure of magnetic reconnection in the magnetosheath. Rotational Discontinuity (RD)/Time-Dependent Intermediate Shock (TDIS) are found to dominate the reconnection layer, which and some weak slow shocks are responsible for the ion heating and acceleration.

Foreshock compressional boundary

We employ 2.5‐D electromagnetic, hybrid simulations that treat ions kinetically via particle‐in‐cell methods and electrons as a massless fluid to study the formation and properties of a newly discovered boundary named the foreshock compressional boundary (FCB). This boundary forms in the ion foreshock and is associated with enhanced densities and magnetic field strengths. At times, but not always, the FCB separates the pristine solar wind plasma from the ion foreshock. In this study, we investigate the dependence of FCB characteristics on solar wind Mach number and cone angle (the angle between flow velocity and interplanetary magnetic field). We show that the strength of the foreshock compressional boundary increases with the Mach number. This enhancement is in turn tied to the density and velocity of the backstreaming ions in the foreshock whose interaction with the solar wind results in ULF turbulence which is ultimately responsible for the formation of FCB. During small cone angles the foreshock compressional boundary is symmetric with respect to the radial direction. As the cone angle increases, the FCB becomes less symmetric and eventually is confined to one side of the foreshock. The strength of the FCB also decreases with increasing cone angle but depending on the Mach number can exist for cone angles of 40° and beyond. A recent study that compared data from a global hybrid simulation of the foreshock with Cluster spacecraft observations showed that encounters with foreshock cavities can be interpreted as back and forth motion of a FCB causing spacecraft to move from the solar wind through the FCB into the foreshock and back into solar wind. An example of a FCB observed by the Cluster spacecraft is presented and shown to be in general agreement with model predictions.

Hybrid simulation of foreshock waves and ion spectra and their linkage to cusp energetic ions

A three‐dimensional global hybrid simulation is carried out to investigate energetic ions and electromagnetic waves in the quasi‐parallel (Q‐∥) bow shock and cusp for a typical interplanetary magnetic field configuration during the cusp energetic particle events. The bow shock, magnetosheath, and dayside magnetosphere form by interaction between the solar wind and geomagnetic dipole field. Ions of solar wind characteristics injected into the system evolve along with electromagnetic waves in a self‐consistent manner. Several important features are yielded from the simulation. Solar wind ions are accelerated by the waves and turbulence at the bow shock and foreshock as indicated by the ion distribution. The shock‐accelerated ions possess an exponential energy spectrum with their differential flux scaled by the field‐aligned distance to the bow shock, consistent with satellite observations. Second, the compressive and transversal waves in the foreshock are strongly correlated with the diffuse ion dynamics. Third, energetic ions in the magnetosheath and cusp downstream from the Q‐∥ shock also exhibit a spectrum similar to those at the shock. By tracing trajectories of cusp energetic ions in the simulation, the origin of these ions is revealed. Their source is predominantly associated with the Fermi acceleration at the Q‐∥ shock and foreshock. Waves in the magnetosheath and cusp, as well as magnetic reconnection at the high‐latitude magnetopause, are relatively insignificant in accelerating cusp energetic particles.

Global hybrid simulations: Foreshock waves and cavitons under radial interplanetary magnetic field geometry

We use global hybrid (kinetic ions and fluid electrons) simulations to study the solar wind interaction with the magnetosphere for a radial (θvB = 0) interplanetary magnetic field (IMF) geometry. Global hybrid simulations provide a collective picture of processes taking place in the foreshock, bow shock, and magnetosheath. Because ions are treated as particles, these codes also give information on ion‐scale microphysics. Under radial IMF geometry, the foreshock forms in front of the dayside magnetosphere, and the plasma convecting downstream is very perturbed. The foreshock is permeated by weakly compressive ultralow frequency (ULF) waves that propagate at angles up to 30° to the ambient field. These weakly compressive waves are generated by field‐aligned ions. Wave‐Particle interaction results in ion scattering, while wave amplitude grows and fluctuations become compressive as they approach the shock. While weakly compressive waves are dominant far from the shock, a second population of ULF fluctuations arises close to it. These fast magnetosonic waves propagate at large angles to the magnetic field and are linearly polarized. We find that, in addition to the ULF waves, large depressions in density and magnetic field magnitude form near the shock. These density cavitons or depressions are bounded by enhanced magnetic fields, and inside them, hot diffuse ions are present. Foreshock density cavitons in our simulations are embedded in regions with ULF activity, which is in contrast to the isolated character of “foreshock cavities” reported previously. We show that density cavitons form due to the nonlinear interaction of the two types of waves present in the foreshock. Wave interaction also modifies the characteristics of weakly compressive waves. A comparison of our results with Cluster observations reveals that the characteristics of weakly compressive waves in our simulation resemble the properties of right‐handed 30‐s waves found in the terrestrial foreshock. Likewise, we show the existence of density cavitons, or depressions, in regions of Earth's foreshock permeated by ULF waves. The properties of these observed Cluster cavitons are similar to the ones we find in the simulations.

Connection between bow shock and cusp energetic ions

[1] A linkage between energetic ions in the bow shock and the cusp is established using a three-dimensional (3-D) self-consistent global hybrid simulation, for a case of the interplanetary magnetic field (IMF) condition favoring high-latitude magnetic reconnection and inter-connection between field lines in the quasi-parallel bow shock and the cusp. A diffuse ion distribution is obtained in the quasi-parallel shock and foreshock region, where large-amplitude waves and turbulence are present. Its energy spectrum is well described by an exponential function as in the case for upstream diffuse ions previously observed. These bow shock accelerated ions, upon being transmitted into the cusp region, form the bulk of cusp energetic ions. By tracing particles from the solar wind to the cusp, it is found that the source of acceleration for the cusp energetic ions is predominantly in the quasi-parallel shock and foreshock region, through a Fermi-type acceleration mechanism. Contribution to the cusp energetic ions from magnetosheath/cusp waves and magnetic reconnection appears relatively minor. Owing to the limitation of the model used, the magnetospheric source for energetic particles in the cusp cannot be fully investigated.

Ion dynamics associated with Alfven wave in the near-Earth magnetotail: Two-dimensional global hybrid simulation

Ion dynamics in the near-Earth magnetotail region is examined during periods of fast Earthward flow with a two-dimensional (2-D) global-scale hybrid simulation. The simulation shows that shear Alfven waves are generated at x ∼ −10 R E, where the strong earthward flow is arrested by the dipole field, and propagate along field lines from the equator to both southern and northern polar ionosphere. Non-gyrotropic ion velocity distributions occur where the large-amplitude Alfven waves are dominant. The simulation indicates that the Alfven waves are generated by interaction of the fast earthward flow with the stationary near-Earth plasma. Beam ions are found to be pitch-angle scattered and trapped in the wave field, leading to the non-gyrotropic ion distributions in the high-latitude plasma sheet boundary. In addition, significant particle heating and acceleration are found to occur behind the dipolarization front due to the effect of wave turbulence.

Structure of Mercury's magnetosphere for different pressure of the solar wind: Three dimensional hybrid simulations

We have carried out a self‐consistent three dimensional global hybrid simulation study examining the interaction of the solar wind with Mercury's magnetosphere. We consider two cases: one with relatively high solar wind pressure, and another with relatively low solar wind pressure. With lower solar wind pressure, the subsolar magnetopause forms at about 1.7 RM (where RM is the planetary radius) and well pronounced cusp regions are formed; also a closed ion ring forms around the planet. In the higher solar wind pressure case the magnetopause is pushed closer to Mercury's surface (∼1.2 RM) and the cusp regions are less pronounced; the ion ring in this case is confined to a region closer to the planet with a smaller radial extent. In both cases reconnection occurs at about ∼2.4 RM down the magnetotail and a plasma sheet is formed. In general the plasma within the magnetosphere is more energetic in the high solar wind pressure case and the ion foreshock contains hotter magnetosheath plasma. Particles originating from the planet disperse through the magnetosphere, with the greatest congregation occurring in the inner magnetospheric drift driven rings in both cases. These planetary particles can also leak upstream into the foreshock region.

Flux transfer events in the cusp

We use 2.5‐dimensional global hybrid (kinetic ions, fluid electrons) simulations to study the evolution of Flux Transfer Events (FTEs) and their interaction with the cusp during their poleward motion. A southward pointing interplanetary magnetic field is considered. It results in low‐latitude, time‐dependent reconnection and the formation of multiple FTEs with a variety of sizes ranging from a few hundred to thousands of km. FTEs are found to travel along the magnetopause surface with velocities exceeding the surrounding magnetosheath by ∼1 local Alfven speed. As a result, a slow mode bow wave and a pile up of plasma and magnetic field precede larger, but not smaller, FTEs. As an FTE encounters the cusp, a secondary magnetic reconnection is initiated which ultimately leads to its disintegration as it moves to higher latitudes. Both the slow mode bow wave and reconnection accelerate plasma into the cusp and enhance the flux of energetic ions in this region. The downward flux of ions at a given energy moves poleward in time, exhibiting characteristics similar to the poleward moving auroral forms previously attributed to FTEs.

Formation of hot flow anomalies and solitary shocks

Interaction of a tangential discontinuity (TD) with the bow shock is investigated by using electromagnetic, global hybrid simulations in which ions are treated kinetically via particle‐in‐cell methods and electrons form a massless fluid. On the basis of previous studies, it was expected that the interaction would result in the formation of a hot flow anomaly (HFA) propagating along the curved bow shock surface. The results are unexpected in two major ways. First, the hot flow anomaly is only formed during the interaction of the TD with the quasi‐parallel side of the bow shock. The lack of a HFA on the perpendicular side is due to the inability of a large fraction of ions to escape into the solar wind, as is required for an HFA to form. Second, the interaction of the TD with the quasi‐perpendicular portion of the bow shock results in a previously unknown, shock structure which we name the “solitary shock.” The solitary shock consists of a finite width (a few ion inertial length), fast magnetosonic shock‐like structure followed by a relatively less compressed, more turbulent plasma with complex and spatially varying properties in the downstream region. We have determined that the formation of the solitary shock after the passage of the TD is due to the new direction of the interplanetary magnetic field. Further, this is not a transitory phenomena and when the interplanetary magnetic field cone angle is large (∼>50°) a significant portion of the bow shock surface is affected. Solitary shocks form in the regions where the motional electric field in the magnetosheath points away from the shock. We demonstrate that solitary shocks differ from regular quasi‐perpendicular shocks due to differences in ion dissipation processes. We also present the results of a detailed survey of the effects of simulation parameters such as cell size, resistivity, system size, and 2.5‐dimensional versus three‐dimensional domains on the solitary shock solutions.

Formation of dayside low-latitude boundary layer under northward interplanetary magnetic field

[1] A three-dimensional (3-D) global hybrid simulation is carried out to study the formation of the dayside low-latitude boundary layer (LLBL) under a purely northward interplanetary magnetic field (IMF). Magnetic reconnection in both northern and southern hemispheres leads to a continued formation of newly closed field lines on the dayside, and a subsequent formation of the LLBL by capture of the magnetosheath ions on the original magnetosheath field lines, as the newly closed field lines shorten. The formation of the LLBL is associated with the tailward spreading of the transmitted ions along the magnetopause as the newly closed flux tubes convect tailward. Meanwhile, compressional waves and transverse waves in magnetic field are generated in the magnetosphere. The ion distributions in the cusp and the boundary layer show the presence of multiple ion beams of the transmitted magnetosheath ions, as well as ion heating.

Macrostructure of collisionless bow shocks: 2. ULF waves in the foreshock and magnetosheath

We perform global hybrid (kinetic ions and fluid electrons) simulations of the solar wind coupling with magnetic dipoles of different strength and study foreshock morphology and its influence on the bow shock and magnetosheath. The advantage of the hybrid code over fluid codes is that it treats ion‐scale microphysics in the context of the global interaction. We study the characteristics of the ultralow‐frequency (ULF) waves generated by kinetic instabilities and find two types of waves: (1) sinusoidal almost parallel propagating waves and (2) highly compressive obliquely propagating fluctuations. In contrast to suggestions made in the past, we show that compressive waves found near the shock do not evolve from the sinusoidal wave population but rather are generated by a different ion population. We find that the sinusoidal waves are generated by field‐aligned backstreaming ions, while the compressive waves near the shock are generated by gyrating ion beams closer to the shock. Our results show that, for low dipole magnetization, noncompressive waves dominate the foreshock, while compressive waves grow just in a very small region and do not have time to evolve, so no quasi‐parallel shock is formed. In contrast, when the system scale size is much larger than an ion inertial length, highly compressive waves convect into the quasi‐parallel region, evolving into large steepened structures that eventually form the shock transition. These waves play an active role in shock dissipation processes and eventually form part of the wave spectra in the magnetosheath. Comparison of our results with observations shows that the characteristics of sinusoidal waves in our simulations resemble the properties of 30‐s sinusoidal quasi‐monochromatic waves in the Earth's foreshock, while compressive waves have similar properties to the observed right‐handed steepened fluctuations. Downstream from the shock, the magnetosheath is permeated by a variety of waves that result from the convection of upstream waves and also from local generation. The wave characteristics are different in the quasi‐parallel and quasi‐perpendicular parts of the magnetosheath.

Global hybrid simulations of energetic particle effects on the n=1 mode in tokamaks: Internal kink and fishbone instability

Global hybrid simulations of energetic particle effects on the n=1 internal kink mode have been carried out for tokamaks. For the International Thermonuclear Experimental Reactor (ITER) [ITER Physics Basis Editors et al., Nucl. Fusion 39, 2137 (1999)], it is shown that alpha particle effects are stabilizing for the internal kink mode. However, the elongation of ITER reduces the stabilization effects significantly. Nonlinear simulations of the precessional drift fishbone instability for circular tokamak plasmas show that the mode saturates due to flattening of the particle distribution function near the resonance region. The mode frequency chirps down rapidly as the flattening region expands radially outward. Fluid nonlinearity reduces the saturation level.

Global hybrid simulations of solar wind interaction with Mercury: Magnetosperic boundaries

The Mariner 10 observations established the presence of a magnetosphere at Mercury with general characteristics similar to that of the Earth despite a much smaller size (i.e., magnetic dipole strength) and the absence of an atmosphere–ionosphere. Observationally, a mapping between the terrestrial and hermean magnetospheric regions has been made which entails increasing the size of the planet by a factor of 8. Recently, we have used the results of global hybrid (kinetic ions, fluid electrons) simulations of solar wind interaction with magnetic dipoles of varying strength to show the existence of a spectrum of magnetospheric structures whose size and level of complexity increases with the dipole moment. A physical parameter which helps characterize these magnetospheres is D p, the distance ahead of the dipole where the magnetic field pressure balances the solar wind dynamic pressure. Expressed in units of ion skin depth, when D p ∼ > 20 the interaction results in the formation of an earthlike magnetosphere. Thus, the magnetosphere of Mercury with D p ∼ 65 should resemble that of the Earth. However, not all magnetospheric processes in D p > 20 magnetospheres are guaranteed to be alike and possess the same spatial and temporal scales. In this paper, we use the results of global hybrid simulations, corresponding to D p = 64, to examine the nature of the boundaries and discontinuities at the hermean magnetosphere and how they compare with the corresponding regions at the Earth.

ULF waves and their influence on bow shock and magnetosheath structures

We perform global hybrid (kinetic ions, fluid electrons) simulations of the solar wind interaction with a magnetized planet to study solar wind coupling for different dipolar field strengths and a variety of interplanetary magnetic field (IMF) geometries. The advantage of the hybrid code over fluid codes is that it treats ion-scale microphysics in the context of the global interaction. The shock and magnetosheath macrostructures are directly tied to ion kinetic processes and microinstabilities that generate ultralow frequency (ULF) waves in the foreshock and in the magnetosheath. We find that the quasi-perpendicular shock can form for obstacles with low magnetization and small size (relative to ion scales). In contrast, the quasi-parallel shock only forms when the scale size of the system is much larger than the ion inertial length, and depends on the generation of compressive waves. The foreshock is permeated by noncompressive waves generated by backstreaming beam ions, and by compressive fluctuations generated by beam-ring ion distributions. As compressive waves convect into the quasi-parallel region they evolve into large structures and form the shock transition. We find that variations of IMF geometry have a deep influence on magnetosheath structures and on the way that the plasma is heated and compressed.

Hybrid simulation study of ion escape at Titan for different orbital positions

Titan, the largest moon of Saturn, forms a unique plasma interaction with the Kronian corotating plasma. Titan’s dense and nitrogen rich atmosphere is the primary source of the nitrogen torus around Titan’s orbit through the subsonic and super-Alfvénic interaction between the corotating plasma of the Kronian system and the exosphere and ionosphere of Titan. We have studied this magnetic interaction of Titan with a global hybrid simulation model and especially the effect of the orbital position of Titan with four different Saturn local time. The hybrid simulation includes the various drifts important in Titan’s plasma environment. Under the assumption of uniform magnetospheric properties along Titan’s orbit, differences were found in the tail structure while the net emission rates varied only little. For all four studied orbital positions the magnetic field maximum was located on the anti-Saturn side of Titan. The iono- and magnetotails were co-aligned and tilted towards Saturn up to 45°.

Three‐dimensional global hybrid simulation of dayside dynamics associated with the quasi‐parallel bow shock

A three‐dimensional global‐scale hybrid simulation is carried out, for the first time, for dynamics of the dayside bow shock‐magnetosphere system associated with the quasi‐parallel bow shock. A case with IMF along the Sun‐Earth line is examined in detail. First, the foreshock waves and the associated shock reformation process are investigated. In particular, the generation and structure of diamagnetic cavities, with a decrease in the magnetic field and density, in the foreshock of the quasi‐parallel shock are discussed. Second, the interaction of the foreshock‐originated pressure pulses with the dayside magnetosphere is simulated. The diamagnetic cavities that are generated in the turbulent foreshock due to the ion beam plasma interaction are found to lead to strong surface perturbations at the magnetopause. Third, the coupling between the pressure pulses and the magnetosphere is studied. The compressional waves are found to mode convert to shear Alfvén waves and kinetic Alfvén waves through the Alfvén resonance process in nonuniform plasmas. The shear Alfvén waves lead to field line resonance, which corresponds to the fundamental odd resonance wave number, and produce field‐aligned currents in the dipole magnetospheric field.

Influence of the solar EUV flux on the Martian plasma environment

Abstract. The interaction of the solar wind with the Martian atmosphere and ionosphere is investigated by using three-dimensional, global and multi-species hybrid simulations. In the present work we focus on the influence of the solar EUV flux on the Martian plasma environment by comparing simulations done for conditions representative of the extrema of the solar cycle. The dynamics of four ionic species (H+, He++, O+, O2+), originating either from the solar wind or from the planetary plasma, is treated fully kinetically in the simulation model in order to characterize the distribution of each component of the plasma, both at solar maximum and at solar minimum. The solar EUV flux controls the ionization frequencies of the exospheric species, atomic hydrogen and oxygen, as well as the density, the temperature, and thus the extension of the exosphere. Ionization by photons and by electron impacts, and the main charge exchange reactions are self-consistently included in the simulation model. Simulation results are in reasonable agreement with the observations made by Phobos-2 and Mars Global Surveyor (MGS) spacecraft: 1) the interaction creates a cavity, void of solar wind ions (H+, He++), which depends weakly upon the phase of the solar cycle, 2) the motional electric field of the solar wind flow creates strong asymmetries in the Martian environment, 3) the spatial distribution of the different components of the planetary plasma depends strongly upon the phase of the solar cycle. The fluxes of the escaping planetary ions are computed from the simulated data and results for solar maximum are compared with estimates based on the measurements made by experiments ASPERA and TAUS on board Phobos-2.

Global hybrid model of the solar wind interaction with the Venus ionosphere: ion escape processes

Comprehensive two-dimensional (2D) global hybrid (particle ions, massless fluid electrons) simulations of the solar wind interaction with the ionosphere of Venus are conducted. We calculate the entire Venus–solar wind interaction region including the ionosphere, ionopause transition layer, magnetosheath, and solar wind regions, by applying boundary-fitted coordinates to the particle-in-cell code. Our model is successful in reproducing several observed features, such as long streamers of ionospheric plasma and detached plasma clouds. In this paper, we investigate the processes in which planetary ions are removed by the solar wind interaction using the comprehensive global hybrid model. While the ion escape process due to the pickup of exospheric particles has been intensively studied by many authors, possible ion loss processes that take place at and below the ionopause, such as viscous process associated with the Kelvin–Helmholtz (K–H) instability, are less well understood. Hence, in this paper, the escape processes are investigated with a particular emphasis placed on the processes occurring at the ionopause. We discuss the relative importance of the escape processes for the case of low solar wind dynamic pressure (unmagnetized ionosphere condition) as well as for the high dynamic pressure case (magnetized ionosphere condition), and show the viscous removal process occurring at the ionopause plays a significant role in the ion escape from Venus. The model also suggests the asymmetrical appearance of “disappearing nightside ionosphere”.

B143 3次元可視化とバーチャルリアリティによる地球磁気圏研究

In the present study, we construct the 3-D data analysis environment for space plasma simulations such as Global MHD simulations, hybrid simulations and full particle simulations. This analysis environment works on a 3-D visualization tool, Application Visualization System; AVS/Express. It provides Earth and Space researchers with research environment to achieve new scientific findings.