用数值模拟的方法研究磁暴主相期间影响环电流分布特征的要素

环电流是距离地心2～7 Re的带电粒子围绕地球西向漂移形成的.环电流的增强将引起全球磁场的降低,反映了地磁暴的强度.磁暴主相期间,对流电场驱动等离子体片中的能量粒子经历E×B漂移与俘获注入环电流,进入损失锥的粒子沉降到大气中.本文采用磁暴主相期间环电流离子分布的模型,结合上述因素研究不同离子能量下对流电场对环电流离子通量的直接影响,以及强弱对流电场下环电流能量离子投掷角的变化,并从物理上阐述造成此种通量分布特性的原因.      

行星际激波对地球磁层的压缩效应分析

2004 年11月9日WIND飞船探测到一个典型的行星际激波. 激波前行星际磁场为持续约50 min的弱南向磁场, 越过激波面, 磁场发生北向偏转且太阳风动压脉冲增强. 在此强动压脉冲增强结构作用下, 磁层被压缩至一个很小的区域. 激波作用于磁层时引起地球同步轨道 各区域高能粒子通量的响应, 但是不同磁地方时的高能粒子通量的响应不同, 表现出双模式扰动, 即在晨昏两侧各能段的电子和质子通量显著增强, 在子夜侧发生类似于亚暴的无色散粒子注入现象. 扰动从向阳面传输到背阳面, 向阳面粒子通量最先增强, 随后背阳面靠近晨昏两侧, 粒子通量开始增强, 最后子夜侧粒子通量表现出无色散高能粒子注入的特点. 另外, 在靠近正午侧, 质子通量先于电子通量发生响应, 在子夜侧电子通量则先于质子通量发生响应. 利用位于向阳面正午两侧的GOES-10 和 GOES-12卫星观测数据发现, 激波作用于磁层时靠近晨侧的磁场变化表现出简单压缩效应, 而靠近昏侧的磁场变化则显然不同, Bx分量减弱, Bz分量几乎减为零, 而By分量则显著增强. 此外, 位于近地磁尾低纬尾瓣区的TC-1卫星观测到激波触发的尾瓣SI现象.      

磁暴期间环电流离子中性原子成像的模拟与观测比较研究

采用已经建立的环电流离子解析模型,结合Chamberlain地冕中性层模型,研究了2004年11月一次大磁暴期间的环电流区域中性原子(ENA)图像.结果表明,模拟的ENA图像与TC-2卫星搭载的中性原子成像仪(NUAUD)的观测图像在方位角或地方时分布、高度或纬度分布和能谱分布方面存在一定的差异.如果依据磁暴发展的不同阶段来选择环电流离子模型的方位角不对称因子和通量最大方向的方位角,增大地冕中性层在低高度区域的密度或者考虑氢(H)以外的其他中性成分,改进注入边界处的离子能谱分布函数,且考虑不同种类环电流离子的比例随磁暴发展可能发生的变化,该模型有望产生更符合观测的模拟ENA图像.     

MMS星座对磁层顶磁通量绳内离子惯性尺度结构的观测

利用MMS观测数据,对磁层顶通量绳内离子惯性尺度（d_i）的结构进行分析研究.结果发现,许多不同尺度（约1d_i至数十d_i）的通量绳内都存在具有d_i尺度的电流 j _m,其方向在磁层顶局地坐标系的-M方向,即与磁层顶查普曼-费拉罗电流同向,由电子在+M方向的运动（ v _em）携带.这些电流结构具有以下特征:磁鞘与磁层成分混合,磁场为开放形态;离子去磁化,电子与磁场冻结;N方向（即垂直于磁层顶电流片方向）的电场 E _n显著增大,幅度达到约20mV·m-1,并伴有明显的尖峰状起伏,该增强和尖峰状起伏的电场对应于霍尔电场.分析表明,电流、电子与离子运动的偏离以及霍尔电场之间遵从广义欧姆定律,三者密切关联.进一步对磁层顶磁重联的探测数据进行分析发现,在很多重联区内也存在与通量绳内相似的结构,其尺度约为d_i量级,其中霍尔电场 E _N、电流 j _M和电子速度 v _eM均与通量绳内对应物理量的方向相同且幅度相近.基于上述观测事实,采用经典FTE通量绳模型,对通量绳内电流、电子运动和霍尔电场的起源进行了初步探讨,认为其来源于磁层顶无碰撞磁重联区内的相应结构,并且后者在离子尺度通量绳的形成过程中起到重要作用.      

2010年4月地球同步轨道相对论电子增强事件分析

为研究2010年4月地球同步轨道相对论电子通量异常增强事件的原因, 选取了2004-2010年之间高速太阳风下7个类似事件进行对比分析. 探讨了多种可能导致此次异常事件的太阳风和地磁条件. 结果表明, 较弱的磁暴使得相对论电子高通量区域更接近同步轨道, 此外, 哨声波加速很可能在2010年4月地球同步轨道相对论电子通量异常增强事件中起到重要作用. 磁暴强度与种子电子的注入深度密切相关, 表现为Dst指数曲线的形态与能量为30～100keV的电子高通量区域的下边缘高度吻合. 能量为30～100keV电子的注入深度影响了能量大于300keV的电子出现的磁层区域. 此事件中, 由于磁暴相对较弱, 种子电子向内磁层注入的深度较浅, 更靠近同步轨道区域, 这使得相对论电子大量出现的区域也靠近同步轨道, 最终导致同步轨道相对论电子通量异常增强. 另外, 2010年4月地球同步轨道相对论电子通量异常增强事件中, 高强度的亚暴提供了充足的种子电子并加强了波粒相互作用, 这也是相对论电子增强的必要条件.      

用场向电流的观测结果研究高纬电离层电场

本文从Triad卫星观测到的场向电流日变化的统计结果出发, 利用场向电流日变化的付里叶级数展开和简单模式法分别求出电导率均匀时和极光带电导率增强时高纬电离层电位的分析解.结果表明, 电场集中在极光区是由2区场向电流引起的.在本文所用的场向电流分布形式下, 加上Pederson电导率的升高。极光区Hall电导率的增大反而有助于电场向中纬穿透.|AL|≥100γ时, 场向电流分布对对流圈位置西向旋转起一定作用, 但极光带Hall电导率的变化是造成大角度旋转的主要原因.Perdson电导率的增大, 对旋转角无影响.结果还表明, 在不考虑电导率日夜不均匀时, 由于场向电流复杂的日变化, 也可出现对流圈的晨昏不对称性.以上的电场分布形态, 与观测的电场形态基本相符。      

磁扰动和磁静时近地等离子体片中脉冲电场的分布

Ｇｅｏｔａｉｌ卫星的电场数据被用于分析近地磁尾等离子体片中电场在磁扰动（Ｄｓｔ＜－２５ｎＴ）和磁静时（Ｄｓｔ＞－２５ ｎＴ的统计分布．结果表明，伴随着地向高速离子流，在Ｘ＞－１６Ｒｅ以内区域出现强电场（高达 ５—８ ｍＶ／ｍ）．磁扰动期间强电场的幅值较磁静时大，并且出现在更靠近地球的位置．较强和较靠近地球的强电场与磁扰动时更薄的等离子体片和更接近地球的等离子体片内边界相联系．观测结果意味着磁扰动期间的亚暴可能更有效地将高能粒子注射到环电流中．这对磁暴和亚暴的关系问题的解决有重要意义．     

木星环绕探测任务中的内带电风险评估

木星拥有类似地球辐射带的辐射带结构,其辐射带质子通量是地球的10倍,高能电子通量比地球高2~3个数量级,且最高能量可达1 Ge V。因此木星探测任务的抗辐射设计是任务成功的关键。选择3种不同倾角大椭圆探测轨道,仿真分析了2种介质在变化能谱下的内带电过程。仿真结果表明,对于环氧树脂(Fr4),由于电阻相对较小,电子通量较大的近木点的充电电荷,会在远离辐射带时泄放,其最大充电电场取决于近木点的电子通量;对于聚酰亚胺(Kapton),由于电阻相对较大,充电电荷不能及时泄放,不同轨道间电荷逐渐累计,最大电场不断增加。另外,环木轨道倾角越大,越有利于降低充电电场。和地球GEO轨道相比,不同电阻介质在环木轨道的充电差异相对地球GEO轨道较小。     

磁尾粒子在晨昏电场作用下的散射与沉降

我们认为,在沉降粒子的形成中,磁尾粒子的散射是极其重要的。本文从粒子的轨道理论人手,通过对近900条粒子轨道的具体计算,研究了磁尾粒子在晨昏电场作用下的投掷角变化。计算结果清楚地显示出了磁尾粒子在晨昏电场的作用下,经历着明显的散射过程。运动经过非小扰动区的磁尾粒子在非小扰动区内经多次反射,磁矩不再是不变的,从而投掷角改变,使得一个原在磁尾为各向同性的投掷角分布,在粒子运动到远离中性线的近地区域时改变成为一个近麦氏分布。我们还研究了这些粒子的空间经历,发现这些离开磁尾进入近地区域的粒子在发生散射的同时还发生了空间分离——晨昏分离、纬向分离以及质子和电子之间的相对空间分离,给出了清晰的粒子沉降图象。      

代数重建法反演环电流分布的初步结果

TC-2卫星携带了高能中性原子成像仪.在理论上可以根据对中性原子的探测通过反演获得磁层中环电流的空间分布.开发了一种新的反演技术和算法来反演环电流的分布.文中将层析成像技术引入反演环电流区离子分布,把磁层空间环电流区域划分为网格并依据层析成像常用的代数重建法反演环电流的分布.反演前后的电流离子分布具有一致性,证实了这种反演方法的可行性.通过对比反演结果与初始分布,模型通过较少的假设和简单的步骤就可以获得全面的环电流信息.      

准平行无碰撞激波上游能量粒子的观测研究

通过Cluster卫星在2005年3月16日观测到的一个准平行激波观测事例,研究了准平行激波上游低频等离子体波动与能量离子之间的关系.卫星观测结果表明,在准平行激波上游,离子微分能通量受到了非线性波动的调制.在磁场强度较小区域,离子微分能通量较高.产生这种现象的可能原因是准平行激波上游的非线性波动可以捕获离子,被捕获的离子在波动中来回弹跳并被电场加速,从而导致磁场强度较小区域离子微分能通量较高.这一观测结果与已有的混合模拟结果相吻合.      

Registration of ELF waves in rocket-satellite experiment with plasma injection

Two rocket experiments KOMBI-SAMA with plasma injection at height 100–240 km were performed in August 1987 in the region of Brazilian magnetic anomaly (L = 1.25). The launching time of the rocket was determined so that plasma injection was at the time when satellite COSMOS 1809 passed as close as possible to magnetic tube of injection. Caesium plasma jet was produced during ≥ 300 s by electric plasma generator separated from the payload. By diagnostic instruments on board of the rocket and the satellite were registered energetic particle fluxes and plasma wave activities stimulated by plasma injection. When the satellite passed the geomagnetic tube intersecting the injection region an enhancement of ELF emission at 140 Hz, 450 Hz by 2 times was registered on board the satellite. An enhancement of energetic particles (E > 40 keV) flux by 4–5 times was registered on board the rocket. Observed ELV emission below 100 Hz is interpreted as generation of oblique electromagnetic ion-cyclotron waves due to drift plasma instability at the front of the plasma jet.     

Some peculiarities of longitudinal distribution of proton fluxes at high latitudes

Dynamical features of proton fluxes at high and middle latitudes were studied based on data measured by Sun-synchronous low-altitude (800 km height) polar-orbiting three NOAA series satellites: POES 15, 16, and 17 during the geomagnetic storm on January, 21–22, 2005. Data from three satellites that passed the Northern hemisphere along different MLTs allow reconstructing the longitudinal distribution of the proton fluxes. Measurements of protons with energies of 30–80 keV and 80–240 keV (the ring current energy range) by 0- and 90-detectors were used to evaluate and compare the longitudinal asymmetry of proton flux distribution measured in the regions equatorward and poleward of the isotropic boundary. It was found that during all the phases of the geomagnetic storm distribution of the maximum flux of precipitating protons (0-detector data) is sufficiently asymmetric. The maximal flux position along MLT is moving from pre-midnight sector in quiet time to post-midnight one before and during SSC and moving back during recovery phase. The longitudinal distribution of precipitation maxima demonstrates the local increase in afternoon sector (approximately at 13:30 MLT) and decrease in the dusk one during SSC. These features are evident consequence of the magnetosphere compression. To identify the origin of the particles, the locations of maximum fluxes have been projected to the magnetosphere. It was determined that during geomagnetic storm main and recovery phases maximum fluxes were measured at latitudes poleward of the isotropic boundary. To evaluate the trapped particle flux asymmetry, the particle fluences (90-detector data) were calculated along the satellite orbit from L = 2 to the isotropic boundary. The total fluences of trapped particles calculated along the satellite orbit show regular asymmetry between dusk and dawn during main and recovery phases. The maximal intensity of proton fluxes of both investigated populations located poleward and equatorward of the isotropic boundary is achieved during SSC. The total flux measured during crossing the anisotropic region can be considered as a proxy for ring current injection rate.     

磁场重联的二维粒子模拟研究

利用二维全粒子模拟方法研究了无碰撞等离子体中的磁场重联过程，得到了不同区域的离子和电子速度分布．计算结果表明，电子和离子在扩散区中的不同动力学特性产生的Hall电流使磁场的y分量By呈现四极形分布．离子和电子的速度分布偏离了初态时的Maxwell分布，呈现非局域的多重分布．同时由于磁场重联而产生的电场使电子在X点附近得到加速和加热，因而在电子的能谱分布中形成-高能尾。     

Circulation of energetic ions of terrestrial origin in the magnetosphere

Energetic ion composition measurements have now been performed from earth orbiting satellites for more than a decade. As early as 1972 we knew that energetic (keV) ions of terrestrial origin represented a non-negligible component of the storm time ring current. We have now assembled a significant body of knowledge concerning energetic ion composition throughout much of the earth's magnetosphere. We know that terrestrial ions are a common component of the hot equatorial magnetospheric plasma in the ring current and the plasma sheet out to ? 23 R_E. During periods of enhanced geomagnetic activity this component may become dominant. There is also clear evidence that the terrestrial component (specifically O⁺) is strongly dependent on solar cycle. Terrestrial ion source, transport, and acceleration regions have been identified in the polar auroral region, over the polar caps, in the magnetospheric boundary layers, and within the magnetotail lobes and plasma sheet boundary layer. Combining our present knowledge of these various magnetospheric ion populations, it is concluded that the primary terrestrial ion circulation pattern associated with enhanced geomagnetic activity involves direct injection from the auroral ion acceleration region into the plasma sheet boundary layer and central plasma sheet. The observed terrestrial component of the magnetospheric boundary layer and magnetotail lobes are inadequate to provide the required influx. They may, however, contribute significantly to the maintenence of the plasma sheet terrestrial ion population, particularly during periods of reduced geomagnetic activity. It is further concluded, on the basis of the relative energy distributions of H⁺ and O⁺ in the plasma sheet, that O⁺ probably contributes significantly to the ring current population at energies inaccessible to present ion composition instrumentation (? 30 keV).     

The collective gyration of a heavy ion cloud in a magnetized plasma

In both the ionospheric barium injection experiments CRIT I and CRIT II, a long-duration oscillation was seen with a frequency close to the gyro frequency of barium and a time duration of about one second. A model for the phenomenon which was proposed for the CRIT 1 experiment is here compared to the results from CRIT II which made a much more complete set of measurements. The model follows the motion of a low-β ion cloud through a larger ambient plasma. The internal field of the model is close to antiparallel to the injection direction v_i but slightly tilted towards the self-polarization direction E_P = −v_ixB. As the ions move across the magnetic field, the space charge is continuously neutralized by magnetic-field aligned electron currents from the ambient ionosphere, drawn by the divergence in the perpendicular electric field. These currents give a perturbation of the magnetic field related to the electric field perturbation by ΔE/ΔB ≈ V_a. The model predictions agree quite well with the observed vector directions, field strengths, and decay times of the electric and magnetic fields in CRIT II. The possibility to extend the model to the active region, where the ions are produced in this type of self-ionizing injection experiments, is discussed.