Ion-acoustic waves in non-Maxwellian magnetospheric electron-positron-ion plasma

Using Boltzmann-Vlasov kinetic model for nonthermal distributed electron-positron-ion plasma of our Earth’s magnetosphere and the solar wind streaming plasma can drive ion-acoustic waves unstable. It is found that the growth rate increases with the decrease of spectral index and increases with the streaming velocity of the solar wind. The numerical results are also presented by choosing some suitable parameters of magnetospheric plasma.     

太阳风电子动力学不稳定性

电子是太阳风粒子中最为重要的组分之一,它可以通过多种机制对太阳风产生影响.太阳风中的电子通常具有温度各向异性和束流两种非热平衡分布特征,这些偏离热平衡分布的特征可以通过波粒相互作用激发电子不稳定性和等离子体波动,激发的等离子体波动又可以通过波粒相互作用调制太阳风粒子的分布,从而加热太阳风中的背景粒子.因此电子动力学不稳定性在太阳风的演化过程中扮演了极为重要的角色.详细介绍了太阳风中常见的电子动力学不稳定性,并基于等离子体动力论,详细介绍太阳风传播过程中所出现的各种不稳定性,尤其是在近日球层和太阳大气区域所出现的电子声热流不稳定性以及低混杂热流不稳定性,并分析其波粒相互作用机制,以便更加深入地研究太阳风传播过程中的电子分布函数演化.     

Distributions of tangential discontinuity in the corotating solar wind structure

Distributions of the tangential discontinuity (TD) in the solar wind sector structure are investigated on the basis of the magnetic field data and the ion plasma parameters from the Explorer 33 satellite from 23 January to 23 March 1968. The TD is separated from the observed field fluctuations by calculating the direction of the plasma flow and also the direction of the minimum field fluctuation with respect to the ambient magnetic field direction.It is found that the TD is formed by the thin layered field-aligned currents (the current sheets), and that the TD is predominantly built up in the leading edge of the solar wind where the compression of the plasma and the magnetic field takes place.It is suggested that the current sheets might be locally generated in the leading edge in the turbulent conditions arising from collisions between the fast- and the slow-stream of the solar sector structure.     

Solar wind-magnetosphere interaction as simulated by a 3-D em particle code

We have studied the solar wind-magnetosphere interaction using a 3-D electromagnetic particle code. The results for an unmagnetized solar wind plasma streaming past a dipole magnetic field show the formation of a magnetopause and a magnetotail, the penetration of energetic particles into cusps and radiation belt and dawn-dusk asymmetries. The effects of interplanetary magnetic field (IMF) have been investigated in a similar way as done by MHD simulations. The simulation results with a southward IMF show the shrunk magnetosphere with great particle entry into the cusps and nightside magnetosphere. This is a signature of a magnetic reconnection at the dayside magnetopause. After a quasi-stable state is established with an unmagnetized solar wind we switched on a solar wind with an northward IMF. In this case the significant changes take place in the magnetotail. The waving motion was seen in the magnetotail and its length was shortened. This phenomena are consistent with the reconnections which occur at the high latitude magnetopause. In our simulations kinetic effects will determine the self-consistent anomalous resistivity in the magnetopause that causes reconnections.Deceased January 24, 1993; R. Bunemanet al. 1993.     

A new numerical method for simulating the solar wind Alfvén waves: Development of the Vlasov-MHD model

A novel scheme of plasma simulation particularly suited for computing the one-dimensional nonlinear evolution of parallel propagating solar wind Alfvén waves is presented. The scheme is based on the Vlasov and the MHD models, for solving the longitudinal and the transverse components, respectively. As long as the nonlinearity is not very large (so that the longitudinal and transverse components are well separated), our Vlasov-MHD model can correctly describe evolution of finite amplitude parallel Alfvén waves, which are typical in the solar wind, both in the linear and nonlinear stages. The present model can be applied to discussions of phenomena where the parallel Alfvén waves play major roles, for example, the solar coronal heating and solar wind acceleration by the Alfvén waves propagating from the photosphere.     

Helical waves in type-1 comet tails

Oscillations of type-1 comet tails with plasma compressibility taken into account are studied. A comet tail is treated as a plasma cylinder separated by a tangential discontinuity surface from the solar wind. The dispersion equation obtained in the linear approximation is solved numerically with typical plasma parameters. A sufficient condition for instability of the cylindrical tangential discontinuity in the compressible fluid is obtained. The phase velocity of helical waves is shown to be approximately coincident with Alfvén speed in the tail in the reference system moving with the bulk velocity of the plasma outflow in the tail. The instability growth rate is calculated.This theory is shown to be in good agreement with observations in the tails of Comets Kohoutek, Morehouse and Arend-Roland. Hence we conclude that helical waves observed in type-1 comet tails are produced due to the Kelvin-Helmholtz instability, and the model under consideration is justified. If so, one may estimate comet tail magnetic field from the pressure balance at the tangential discontinuity; it turns out to be of the order of the interplanetary magnetic field.     

A global 3-D simulation of interplanetary dynamics in June 1991

The global dynamics of the solar wind and interplanetary magnetic field in June 1991 is simulated based on a fully three-dimensional, time-dependent numerical MHD model. The numerical simulation includes eight transient disturbances associated with the major solar flares of June 1991. The unique features of the present simulation are: (i) the disturbances are originated at the coronal base (1R _s) and their propagation through inhomogeneous ambient solar wind is simulated out to 1.5 AU; (ii) as a background for the transients, the global steady-state solar wind structure inferred from the 3-D steady-state model (Usmanov, 1993c) is used. The parameters of the initial pulses are prescribed in terms of the near-Sun shock velocities (as inferred from the metric Type II radio burst observations) relative to the preshock steady-state flow parameters at the flare sites. The computed parameters at the Earth's location for the period 1–18 June, 1991 are compared with the available observations of the interplanetary magnetic field, solar wind velocity, density, and with variation of the geomagnetic activityK _pindex.     

Preferred bartels days of high-speed plasma streams in the solar wind

An analysis of 346 high-speed solar wind streams observed at 1 AU during 1964–75 is presented. The analysis shows that a two-sector structure was the dominant feature of the interplanetary magnetic field associated with the high-speed solar wind plasma. The high-speed streams occurred at preferred Bartels days: Positive polarity streams were most frequent near Bartels day 4, negative polarity streams were most frequent near Bartels day 17. Since the solar wind carries with it the photospheric magnetic polarity of the solar source region, the observed distribution of Bartels days must indicate a fundamental property of the distribution of the solar sources of high-speed plasma streams. The observations are explained in terms of a tilted dipole model of the solar-interplanetary field.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.     

Comparative investigation of the terrestrial and Venusian magnetopause: Kinetic modeling and experimental observations by Cluster and Venus Express

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.     

Deflection of coronal mass ejection in the interplanetary medium

A solar coronal mass ejection (CME) is a large-scale eruption of plasma and magnetic fields from the Sun. It is believed to be the main source of strong interplanetary disturbances that may cause intense geomagnetic storms. However, not all front-side halo CMEs can encounter the Earth and produce geomagnetic storms. The longitude distribution of the Earth-encountered front-side halo CMEs (EFHCMEs) has not only an east–west (E–W) asymmetry  (Wang et al., 2002), but also depends on the EFHCMEs' transit speeds from the Sun to 1 AU. The faster the EFHCMEs are, the more westward does their distribution shift, and as a whole, the distribution shifts to the west. Combining the observational results and a simple kinetic analysis, we believe that such E–W asymmetry appearing in the source longitude distribution is due to the deflection of CMEs' propagation in the interplanetary medium. Under the effect of the Parker spiral magnetic field, a fast CME will be blocked by the background solar wind ahead and deflected to the east, whereas a slow CME will be pushed by the following background solar wind and deflected to the west. The deflection angle may be estimated according to the CMEs' transit speed by using a kinetic model. It is shown that slow CMEs can be deflected more easily than fast ones. This is consistent with the observational results obtained by Zhang et al. (2003), that all four Earth-encountered limb CMEs originated from the east. On the other hand, since the most of the EFHCMEs are fast events, the range of the longitude distribution given by the theoretical model is E40°,W70°, which is well consistent with the observational results (E40°,W75°).     

Three-dimensional computer modeling of dynamic reconnection in the magnetotail

Magnetospheric substorm in the magnetotail region is studied numerically by means of a three-dimensional MHD code. The analytic solution for the quiet magnetotail is emloyed as an initial configuration. The localized solar wind is modeled to enter the simulation domain through the boundaries located in the magnetotail lobe region. As a result of the interaction between the solar wind and the magnetosphere, the magnetic field lines are stretched, and the plasma sheet becomes thinner and thinner. When the current-driven resistivity is generated, magnetic reconnection is triggered by this resistivity. The resulting plasma jetting is found to be super-magnetosonic. Although the plasmoid formation and its tailward motion is not quite clear as in the two-dimensional simulation, which is mainly because of the numerical model chosen for the present simulation, the rarification of the plasmas near thex-point is observed. Field-aligned currents are observed in the late expansive stage of the magnetospheric substorm. These field-aligned currents flow from the tail toward the ionosphere on the dawn side and from the ionosphere toward the tail on the dusk side, namely in the same sence of the region 1 current. As the field-aligned currents develop, it is found that the cross-tail current in the Earthside midnight section of the magneticx-point is reduced.     

A simulation study of the solar wind including the solar rotation effect

An axisymmetric solar wind structure including the solar rotation effect is studied by the method of MHD computer simulation. For the case of the radial magnetic field configuration, the simulation result is fairly well coincident with the steady-state solution. For the case of the dipole magnetic field configuration, the properties of the solution depend on the ratio of the gas pressure to the magnetic pressure-ratio) in the model. If the-ratio is small, a clearly defined stagnation region appears in the wind, in which the flow speed is very small and the azimuthal magnetic field is very weak because of the corotation of the plasma. If the-ratio is greater than 1, the plasma is not effectively trapped by the magnetic field so that the stagnation region is not clearly defined in the solution.     

Features of the Martian Magnetic Field Structure

Based on the single-fluid MHD model of Mars space simulation, this paper has studied the magnetic field structure in the near-Mars space and investigated the influence of Martian crustal magnetic anomalies on the magnetic field structure. In the process of the solar wind interaction with Mars, the bow shock and magnetic pile-up region are produced. The interplanetary magnetic lines are curved and deformed while they are towed toward the two poles by the solar wind. The majority of magnetic lines bypass the two poles, then leave behind a ‘V-shaped’ structure in the magnetotail behind Mars. In the crust of Mars, the local magnetic anomalies have a noticeable influence on the magnetic field structure. The magnetic anomalies at different positions and in different intensities interact with the solar wind to form the mini-magnetospheres of different structures and morphologies, such as the towed mini-magnetosphere and the mini-magnetosphere with open magnetic lines. The local magnetic anomalies have changed the near-Mars magnetic field structure, and probably changed the plasma distribution as well.     

Interfacing MHD Single Fluid and Kinetic Exospheric Solar Wind Models and Comparing Their Energetics

An exospheric kinetic solar wind model is interfaced with an observation-driven single-fluid magnetohydrodynamic (MHD) model. Initially, a photospheric magnetogram serves as observational input in the fluid approach to extrapolate the heliospheric magnetic field. Then semi-empirical coronal models are used for estimating the plasma characteristics up to a heliocentric distance of 0.1 AU. From there on, a full MHD model that computes the three-dimensional time-dependent evolution of the solar wind macroscopic variables up to the orbit of Earth is used. After interfacing the density and velocity at the inner MHD boundary, we compare our results with those of a kinetic exospheric solar wind model based on the assumption of Maxwell and Kappa velocity distribution functions for protons and electrons, respectively, as well as with in situ observations at 1 AU. This provides insight into more physically detailed processes, such as coronal heating and solar wind acceleration, which naturally arise from including suprathermal electrons in the model. We are interested in the profile of the solar wind speed and density at 1 AU, in characterizing the slow and fast source regions of the wind, and in comparing MHD with exospheric models in similar conditions. We calculate the energetics of both models from low to high heliocentric distances.     

Substorm recovery phase

The recovery phase of the magnetospheric substorm is studied numerically by means of a two-dimensional time-dependent nonlinear resistive MHD code. The initial configuration was chosen from the earlier numerical model in which the magnetospheric substorm was driven by the solar wind plasmas. In order to study the recovery phase, the entering solar wind energy flux was reduced when the magnetospheric substorm was in its expansive phase. The system was found to respond instantly to this change and the result showed many characteristic features related to the recovery phase including the tailward motion of thex-point of the reconnected magnetic field lines and the restoration of a tail-like configuration of the magnetic field. Thex-point moved at almost the same speed of the plasma flow in the upstream region, which was considerably smaller than the speed of the plasma jetting or the speed of the plasmoid. As the recovery phase progressed, the plasma jetting across thex-point was reduced very much in the Earthside region. Although the plasma flow was generally in the Earthward direction in the Earthside region of thex-point, the tailward flow was also found near thex-point. The current density was reduced near thex-point and the neutral sheet was broadened in the recovered region. The plasma sheet also became thick in this region. During the recovery of the substorm, the energy conversion rate, both in the form of plasma acceleration and the Joule heating, was reduced. These results on the recovery phase together with the earlier simulation result on the expansive phase indicate that driven reconnection can be a viable mechanism for the magnetospheric substorm including the recovery phase.     

A comprehensive view of solar-terrestrial relationships in terms of a chain of four dynamo-powered plasma acceleration processes

This paper emphasizes an effort to link processes which relate solar activity and magnetospheric disturbances in terms of energy transfer through a chain of four elements. In this view, each element is explicitly thought to be powered by a dynamo, namely the solar wind generation dynamo, the solar flare dynamo, the solar wind-magnetosphere dynamo and the auroral dynamo, respectively. Each dynamo powers a plasma acceleration process by the Lorentz force and the plasma flows thus generated are the solar wind, the flare-generated solar wind disturbance, the magnetospheric plasma convection and the ionospheric convection, respectively. Each plasma flow conveys the energy from one element to the next in the chain. Some of the kinetic energy of the photospheric plasma is eventually deposited in the polar ionosphere as heat energy.     

Generation of rotational discontinuities by magnetic reconnection associated with microflares

Magnetic reconnection can take place between two plasma regions with antiparallel magnetic field components. In a time-dependent reconnection event, the plasma outflow region consists of a leading bulge region and a trailing reconnection layer. Magnetohydrodynamic (MHD) discontinuities, including rotational discontinuities, can be formed in both the bulge region and the trailing layer. In this paper, we suggest that the rotational discontinuities observed in the solar wind may be generated by magnetic reconnection associated with microflares in coronal holes. The structure of the reconnection layer is studied by solving the one-dimensional Riemann problem for the evolution of an initial current sheet after the onset of magnetic reconnection as well as carrying out two-dimensional MHD simulations. As the emerging magnetic flux reconnects with ambient open magnetic fields in the coronal hole, rotational discontinuities are generated in the region with open field lines. It is also found that in the solar corona with a low plasma beta ( 0.01), the magnetic energy is converted through magnetic reconnection mostly into the plasma bulk-flow energy. Since more microflares will generate more rotational discontinuities and also supply more energy to the solar wind, it is expected that the number of rotational discontinuities observed in the solar wind would be an increasing function of solar wind speed. The observation rate of rotational discontinuities generated by microflares is estimated to be dN _RD/dt - f/63 000 s (f > 1) at 1 AU. The present mechanism favors the generation of rotational discontinuities with a large shock normal angle.     

A Simulation Method for Semicollisional Plasmas

We present a one dimensional kinetic simulation model which can be used to simulate the stationary state of a semicollisional plasma. Results of a simple simulation are presented and compared to Fokker-Planck calculations. The model is particularly well suited for the diluted solar atmosphere. This revised version was published online in July 2006 with corrections to the Cover Date.     

Solar wind flow associated with stream-free sector boundaries at 1 AU

The correlations between the plasma characteristics of the solar wind flow in the vicinity (± 12 hr) of stream-free sector boundaries near Earth are examined using the composite data base of interplanetary plasma for the period 1965–1980. We confirm the result of Lopez et al. (1986) of an inverse relationship of the proton temperature (T _p) with the momentum flux density (NV ²) in the low speed wind at 1 AU. The coefficients of lines of best fit to the T _pvs NV ²(as well as T _pvs V) distribution in our sample are, however, significantly different from those of the undifferentiated sample of low speed wind considered by Lopez et al. such that T _pis, in general, lower than expected. We find further that the proton number density (N) varies as the inverse cube of the flow speed (V) indicating an invariance of the kinetic energy flux density (NV ³) relative to velocity structure in the plasma flow around stream-free boundaries. These average relationships, which are unaffected by interplanetary dynamical processes, are suggested to be due to sub-sonic addition of momentum and energy to the solar wind flow from the source structures, namely coronal streamers.