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MAGNETOSPHERIC PHYSICS IN CHINA: 2000-2002

CAO Jinbin ,  LIU Zhenxing

Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing 100080, China

and PU Zhuying

School of Earth and Space Sciences, Peking University, Beijing 100871, China

 

ABSTRACT

This brief report presents the latest advances of the magnetospheric physics researches in China during the period of 20002002, made independently by Chinese space physicists and through international cooperation. The related areas cover almost every aspect of magnetospheric physics. 

 

I. SPACE EXPLORATION

Double Star Program (hereafter referred to as DSP) is a very important space mission to explore the geospace plasma environment. DSP is designed to observe and study the global response of geospace plasma environment to solar activities and interplanetary disturbances, dynamic processes in boundary layers of magnetosphere, as well as magnetospheric substorms and magnetic storms, and furthermore to establish relevant models. DSP shall cooperate closely with the Cluster mission of European Space Agency to form a six-point exploration of geospace first time in history. The implementation of DSP will greatly advance the magnetospheric physics researches in China and globally. Details of DSP are described in “A Brief Introduction to Geospace Double Star Program” by. LIU Zhenxing in this issue.

 

II. GEOMAGNETIC STORMS

Study for geomagnetic storms has been one of the most active research areas of magnetospheric physics in China. Chen et al.[1] studied latitudinal characteristics of the geomagnetic field during the geomagnetic storm on July 15-16, 2000. This storm is a response to the solar Bastille flare on July 14. Generally, the geomagnetic disturbances at middle and low latitudes during a storm are mainly caused by three magnetospheric-ionospheric current systems, such as the ring current system (RC), the partial ring current and its associated region II field-aligned currents (PR), as well as the region I field-aligned currents (FA). The following results have been shown. (1) The northward turning of IMF-Bz component started the sudden commencement of the storm, and its southward turning caused the main phase of the storm. (2) The PR- and FA-currents varied violently in the main phase. In general, the field of the FA-current was stronger than that of the PR-current. (3) In the first stage of the recovery phase, the RC-field gradually turned to anti-parallel to the geomagnetic axis from a 15o deviation, and the local time ( ) pointed by the RC-field kept at 16:00. Then,  rotated with the stations, and the RC-field was not anti-parallel to the geomagnetic axis, but with a 5o-10 o deviation. These facts suggest that the warped tailward part of the ring current decays faster than the symmetric ring current.

Chen et al.[2] also studied the ionospheric equivalent currents in the high latitude and the auroral electrojet system during the geomagnetic storm on July 15-16, 2000. The currents and the electrojet are analyzed by using geomagnetic data from IMAGE chain. Large-scale vortices of equivalent currents are observed during the storm. The vortices in the dusk side of ionosphere correspond to a four-celled pattern of plasma convection associated with NBZ, region I and region II field-aligned currents. Only one vortex can be found in the dawn side ionosphere after interplanetary magnetic field (IMF) turns southward. In the initial phase of the storm, the center of eastward electrojets tends to shift equatorward. It arrives at 58.62o latitude of corrected geomagnetic coordinates (CGM). Westward electrojets are strong in the main phase. The center of westward electrojets in this period moves equatorward and may be beyond the most southward station (56.45 o) of the chain.

Shen et al.[3] studied the storm-substorm relationship by developing a relationship between AL and Dst indecies based on the physics of the process. They argued that the magnetospheric electric field caused both the westward auroral electrojet and the particle injection into the ring current region. The adiabatic acceleration of ring current particles and the thermal properties of the ring current source were included in the calculations. The injection function deduced was found to be a function of AL index and the solar wind dynamical pressure . For , the injection function was proportional to  with a time lag ; while for large  the injection function was proportional to . This result was consistent with previous statistical results. They also found that the ring current decay time was a function of AL index. A algorithm was then obtained which predicted Dst from AL and the solar wind dynamical pressure . The correlation coefficients between Dst index predicted by the algorithm and the observed Dst index were 0.84 and 0.80 for the years 1998 and 1999, respectively.

Fu et al.[4] studied the ion composition variation in the inner magnetosphere during the storms in 1991 by using data sets obtained from the MICS instrument onboard CRRES. The observations were made during the second half of the CRRES mission which was near maximum of solar cycle 22. Four selected storms are subjects of detailed case studies; statistical results are based on a group of moderate (50<Dst<100 nT) and large storms (Dst>100 nT) observed in 1991. The case studies show that energetic particle enhancements occur at very low equatorial altitudes (L=34) during large storms with significant delays relative to the storm SSC times (of about 20 hours). The average time duration of the particle enhancements is about 47 hours. By studying the time variation of energy spectrograms of H+, it is found that lower-energy (E<100 keV) and higher-energy (E>100 keV) protons show different time profiles during the development and decay of the ring current. The low-energy part shows a dramatic intensification and a rapid decay. However, its relative contribution to the ring current defined by the density ratio N ( )/N during the storm maximum is almost constant. On the other hand, high-energy protons first exhibit a flux decrease followed by a delayed increase. The density ratio N ( )/N shows an anti-correlation with the storm intensity. It is confirmed that the ionospheric origin particles (e.g. O+) are important constituents of the storm time ring current. The fractional number density of O+ ions increases with the intensity of the storm. The statistical results demonstrate that the energy density of O+ is a steep function of Dst for moderate storms. However, it seems to increase very slowly with Dst, or even almost independent of Dst for large storms (Dst∣≥120 nT). The ratios of solar wind origin He++ density to the total density show no obvious difference among large storms. The same appears for He+ ions.

Fu et al.[5] also studied the Ion composition variations in the ring current during storm time by using data set obtained from CRRES/MICS. Case study of the twin storms on July 1991 and statistical study of 12 intense storms in the same year both indicate that the storm-time ring current particles can be divided into two groups. One of them consists of O+, low energy H+ and He++ which are originated from the solar wind (SOP). It is shown that in quiet time the major particles of the ring current are SOP, whereas during the main phase of large storms the main component of the ring current particles is the IOP. Ring current particles of intense storms can be injected to low L-altitudes(L3-4). It is confirmed that the contribution of IOP to the ring current increases with Dst value. In large storms IOPs are the major constituents of the ring current, even up to 80% of the total number density at the Dst Maximum. There is a clear evidence showing that it is the rapid enhancement and reduction of the O+ flux of the ring current that lead the Dst index to rapidly decrease during the main phase and to quickly recover in the early recovery phase. Further analyses indicate that during weak storms (Dst >-50 nT) the contribution of O+ ions to the ring current is negligible.

Xie et al.[6] studied the injection of storm-time ring current ions with 3-D test particle trajectory calculations (TTS). Two major and important new results were presented in their paper. It was found that a new seperatrix existed between open and close trajectories of drifting particles and that the shielding electric field played an important role for the formation of the closed symmetric ring current.

III. MAGNETOSPHERIC SUBSTORMS

Pu et al.[7] studied the global magnetic reconnection–current disruption–ionosphere-magnetosphere coupling model for substorms. Theoretical studies and observational analyses show that decreased earthward flows cause the magnetospheric configuration modes unstable in the near earth magnetotail region and significantly enhance its growth rate. Midtail reconnection, near earth magnetotail current disruption and ionosphere-magnetosphere coupling can cooperatively trigger the expansion phase of substorms. This model is in agreement with a lot of observations.

Pu et al.[8] described a global synthesis depolarization model combining coupled processes in the midtail, inner tail and auroral ionosphere. In the late growth phase, magnetic reconnection releases the magnetic energy stored in the magnetotail. Magnetic flux and energy are transported earthward and tailward. As the earthward flow slows down in the near-earth plasma sheet (NEPS), it compresses the magnetic field and plasmas near the earthward inner edge of the NEPS and pushes them further inward (earthward and equatorward). This sets up a favorable condition for generating the drift ballooning mode (DBM) instability in the inner tail. The unstable DBMs generate coupled Alfven–slow-magnetosonic-waves and field–aligned–currents (FACs), resulting in a turbulent state in the equatorial region and enhancing the ionospheric conductance . As soon as and FACs increase to a threshold level, the substorm current wedge is formed, leading to an explosive intensification of the auroral electrojet and magnetic field depolarization at substorm onset. Moreover, they regard the “substorm trigger phase” (Ohtani et al., Planet. Space Sci. 37 (1989), 579–588) as the interval during which the inner tail is being further compressed inward and the DBMs explosively develop to trigger magnetic field depolarization. They suggest that the dawn-dusk electric field Ey in the inner tail may either appear somewhat later than or simultaneously with magnetic reconnection in the midtail. It seems that a variety of expansion onset features can be explained in terms of this synthesis depolarization model.

A Synthesizing Tail Reconnection and Current Disruption Model for Substorm Initiation was also studied by Pu et al.[9]. It is found that the decelerated earthward flow created by magnetic reconnection in the mid tail causes the ballooning mode instability (BMI) to explosively grow near the inner edge of the plasma sheet. The BMI generates coupled Alfven-slow magnetosonic waves and field-aligned currents, resulting in substorm expansion onset. This synthesizing global substorm scenario seems to be consistent with a variety of substorm features observed in the magnetotail and on the ground.

Chen et al.[11] studied the acceleration of electron by a field-aligned electric field during a magnetospheric substorm in the deep geomagnetic tail by means of a one-dimensional electromagnetic particle code. It was found that the free acceleration of the electrons by parallel electric field is obvious, kinetic energy variation is greater than electromagnetic energy variation in the presence of parallel electric field. But magnetic energy is greater than kinetic energy variation and electric energy variation in the absence of parallel electric field. More mode waves in the presence of parallel electric field are generated than that in the absence of parallel electric field.

In a succeed paper Korth and Pu[10] studied the magnetic field configuration and field-aligned acceleration of energetic ions during substorm onsets observed by Chen et al. [11] . They present an interpretation of the observed field-aligned acceleration events measured by GEOS-2 near the night-side synchronous orbit at substorm onsets (Chen et al., 2000). They show that field-aligned acceleration of ions (with pitch angle asymmetry) is closely related to strong short-lived electric fields in the Ey direction. The acceleration is associated with either rapid depolarization or further stretching of local magnetic field lines. Theoretical analysis suggests that a centrifugal mechanism is a likely candidate for the parallel energization. Equatorward or anti-equatorward energization occurs when the tail current sheet is thinner tailward of earthward of the spacecraft, respectively. The magnetic field topology leading to antiequatorward energization corresponds to a situation where the near-Earth tail undergoes further compression and the inner edge of the plasma sheet extends inwards as close as the night-side geosynchronous altitudes.

Hong et al.[12] studied nightside auroral arcs in high latitudes during substorms and their relationship with IMF. Observations of all-sky imaging, geomagnetic field, Pi2 pulsation from Zhongshan station and IMF data from Wind satellite have been used to investigate 7 cases of nightside arcs intensification and fading in high latitudes. The characteristics of auroral arcs are as follows: All of arcs intensify briefly, then fade, and last for 10–20 min; the arcs nearly align along the Sun–Earth direction with pronounced duskward drift; most of the arcs occur after the interplanetary magnetic field (IMF) turns southward corresponding to the substorm growth or expansion phase, and IMF Bx>0, IMF By<0; the dawn-dusk drift of arc is in the opposite direction of IMF By; these arcs have no close relation to magnetic activities and Pi2 pulsation. The present results suggest that the brief intensification of auroral arcs may be due to IMF Bx>0 which favors magnetic reconnection in the southern lobe of the magnetotail, the arc fading may be attributed to southward IMF Bz, and the dawn-dusk motion of the arcs may be driven by the effect of the IMF By component on the motion of the reconnection site.

Hong et al.[13] further studied the auroral substorm response to solar wind pressure shock. Two cases of auroral substorm have been studied with Polar UVI data, which were associated with solar wind pressure shock arriving at Earth. The global aurora activities started about 1–2 min after pressure shocks arrived at dayside magnetopause, then nightside auroras intensified rapidly 3–4 min later, and auroral substorm onset. The observations in synchronous orbit indicated that the compressing effects on magnetosphere were observed in their corresponding sites about 2 min after the pressure shocks impulse magnetopause. They propose that the auroral intensification and substorm onset possibly result from hydromagnetic wave produced by the pressure shock. The fast-mode wave propagates across the magnetotail lobes with higher local Alfven velocity, magnetotail was compressed rapidly and strong lobe field and crossing tail current were built in about 1–2 min, and furthermore the substorm were triggered due to an instability in current sheet.

Jin et al.[14] conducted a 2.5 dimensional MHD simulation of multiple–plasmoid -like structures in the course of a substorm on the basis of equilibrium solutions of the quiet magnetotail. Two types of distributions of the By component and the plasma density (Type I and Type II) are used as the initial states of the simulation studies. The results of all cases with different initial conditions illustrate that the formation of plasmoids occurs intermittently and repeatedly. Also, all the plasmoids are high-density and high-temperature regions in comparison with the ambient environment; that means the plasma with high-temperature is embedded in plasmoids formed repeatedly. These results are in line with features of multiple-plasmoid-like structures observed on January 15, 1994, with Geotail at 96 RE in the tail. Thus it can be concluded that a large amount of the energy stored in the magnetotail is gradually dissipated by ejecting multiple plasmoids in the course of substorms. In this simulation it is shown that the lasting inflow caused by the electric field E imposed on the boundary of the simulation box controls the recurrent formation of plasmoids. Further, as additional evidence, the multiple-plasmoid-like structures have been detected by Geotail in conditions of high-speed solar wind streams and southward IMF. Therefore our simulation suggests that the solar wind and the IMF have close control over the magnetotail dynamic process. The features of a classic plasmoid (bipolar Bz) and a traveling compression region (TCR) in the (X, Z) plane can be seen in all cases. Taking the time evolution of the By component into account, two kinds of plasmoid-like structures with a flux rope core and with both By and Bz bipolar signatures can be reproduced for the Type I and Type II conditions, respectively. Therefore the occurrence of various magnetic structures in the magnetotail might be related to nonsteady driven reconnection with different initial distributions of the By component.

IV. SOURCE AND LOSS OF MAGNETOSPHERIC IONS

Zong et al.[15] studied the escape of magnetospheric O+ oxygen ions into the magnetosheath: mechanism and effects. During the magnetic storm on January 10, 1997, Geotail spacecraft observed a manetospheric O+ burst event just on the outside of the dayside magnetopause. Energetic oxygen ions were seen to flow downward in the magnetosheath. The appearance of this O+ burst event was in close connection with the strong southward interplanetary magnetic field. The Principle Axis Analysis (PAA) indicates that the dayside magnetopause during this time period was a rotational discontinuity, magnetic reconnection took place in the magnetopause current sheet. Observations showed that reconnection was of the quasi-steady state type and that the O+ flow has a southward component. Both normal and inverse energy (velocity) dispersion were seen in the O+ flux enhancement process which were caused by the Time Of Fly (TOF) effect of oxygen ions escaping along the reconnected magnetic field lines. Only O+ can be transported by the gradient drift from the magnetosphere into the reconnection region, therefore they can be continuously detected in the magnetosheath. The escape rate of O+ in this event is estimated to be 0.61×1023s-1, which was approximately 33% of the total input rate of the ring current oxygen ions. It is the escape of a great number of oxygen ions from the duskside of the inner magnetosphere (ring current region) that leads to the pronounced asymmetry in the ring current ASY-H index.

V. THE RADIATION BELT

Pu et al.[16] studied the drift shell tracing and secular variation of inner radiation environment in the south Atlantic anomaly region. The drift shell tracing method is proposed to investigate the secular variation of trapped particle fluxes in the low Earth orbits (but much higher than the atmospheric cut-off altitude) through NASA standard radiation models. The preliminary results show that over the past three decades fluxes of high-energy particles at the altitudes1000 km in the South Atlantic Anomaly (SAA) significantly increased, the center region of particle SAA apparently moved westward, and the drift shells of trapped particles considerably descended. The relevant physical problems are briefly discussed.

VI. FIELD ALIGNED CURRENTS

Chen et al.[17] studied the generation of pressure pulses, traveling convection vortices, and field-aligned currents in the magnetosphere by response to interplanetary tangential discontinuity. Three-dimensional magnetohydrodynamic (MHD) simulations are conducted to study the response of the magnetosheath and the magnetosphere to an interplanetary tangential discontinuity (TD). Fast mode waves and a slow mode-like wave structure are generated in the magnetosheath by interactions between the interplanetary TD and the bow shock. These waves further propagate and disturb the dayside magnetosphere. The impinging of the pulse on the magnetosphere leads to the generation of traveling convection vortices, Alfven waves and associated field-aligned currents (FACs). During the fast mode waves propagate downstream out of the dayside region in about 2 minutes, twin vortices are generated in the magnetospheric equatorial plane and can be mapped along magnetic field lines to high latitudes. The associated FACs are also set up in the same time. As time progresses, a large amplitude density pulse, made of a packet of slow mode-like waves, comes to compress magnetopause significantly for several minutes. Since the magnetosphere must find new equilibrium as it is compressed, this equilibrium process would create new vortices as well as new FACs.

Cao et al.[18,19] studied the response of field aligned current in the magnetopause to the interplanetary By. Recent satellite observations and ground-based observations show that the interplanetary has a strong influence on the FAC of the earth. The influence of interplanetary By component on the FAC in the reconnection region of magnetopause is studied by means of 3D MHD simulation. The interplanetary By influence the reconnection process and the related FAC of the simulation region by dayside boundary magnetic field Bby at x= -Lx. Results show the rapid increase of  Bby can increase FAC remarkably , sometimes about one order larger. Larger Bby can also maintain FAC at a high level. In addition, the asymmetry of By distribution in the reconnection region caused by Bby give rise to the asymmetry of FAC. These results are in good agreement with satellite observations. Because FACs at the inner side of the reconnection layer flow into the polar ionosphere along the magnetic field, the results are very important to the better understanding of coupling of solar wind – magnetosphere – ionosphere.

VII. MAGNETIC RECONNECTION

Liu et al.[20,21,23] studied the transient reconnection caused by the impact and sudden stop of non-homogeneous dynamic pressure. They proposed that local and transient reconnection in the plasma boundary layer can be caused by the impact and switch-off of non-homogenous dynamic pressure. The two-dimensional compressible magnetohydrodynamic (MHD) simulation is used to investigate the reconnection processes in these two cases. It is found that the state of transverse flow and its lasting period play an important role in reconnection processes. When the transient flow is homogeneous, it does not cause reconnection, the magnetic field lines are only pushed downstream. When the transverse flow is sheared one, magnetic reconnection always occurs during the impacting period, and a new “reverse K” pattern reconnection configuration is formed in the plasma boundary layer becomes very unstable, many kinds of fluid vertex, flow pattern and reconnection configuration are generated in the current sheet region, the reconnection configurations are very different from that for impact phase. We suppose that the sudden stop of an external flow action may be an important trigger mechanism of energy conversion and magnetic reconnection.

According to the simulation results, an alternative transient reconnection model has been proposed, it can be called “transient shear flow driven reconnection” model. This model is quite different from MXR model, VIR model and BSXR model. The reconnection mechanism proposed in the model may be existed in some regions of space plasma, such as the subsolar point region of dayside magnetopause boundary layer. This reconnection model can be used to investigate the formation and structure of FTEs, oscillation of magnetopause, vortex structure and the excitation of wave in the dayside magnetopause boundary layer.

Jin et al.[23] numerically studied the asymmetric driven reconnection processes in the vicinity of the magnetopause by using a two-dimensional compressible MHD code. The initial magnetic field configuration is assumed to be in a mechanical equilibrium state. The cases with identical temperatures( / =1.0) and four different ratios of magnetic field strength ( = / =1.0, 1.5, 2.0, 2.5 ); and the case with / =2.0 and =1.5 are investigated ( , and , are the initial magnetic strength and temperature outside the current sheet on the magnetosphere and the magnetosheath, respectively ). When the magnetic field on magnetosheath side is set as southward, a recurrent formation of multiple magnetic bubbles with various scales occurs under the action of the inward plasma flow imposed at the left and right boundaries. In the simulation, some bubbles coalesce into a bigger one and then it is convected out of the simulation domain, the others are convected through the top boundary all alone. Thus, the plasmoid events with different scales and different time intervals take place intermittently and the impulsive features of magnetic reconnection are clearly shown. The multiple magnetic islands are all high-temperature and large-density regions in comparison with the ambient environment. The bipolar signatures or fluctuant variations of normal magnetic field component are generated by the formation of multiple magnetic islands. This result is similar to the FTEs signature.

Jin et al.[24] investigated the mechanism for the occurrence of two types of various magnetic structures in the magnetotail. As well known the magnetic cross-tail component  in the magnetotail is in direct proportion to the interplanetary magnetic field (IMF)  component. And the polarity of IMF and plasmoid / flux rope components do indeed agree. These results indicate that the IMF penetrates plasmoids and the magnetic structures must therefore be three-dimensional. In this paper the dynamical processes of magnetotail in the course of a substorm are studied using a MHD code with two-dimensions and three components on the basis of two types of initial equilibrium solutions of the quiet magnetotail. The numerical results of two cases illustrate various features of time evolution of component that correspond to two kinds of plasmoid-like structures, one with a flux rope core and the other one resembles a “closed loop” plasmoid. Therefore, the occurrence of various magnetic structures in the magnetotail might be related to non-steady driven reconnection with different initial distributions of the component.

Cui and Jin[25] studied the evolution of magnetic helicity of various magnetic structures in the magnetotail. The results illustrate that, in the driven magnetic reconnection process generated by the dawn-dusk electric field in the magnetotail, the transportation of magnetic helicity flux via the boundaries of system is the direct cause of the change of the total magnetic helicity in the system. The various initial distributions of magnetic helicity density and the transportation of magnetic helicity flux may lead to various evolutions of magnetic helicity density in the neutral sheet region, and could result in the formation of various magnetic structures.

Yang and Jin[26] carried out the numerical simulation on the earthward propagating plasmoids. The correlation between plasmoids and geomagnetic substorms has been demonstrated by satellite observations. In addition to the tailward propagating plasmoids with NS bipolar signatures, earthward propagating plasmoids are found, which are characterized by plasma sheet S-N bipolar events and lobe SN bipolar signatures. The occurrence rate of the SN bipolar events is apparently lower than that of the NS events. The SN bipolar events occur primarily during quiet geomagnetic and IMF Bz north conditions, and they are correlated with small, isolated geomagnetic substorms. A 2.5 dimensional MHD simulation on the earthward propagating plasmoids has been carried out, on base of the specific distribution of the cross-tail electric fields Ey component during geomagnetically quiet times (IMF Bz is southward and |By |Bz), The simulation results of the two cases showed the major signatures of the flux rope magnetic structures and plasmoids with complex closed magnetic field lines. The similar signatures to tailward propagating plasmoids could be consistent with IMP 8 observations about the earthward propagating plasma sheet SN bipolar events. The results also displayed a similar configuration to the schematic given by Schindler, which may explain the low occurrence frequency of the SN events. It can be found in this report that the dynamic growth of the magnetic reconnection becomes considerably suppressed for a strong cross-tail magnetic field By. In conclusion, the magnetic reconnection can be the genetic mechanism of the plasma acceleration and heating in the magnetotail in both geomagnetically active and quiet times, namely, under both southward and northward (with |By |Bz) IMF conditions.

Liu and Li[27] studied the relationship between southward IMF events and geomagnetic storms. The solar wind-magnetosphere coupling is investigated in terms of forty-three moderate and intense storms ( < -50 nT) and the associated southward IMF events occurring during 1972 to 1982. The solar wind data is taken from spacecraft IMP-8 and the geomagnetic storms are described by and AE. It is shown that: (1) There are 11 events (25.6% of the total 43 events) which closely follows a shock, 18 events (42%) which occur in the downstream of a shock, and the remainder 14 events bear no relation to the interplanetary shocks. Most of these events are accompanied with the increase of the magnetic field intensity and the dynamic pressure of the solar wind; (2) Only substorms exist when the interplanetary dawn-to-dusk electric field is less than 4 mV/m, but both of magnetic storms and substorms occur simultaneously if the is greater than 5 mV/m; (3) The ring current energization is related to the dynamic pressure ( ), rather than the density or the velocity alone; (4) While the southward IMF component  is crucial, the relative strength of ,  to the  also play a certain role in the energy transfer from solar wind to the magnetosphere. The energy coupling between the solar wind and the magnetosphere is enhanced if the relative strength is larger.

Cao et al.[28] studied the dynamic process of magnetopause magnetic reconnection. The effects of hall term, pressure gradient terms, electric resistivity term and convection term in the generalized Ohm's law to the magnetic reconnection of dayside magnetopause are studied by means of 3-D MHD electromagnetic code. The hall term, pressure gradient term, and electric resistivity term are of the same order, and are smaller than the convection term. The hall term can accelerate magnetic reconnection and play one more important role than the pressure gradient term. This is because in the Farady's equation the curl of pressure gradient term is smaller than that of the hall term. The pressure gradient term slows magnetic reconnection, changes the core field, reduces the outflow velocity and field-aligned current in the case of high plasma Beta and large ion inertial length.

Cai et al.[29] proposed a nonlinear generation mechanism of whistler turbulence at the magnetopause. The plasma maser effect in the existence of enhanced kinetic Alfven wave turbulence is investigated as the generation mechanism of the whistler turbulence at the magnetopause. The numerical results of the growth rate show that on the scale of neither the ion inertial length nor the electron inertial length, the whistler waves can be excited, and the peak of the maximum growth rate occurs on the scale of the effective ion Larmor radius. The kinetic Alfvén wave dynamics associated with the scale length of the effective ion Larmor radius is intrinsically important in the magnetic reconnection and leads to the generation of whistler turbulence. The theoretical model of the generation of whistler turbulence is useful for explaining the observations of intense electromagnetic fluctuations at the magnetopause.

Wang et al.[30] studied the Alfven waves in the magnetic reconnection regions and the acceleration of newborn ions by means of 2D-hybrid simulation code, magnetic driven reconnection in the low plasma  case. Results show that the Alfven waves can be produced in the magnetic reconnection process. The newborn ions can be pitch angle scattered by Alfven waves and have the shell distribution. Some newborn ions are accelerated and the maximum energy is about 4 . The acceleration takes about  and is thus extremely rapidly acceleration process

VIII. INSTABILITIES IN THE MAGNETOSPHERE

Lu et al.[31] studied the Kelvin-Helmholtz instabilities driven by sheared ion flows in the plasma sheet of the geomagnetic tail in the presence of oxygen ions. In the presence of ionospheric oxygen ions in the magnetotail plasma sheet region they investigate the Kelvin-Helmholtz instability driven by sheared ion flow with low frequency surface wave perturbation. By considering proton and oxygen ion flows of the same bulk speed and keeping velocity curl term in the MHD equations the dispersion relation of perturbed wave propagating along magnetic field is obtained. In plasma sheet boundary layer, they find that with increasing abundance of oxygen ions the critical wavelength for the instability is enhanced to about 20 . For a certain abundance of oxygen ions, the profile of the critical shear versus wavelength has a minimum. The wavelength corresponding to the minimum critical shear is called most-unstable wavelength. The instability tends to happen in a small range around the most-unstable wavelength. The larger the abundance of oxygen ions is, the smaller the critical shear minimum and the longer the most-unstable wavelength will be. The growth rate for the instability increases with the shear rapidly to its saturation, which approaches the gyro frequency. The presence of ionospheric oxygen ions may result in the plasma sheet boundary layer instability. Our results may be helpful in understanding the low-frequency magnetic pulsation and the substorm processes.

The sheared ion flow instability is studied in an inhomogeneous plasma background of magnetopause boundary layer at the high latitude magnetotail by Lu et al.[32,33]. By considering tail-aligned currents, they find that the instability excitation strongly depends on the disturbed wavelength. The quasi-critical wave number for the instability is obtained. For relative long wavelength perturbation the instability tends to be excited at the inner edge of the boundary layer. The stable surface wave at magnetopause and the K-H instability at the inner edge of the boundary layer can exist at the same time, and that may help to transfer the momentum toward magnetosphere continuously.

Shen and Liu[34] studied the linear and nonlinear properties of Kelvin-Helmholtz resistive instability in compressible plasmas. Kelvin-Helmholtz (K-H) instability and resistive instability are two important processes occurring in plasma boundaries. They have investigated the coupling mode between K-H instability and resistive instability or K-H Resistive instability (KHRI) by using a compressible MHD approach. Some characteristic effects of the plasma compressibility have been revealed in this study. The results are as follows. (1) KHRI is strongly dependent on the shear flow velocity. KHRI is capable to take place only in two limited ranges of shear flow velocity. (2) When the flow shear strength increases, the linear growth rate of KHRI increases monotonously. (3) The coupling mode appears only in a limited range of wave number. (4) At the nonlinear stage of KHRI, various kinds of shocks, i.e., slow shocks, parallel shocks and fast shocks may occur according to the shear flow velocities.

Wang et al.[35] studied the characteristics of low frequency electromagnetic waves generated by high energy ion beam by computer simulation and analytic method. Results show that in the linear phase, non-resonant mode of short wavelength is excited. In the nonlinear phase, long wavelength mode dominates, short wavelength mode gradually decreases and waves display the features of Alfven waves. These results can be used to explain the turbulence phenomena in the solar wind.

Cao et al.[36] studied the phase angle diffusion, pitch angle diffusion and energy diffusion of newborn ions in the self-consistently generated fields by means of one-dimensional electromagnetic hybrid code. For newborn ions, the time for phase angle diffusing to 2p is a little shorter than the time for pitch-angle scattering to a relatively thin complete shell and it is much shorter than the energy scattering time for broadening of the shell toward a thermal distribution. The speed of phase angle diffusion increases monotonously with the injection velocity, but it does not change in the same way with the injection rate. The phase angle diffusion of heavier injected ions is slower than that of lighter injected ions. The complete pickup process of newborn ions should consist of four stages: (1) the creation of the newborn ions and macro perpendicular pick up due to the motional electric field; (2) phase angle diffusion; (3) pitch-angle diffusion; and (4) energy diffusion.

Chen et al.[37] studied the characteristics of low frequency plasma wave propagation affected by parallel current. The MHD (Magnetohydrodynamic) wave propagation in the plasma region with the current which is parallel to background magnetic field has been studied, the effect of parallel current on the MHD wave dispersive relation has been explained in this paper, the characteristics of parallel current instability which is generated in the propagation of long wave length MHD has been pointed particularly.

IX. THE GEOMAGNETIC FIELD RESEARCHES

Lu et al.[38] represented 2.5-dimensional MHD simulations of the multi-component plasma sheet with the velocity curl term in the magnetic equation. The simulation results can be summarized as follows: (1) There is an oscillation of the plasma sheet with the period in the order of 400 s ( Pc 5 range); (2) The magnetic equator is a node of the magnetic field disturbance; (3) The magnetic energy integral varies antiphase with the internal energy integral; (4) Disturbed waves have a propagating speed in order of 10 km/s earthward; (5) The abundance of oxygen ions influences amplitude, period and dissipation of the plasma sheet oscillation. It is suggested that the compressional Pc 5 waves, which are observed in the plasma sheet close to the magnetic equator, may be caused by the plasma sheet oscillation, or may be generated from the resonance of the plasma sheet oscillation with some Pc 5 perturbation waves coming from outer magnetosphere.

 

X. THE ENERGETIC NEUTRAL ATOM IMAGE

Shen et al.[39] carried out the simulations on the ENA observation properties. The polar satellite of Double Star (DS) of China will be equipped with an ENA detector to observe the evolution of the ring current during magnetic storms and substorms. In order to offer scientific bases for the manufacture of ENA detector and also to make preparation for the future ENA observation data analyses and theoretical research, they have carried out the simulation investigations on the ENA observation properties. Ring current ion flux is assumed as a Kappa distribution and only adiabatic movements are considered, thus the ion flux is obtained theoretically. The calculations have shown that, (1) the stronger the storms, the higher the ENA flux from the ring current; (2) the higher the ENA energy, the lower the ENA flux from the ring current directions; (3) the ENA He flux is rather low, even when the Kp index is high; (4) the injection front can be determined by ENA measurements.

XI. PARTICLE EVENTS IN THE MAGNETOSPHERE

Zhou et al.[40] investigated MeV Electron-Flux Enhancement Events at the Geosynchronous Orbit in April-May 1998. The time and energy response characteristics of the relativistic (MeV) electron-flux enhancement events at the geosynchronous orbit are analyzed and compared to the interplanetary disturbances and magnetic storms by using the high resolving data for the satellite GOES-9 electron fluxes and the spacecraft ACE solar-wind parameters in April-May 1998. The results show that there was the daily variation for the MeV electron flux at the geosynchronous orbit, the maximum flux at about the noon and minimum flux at about the midnight. In two great events starting at the geosynchronous orbit on April 22 and May 5, 1998, the rising time scales of the noon maximum flux up to the peak flux for electrons of the energy >2 MeV are for about 4 days and 1 day, respectively. The continued times of the noon maximum fluxes (greater than the background level) are 13 days (from April 22 to May 4) and 16 days (from May 5 to 20), respectively. The energetic ranges for the MeV electron-flux enhancement events are not same exactly. The rising phases for two great events in April-May correspond to the recovery phases of storms and are closely associated with the solar-wind dynamic pressure pulse, the high-speed stream pulse, and the negative Bz component of the interplanetary magnetic field (IMF).

XII. THE NEUTRAL SHEET

Li and Xu[41] studied the neutral sheet surface by the observations of ISEE-2, IMP-8, AMPTE/IRM, and INTERBALL satellites. A neutral sheet model has been developed, with parameters optimized based upon a large number of magnetometer data from the ISEE-2, IMP-8, AMPTE/IRM, and INTERBALL satellites. In this model, the equatorial-neutral sheet is best fitted by a smooth curved surface in the region up to 40 Re, with no slope discontinuities between the neutral sheet and the equatorial plane, or on the flanks of the neutral sheet. The cross-section of the magnetotail is divided into approximately equal areas above and below the equatorial-neutral sheet inside the magnetopause of the Sibeck model. An exponential dependence on Xgsm is adopted to express the influence of the tilted equatorial plane on the position of the neutral sheet, since it is found this influence weakens with increasing Xgsm.

XIII. THE INTERACTION BETWEEN SPACE PLASMAS AND SPACECRAFTS

Cao et al.[42] studied the effects of electron ring-beam on the satellite surface potential and plasma sheath by means of 2.5-D electrostatic code. The simulation results show that the satellite is charged negatively when there exists electron beam, and the increase of the density and velocity of electron beams increase the absolute value of satellite negative potential. When electron ring-beams are injected in an oblique angle with respected to the magnetic field, the effects of electron ring-beams decrease when the oblique injection angle increases from 0° to 90°. The plasma sheath becomes wider when there is electron ring-beam and its shape changes from “tear drop” to “fan”.

Cao et al.[43] studied the plasma sheath of moving spacecrafts in magnetized plasma of low earth orbit. The relative motion is simulated by a plasma flowing past the probe and therefore a convection electric field is imposed. The results show that, in the LEO orbit, the spacecraft is quickly (at time t = 54 , is electron cyclotron frequency) charged up to the equilibrium negative potential. The increase of magnetic field makes absolute value of the negative potential decrease. The plasma sheath is symmetric around the spacecraft if the motion of the plasma is not considered, and is about several Debye length. The motion of the plasma will make the plasma sheath extend in the wake region.

Cao et al.[44] also studied the effects of ion beam on the satellite charging process during space weather disturbance time. The results show that the ion flow parallel to the magnetic field can raise significantly the satellite potential. The electron collection ability increases with the increase of satellite potential. When the ion flow ceases to be injected, the satellite potential returns rapidly to the value in the case of no ion flow. When ion flows are injected in an oblique angle with respected to the magnetic field, the effects of ion flows decrease when the oblique injection angle increases from 0° to 90°.

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