Entry of dust particles into planetary magnetospheres

While interplanetary dust constitutes a primary source of cosmic particulate matter in planetary magnetospheres, the debris produced by its impact with small satellites and ring material provides an important secondary source. Internal processes, such as volcanic activity, particularly in the smaller satellites, could result in a third source. In the case of the terrestrial magnetosphere there are also artificial (internal) sources: 1–10μ sized A?₂O₃ particles injected by solid rocket mortar burns between near earth and geosynchronous orbit constitute one such source, while the fragments of larger bodies (artificial satellites) due to explosions (e.g., “killer satellites”) and collisions constitute another. Finally, if we include the purely induced cometary magnetosphere among planetary magnetospheres, the injection of cometary dust into it due to entrainment by the outflowing gases constitutes another source.As a result of being immersed in a radiative and plasma environment these dust grains get electrically charged up to some potential (positive or negative). Particularly in those regions where the magnetospheric plasma is hot and dense and their own spatial density is low, the dust grains could get charged to numerically large negative potentials.While this charging may have physical consequences for the larger grains, such as electrostatic erosion (“chipping”) and disruption, it also can effect the dynamics of the smaller grains. Indeed, the small but finite capacitance of these grains, which leads to a phase lag in the gyrophase oscillation of the grain potential, could even lead to the permanent magneto-gravitational capture of interplanetary grains within planetary magnetospheres in certain situations. Here we will review the sources of dust in planetary magnetospheres and discuss their physics and their dynamics under the combined action of both planetary gravitational and magnetospheric electromagnetic forces.     

Size distributions of circumplanetary dust

Dust rings have been observed around each of the giant planets and may also exist around Mars. The particles comprising these rings have short lifetimes due to a number of processes including exospheric and plasma drag, Poynting-Robertson drag, sputtering, collision with other circumplanetary particles, and the Lorentz force for charged grains. The supply of dust is maintained by collisions between macroscopic ring particles and bombardment of moons and ring particles by interplanetary impactors. All of the processes that act to remove or alter the circumplanetary dust grains are functions of particle size, so the initial size distribution of the grains released from an impact onto a moon or ring particle is modified. The size distribution of the impact ejecta can be described by a power-law of the form n(r)dr ∝r^−qdr where n(r)dr is the number of particles in the size range [r,r + dr] and q is the power-law index. For hypervelocity impact excavation, q ≈ 3.5. Drag acts more efficiently on smaller grains resulting in a reduction in q of 1. Other dynamical processes can lead to particle-size dependent collision rates with other circumplanetary objects. These processes can lead to local steepening of the size distribution (increase in q) and to truncation of the dust size distribution to a narrow range of sizes.     

Stability and mass flow in Saturn's rings

Besides gravitational effects, interesting electrodynamical processes could also take place in the vicinity of the rings of Saturn. In part, this is because of the electrostatic charging of the ring particles by the magnetospheric and ionospheric plasma, and in part, the generation of impact plasma by meteoroid bombardment at the ring plane could lead to strong coupling between the rings and the ionosphere via a variety of current systems. The mass transport and angular momentum transfer in association with the ring-ionosphere coupling may cause quite large changes in the ring configuration over the age of the solar system. The presence of the sharp boundary between the B and the C rings perhaps is a good example. To highlight these new developments, we describe several of the electrodynamical mechanisms (with emphasis on their corresponding electric fields and current systems) which have been postulated to be of importance in determining the mass transport of the ring system. Further points are made that, besides mass exchange between the rings and the planetary atmosphere, the mass injection from the rings could also have significant effect on the mass and energy budget of the magnetosphere, maintenance of the E ring, the Titan hydrogen torus as well as aeronomic process in the upper atmosphere of Titan.     

Submicron lunar ejecta in the magnetosphere associated with meteor showers

Recent studies of the lunar ejecta from lunar impacts of interplanetary dust particles indicate that during favorable lunar phases, over 80% of the submicron ejecta enters the earth's magnetosphere. This “pulse” of lunar ejecta produced by the sporadic meteor background will follow the random variations of the sporadic flux. An additional enhancement of this flux can be related to major meteor showers. Since the annual periods of these showers occur during varying lunar phase angles, magnetosphere ejecta flux associated with major showers will vary depending on the coincidence of shower periods and favorable lunar phase angles. The results of an analysis of the “pulse” of ejecta flux in the magnetosphere during the Quadrantids, Geminids, Leonids, and Perseids meteor showers are presented. These results are compared to the satellite measurements of 1959 Eta and HEOS II.     

Plasma effects in the formation,evolution and present configuration of the Saturnian ring system

The Voyager 1 and 2 observations of the fine structure of the Saturnian ring system demonstrate the importance of electric forces in controlling the dynamics of fine (charged) dust in the rings. A new theory (“gravito-electrodynamics”) which combines the electric and the gravitational forces on these grains leads to natural explanations of a number of observed ring phenomena. If plasma processes play a significant role in the dynamics of the ring system at the present time, it is difficult to avoid the conclusion that they also played an important and perhaps crucial role at cosmogonic times during the emplacement and subsequent condensation of the initial dusty plasma. We believe that the Saturnian ring system represents a “time-capsule” containing vital clues about the physical processes operating during the early stages of its formation. We will show that both its overall structure as well as its fine structure, as determined by Voyagers 1 and 2, indicate the crucial importance of plasma processes in its formation and subsequent evolution.     

Particle erosion mechanisms and mass redistribution in Saturn's rings

A variety of physical processes can erode the surfaces of planetary ring particles. According to current estimates, the most efficient of these over the bulk of Saturn's rings is hypervelocity impact by 100 micron to one centimeter radius meteoroids. The atoms, molecules, and fragments ejected from ring particles by erosion arc across the rings along elliptical orbits to produce a tenuous halo of solid ejecta and an extensive gaseous atmosphere. Continuous exchange of ejecta between different ring regions can lead to net radial transport of mass and angular momentum. The equations governing this ballistic transport process are presented and discussed. Both numerical and analytic studies of idealized ring systems illustrate that ballistic transport can cause significant mass redistribution in the rings, especially near regions of high density contrast, such as the inner edges of the A and B rings. Ejecta exchanges can also alter local particle sizes and compositions and may produce pulverized regoliths at least several centimeters deep. The meteoroid erosion rate is so high that significant global torques and mass loss are possible on times shorter than a solar system life time.     

Formation of Saturn's spokes

The scattering properties of the spokes in Saturn's rings suggest that they consist of micron-sized dust particles. We suggest that these grains are elevated above the ring plane by electrostatic charging. We show that electrostatic levitation requires a sufficiently large plasma density near the rings. If the plasma density near the rings exceeds a few 10² cm^?3 levitation may occur at significant rates in the strong electric fields which exist in the wall-sheaths at the ring. Once the dust particles are elevated they drift relative to the plasma (except at synchronous orbit). This relative motion constitutes a current which causes a polarization electric field if the plasma is azimuthally inhomogeneous. The dense plasma will drift radially in response to this electric field and cause levitation of more dust particles as it moves along. It leaves a radially aligned trail of elevated dust particles—the spokes. One way of producing dense plasma is by meteor impact on the rings. We discuss the mechanisms of ring charging, electrostatic levitation and the currents in the plasma-dust mixture. We show that for reasonable conditions spokes of more than 10,000 km radial length can be formed in less than five minutes. We also show that under the same conditions the electrostatic levitation model predicts a dust grain population which peaks at a size of 0.6 microns and can reach optical depths of 0.1.     

由卫星的历表位置测定恒星位置

利用云南天文台1m望远镜上的1号CCD的SRT（分离读出技术）观测天王星卫星的资料,相对卫星的历表位置测定了暗恒星的位置,测定精度与目前国外天然卫星位置测定时的最好精度相当．     

Outstanding issues in understanding the dynamics of the inner plasma sheet and ring current during storms and substorms

The paper deals with five selected issues of the dynamical coupling of the near-Earth plasma sheet and magnetosphere, (1) substorm initiation, (2) dipolarization, (3) pressure release of the outer magnetosphere via the auroral energy conversion process, (4) magnetization of the very high beta plasma assembling at the inner edge of the tail, and (5) penetration of energetic particles into the ring current below L 4. One outstanding and strongly debated subject is not discussed here, the origin of the substorm current wedge. The main conclusions (or personal preferences) are: (1) the substorm is initiated by formation of a near-Earth neutral line; (2) dipolarization occurs through magnetic flux transport by the earthward reconnection flow and not by current diffusion; (3) the auroral energy conversion process, the “auroral pressure valve”, contributes substantially to the pressure release during the substorms; (4) high beta ( 10) plasma breaks up into smaller scale blobs under slow magnetization; and (5) deep and prolonged penetration of hot plasma sheet plasma into the middle magnetosphere produces currents and electric fields which lead to the growth of the storm-time ring current.     

IMF北向时太阳风粒子向磁层输运的试验粒子模拟研究

以ACE卫星实时观测数据驱动的全球磁流体模拟为背景场,选取2003年10月22－24日行星际磁场（IMF）持续北向的事件,使用试验粒子方法,对太阳风粒子向磁层输运的过程进行模拟研究,分析北向IMF下太阳风粒子注入磁层过程中粒子在磁层内的空间分布和时间演化特征。IMF北向期间,进入环电流区域的粒子在晨侧区域的密度大于昏侧,且晨侧的粒子分布范围更广。背阳面磁鞘中的太阳风粒子可以通过低纬边界层进入磁层,但很难通过南北侧磁层顶进入磁层。进入磁尾的太阳风粒子聚集形成冷而密的等离子体片（CDPS）,模拟中CDPS的空间分布和密度大小与观测数据符合。在IMF长时间北向期间,磁尾的粒子数量呈现随时间增长的趋势,并存在约20 min的小幅度准周期变化和约5~6 h的较大幅度的准周期变化。      

Some comments on the F ring-Prometheus–Pandora environment

The system formed by the F ring and two close satellites, Prometheus and Pandora, has been analysed since the time that Voyager visited the planet Saturn. During the ring plane crossing in 1995 the satellites were found in different positions as predicted by the Voyager data. Besides the mutual effects of Prometheus and Pandora, they are also disturbed by a massive F ring. Showalter et al. [Icarus 100 (1992) 394] proposed that, the core of the ring has a mass which corresponds to a moonlet varying in size from 15 to 70 km in radius which can prevent the ring from spreading due to dissipative forces, such as Poynting–Robertson drag and collisions. We have divided this work into two parts. Firstly we analysed the secular interactions between Prometheus–Pandora and a massive F ring using the secular theory. Our results show the variation in eccentricity and inclination of the satellites and the F ring taking into account a massive ring corresponding to a moonlet of different sizes. There is also a population of dust particles in the ring in the company of moonlets at different sizes [Icarus 109 (1997) 304]. We also analysed the behaviour of these particles under the effects of the Poynting–Robertson drag and radiation pressure. Our results show that the time scale proposed for a dust particle to leave the ring is much shorter than predicted before even in the presence of a coorbital moonlet. This result does not agree with the confinement model proposed by Dermott et al. [Nature 284 (1980) 309]. In 2004, Cassini mission will perform repeated observations of the whole system, including observations of the satellites and the F ring environment. These data will help us to better understand this system.     

三维试验粒子轨道法在磁层粒子全球输运中的应用

根据磁层粒子动力学理论, 通过偶极磁场模型验证利用三维试验粒子轨道方法模拟近地球区(r < 8R_e)带电粒子运动特征的可靠性. 在此基础上, 以太阳风和磁层相互作用的全球MHD模拟结果为背景, 利用三维试验粒子轨道方法, 对非磁暴期间南向行星际磁场背景下太阳风离子注入磁层的情形进行数值模拟, 并对北向行星际磁场背景下太阳风离子注入极尖区以及内磁层的几种不同情形进行了单粒子模拟. 模拟结果反映了南向和北向行星际磁场离子向磁层的几种典型输入过程, 揭示出行星际磁场南向时太阳风粒子在磁层内密度分布的晨昏不对称性以及其在磁鞘和磁层内的大致分布, 并得出统计规律. 模拟结果与理论预测和观测结论相一致, 且通过数值模拟发现, 行星际磁场北向时靠近极尖区附近形成的非典型磁镜结构对于能量粒子经由极尖区注入环电流区域过程有重要的影响和作用.      

Highly clumpy structure of the thermal composite supernova remnant 3C 391 unveiled by Chandra

The nature of the internal thermal X-ray emission seen in “thermal composite” supernova remnants is still uncertain. Chandra observation of the 3C391 shows a southeast–northwest elongated morphology and unveils a highly clumpy structure of the remnant. Detailed spatially resolved spectral analysis for the small-scale features reveals normal metal abundance and uniform temperature for the interior gas. The properties of the hot gas comparatively favor the cloudlet evaporation model as a main mechanism for the “thermal composite” X-ray appearance, though radiative rim and thermal conduction may also be effective. A faint protrusion is found in Si and S lines out of the southwest radio border.     

1993年9月12—14日磁暴期间磁层—电离层耦合分析

采用具有明确物理意义的多个地磁指数，以及地面台站链观测的地磁和电离层参数，对一次典型磁暴期内从极光区到赤道附近电离层电流、电场演化发展的耦合过程作了具体分析．结果表明，地磁指数和观测参数能较好地说明磁层－电离层耦合理论结果的主要特征．     

On the importance of fluff on the electric behaviour of cosmic grains

A model is created to describe the effects of “fluff” on the potential and electric field on and close to a charged spherical body embedded in a plasma. The consequences are investigated for dust grains biased at positive or negative potentials, but large enough for electron or ion field emission to be active, especially grains in magnetospheric plasmas. Electron emission reduces the floating potential, whereas ion emission destroys the fluff or even the grain itself. Effects of encounters are discussed. The model also characterizes the levitation of small solid particles from larger bodies.     

磁层高能粒子运动的Fokker Planck方程位置项扩散系数研究

利用准线性理论计算了磁层高能粒子运动的Fokker Planck方程在可观测相空间的位置项扩散系数，并与绝热不变量径向扩散系数进行对比分析.研究发现:位置项扩散系数随径向距离呈现R6的比例关系快速增大.相同径向距离条件下，由于空间位置项z分量的作用，高纬度地区的位置项扩散系数小于低纬度地区.通过与径向扩散系数对比发现，两者具有相同的量级，但两者的相对大小需要根据具体的扰动形态进行分析.此研究对使用测试粒子模拟磁层高能粒子运动，尤其是根据引导中心理论，利用蒙特卡洛方法求解磁层高能粒子运动的Fokker Planck方程，建立磁层空间高能粒子运动的精细化模型具有重要意义.      

A fine mist of very small comet dust particles

A comet nucleus considered as an aggregate of interstellar dust would produce a mist of very finely divided (radius ～ 0.01 μm) particles of carbon and metal oxides accompanying the larger dust grains. These small particles which are very abundant in the interstellar dust size spectrum would provide substantial physical effects because of their large surface area. They may show up strongly in particle detectors on the Halley probes. A strong basis for serious consideration of these particles comes from the other evidence that interstellar dust grains are the building blocks of comets; e.g. (1) the explanation of the “missing” carbon in comets; (2) The S₂ molecule detection which suggests that the comet solid ice materials have been previously subjected to ultraviolet radiation (as are interstellar grains) before aggregation into the comet; (3) the predicted dust to gas ratio.     

Controlling factors of Region 2 field-aligned current and its relationship to the ring current: Model results

One essential component of magnetosphere and ionosphere coupling is the closure of the ring current through Region 2 field-aligned current (FAC). Using the Comprehensive Ring Current Model (CRCM), which includes magnetosphere and ionosphere coupling by solving the kinetic equation of ring current particles and the closure of the electric currents between the two regions, we have investigated the effects of high latitude potential, ionospheric conductivity, plasma sheet density and different magnetic field models on the development of Region 2 field-aligned currents, and the relationship between R2 FACs and the ring current. It is shown that an increase in high latitude potential, ionospheric conductivity or plasma sheet density generally results in an increase in Region 2 FACs’ intensity, but R2 FACs display different local time and latitudinal distributions for changes in each parameter due to the different mechanisms involved. Our simulation results show that the magnetic field configuration of the inner magnetosphere is also an important factor in the development of Region 2 field-aligned current. More numerical experiments and observational results are needed in further our understanding of the complex relationship of the two current systems.     

小天体环的轨道动力学

随着天文观测的深入,发现不仅土星、木星等气态巨行星环绕有光环,某些小天体也环绕有光环。针对目前发现环的半人马型小天体女凯龙星（10199 Chariklo）,研究其引力场中粒子（视为质点）的轨道动力学,分析赤道椭率和小天体自转对环中粒子轨道运动的影响,通过庞加莱截面中的KAM环迭代,得到第一类周期轨道和1∶3共振周期轨道,并通过对比和分析得到小天体光环位置与小天体自转平运动共振的关系。研究结果表明:女凯龙星内环中的粒子最可能处在满足径向振荡幅值范围的第一类周期轨道及其附近的准周期轨道上,但不能排除处在1∶3共振周期轨道及其附近的准周期轨道上;外环中的粒子不可能处在1∶3共振周期轨道上,只可能处在第一类周期轨道及其附近的准周期轨道上。      

Radio occultation by Saturn's rings: Observations of structure and particle size with Voyager 1

Voyager 1 radio occultation study of Saturn's rings gives detailed information regarding the rings' radial structure and particle sizes. Structure within the rings is mapped to a radial resolution of few hundred m in the tenuous parts of ring C and the Cassini Division, and few km over ost of ring A. Fine resolution profiles reveal extremely sharp edges, very narrow gaps, and a host of wave phenomena. Particle size distributions obtained from occultation data within several ring regions are roughly consistent with an inverse cube power law with upper size cutoff in the 5 to 10 m radius range.