磁层亚暴膨胀相的近地触发模型

本文指出现有亚暴的中性线模型其源区在赤疲乏面上离地球太远；以ＧＥＯＳ－２的观测资料为依据，提出了亚暴膨胀相的一个近地触发模型－气球模不稳定性模型，该模型认为，在增长相期间到达Ｒ≈（６－１０）ＲＥ的近地等离子体片内边缘区，出现指向地球方向的离子压强梯度，越尾电流强度增大，磁力线向磁尾拉伸。当等离子体片变薄，电子沉降增强，极光带电离层电导率骤增时，气球模不稳定性在近地等离子体片内边缘区被激发，场向电流     

磁尾瓣区的场向电流

本文利用ISEE-2卫星的磁场和粒子资料(电子:75keVδ<1300keV,质子:170keVp<400keV),发现在磁尾远离等离子片的尾瓣区,常常同时探测到粒子脉冲和横向磁场扰动,表明有场向电流片存在。电流片的积分强度在3.3—21mA/m之间,与Frank等在磁尾等离子片边界上测量到的场向片电流积分强度可相比较。电流片总是成双成对,电流片的强度与AE指数或亚暴的关系密切。和磁层其他区域不同,在磁尾瓣区,经常探测到△B_x和△B_y同时存在,且△B_x和△B_y可相比拟的情形,它们可以用运动的线电流或不均匀密度的电流片来解释。      

等离子体彗尾动力学的数值研究

本文基于可压缩磁流体动力学模型,数值研究了尾瓣巾具有超Alfven速流动的等离子体彗尾的动力学特征。结果表明,等离子体片和尾瓣之间的剪切等离子体流动将会激发流动撕裂模不稳定性,引起彗尾等离子体片中发生磁场重联,形成磁岛和高密度的等离子体团。进而模拟了太阳风引起的局部驱动力对等离子体彗尾中磁场重联的影响,其特征时间远大于流动撕裂模。我们认为一些观测到的等离子体彗尾中的四块和彗尾截断事件可能主要与彗尾中剪切等离子体流动所引起的流动撕裂模不稳定性有关。      

高纬磁层顶尾部边界层的离子流剪切不稳定性

研究了高纬磁层顶层部边界层在非均匀磁场条件下的离子流剪切不稳定性，由于考虑了尾向电流，我们发现离子流剪切不稳定性的激发与扰动波长有关，并得到了激发该不稳定性的准临界波长的表达式，对于磁层顶存在的波长为地球半径量级的表面波扰动，离子流剪切不稳定性倾向于发生在磁层顶边界层的内边界，沿磁层顶外边界传播的稳定的表面波和其内边界的离子流剪切不稳定性同时存在，这将有助于解释磁鞘太阳与动量持续不断地向地磁层的传输。     

磁尾静电离子束流-密度漂移不稳定性的数值研究

本文研究了磁尾等离子体片边界层宽频带静电噪声的产生机制.模型等离子体由暖的背景电子、暖的向地球方向的离子束流和较冷的向尾方向的离子束流组成.结果表明,静电离子束流-密度漂移不稳定性可以在比较宽的频率范围内激发,在低频区大传播角方向上增长率最大,在高频区小传播角方向上增长率也比较大。最大增长率的方向取决于离子束流和密度漂移的速度比值.这些结果与磁尾观测到的宽频带静电噪声特征符合一致.      

有限β等离子体中密度和磁场不均匀驱动的动理学Alfven波

在分析有限β等离子体中的密度、磁场不均匀引起的漂移波不稳定性的基础上，剖析了漂移波不稳定性对动理学Alfven波激发的作用．动理学理论能正确地处理有限拉莫半径效应和波粒共振相互作用，本文根据带电粒子在电磁场中的运动特性，采用Vlasov方程描述离子运动，运用漂移动理学方程对电子运动进行描述．密度不均匀和磁场不均匀对产生漂移不稳定性的对比分析表明：在有限β等离子体中，密度不均匀比磁场不均匀更易激发漂移不稳定性，且密度不均匀激发漂移不稳定性中的能量转换和转移更为强烈．这种能量的转换为动理学Alfven波的激发提供了物理基础．所得数值解表明：动理学Alfven波在磁层中能广泛地被激发产生，特别是在磁层空间的极尖区、磁层顶和等离子体片边界层等具有明显的不均匀性区域中更容易被激发产生．本文的研究结果进一步表明动理学Alfven波对磁层空间中能量传输具有重要作用．     

磁尾的平衡结构

考虑到等离子体片和等离子体幔的存在,我们从Vlasov方程出发得出一个自洽的二维磁尾平衡模式。在这个模式中,假设等离子体处于平衡状态,压强是各向同性的,还假设沿磁尾方向各种参数的变化率较垂直于等离子体片方向的变化率要小得多。晨昏方向的变化则完全忽略不计。这个模式体现出磁尾的一些主要结构要素:(ⅰ)高温等离子体组成的等离子片;(ⅱ)等离子体片中的闭合磁力线;(ⅲ)等离子体极其稀薄的磁尾瓣;(ⅳ)等离子体幔;(ⅴ)磁层顶;(ⅵ)张开的磁尾。讨论了等离子体幔的存在对磁尾平衡结构的影响,结论是等离子体幔的存在使磁尾的形状更趋于尾形,于是,至少部分克服了Birn模式的困难。      

行星际磁场对磁尾场向电流发生率的影响

利用Cluster卫星的磁场和等离子体探测数据, 研究了行星际磁场(IMF)时钟角(clock angle) Φ和锥角(cone angle) θ对磁尾等离子体片边界层(PSBL)区场向电流发生率的影响. 当时钟角Φ >0时, 磁尾场向电流 的发生率较高, 这表明磁尾场向电流的发生与昏向太阳风条件更为密切; 当 90°<|Φ|<180°时, 场向电流的发生率较高, 这表明 场向电流的发生与南向IMF更为密切. 当锥角θ <30°时(即IMF与 日地连线夹角较小时)场向电流的发生率较低. 而当θ> 30°时, 场向电流在90°<|Φ|<180°的情况下发生率明显增大, 这说明南向IMF情况下, 场向电流发生率明显增大. 但是当|Φ|<90°时 (北向IMF情况下), 尽管θ很大, 场向电流的发生率并未明显增大. 当θ>70°时, 且在140°< < i>Φ<160°的行星 际磁场条件下, 磁尾等离子体片边界层区场向电流的发生率最大.      

晨昏电场对磁亚暴触发的影响

本文讨论了晨昏电场存在时磁尾等离子体片内撕裂模不稳定性的激发和增长问题。得到的结果可以用来解释晨昏电场对磁亚暴的触发以及加快等离子体片贮存能量的耗散等现象。我们推导了存在电场漂移时的撕裂模方程,并在均匀电流片模型下求解了该方程的解析解。结果表明触发不稳定性所允许的临界电流片宽度与电场大小近似成正比,这表明考虑晨昏电场效应后磁亚暴更易被激发。此外对于厚度相同的电流片来说,长波撕裂模的增长率也随晨昏电场的加强而增大。      

太阳纯电子事件和质子-电子事件的电子能谱的机制

本文计算、分析了太阳耀斑加速电子在日冕中传输时激发的等离子体尾场的效应,认为耀斑电子的高能成份激发的尾场,能够加速低能耀斑电子,低能耀斑电子的能量增值可达几十keV至上百keV,这种尾场加速将软化约100keV以下的能量范围内(探测阈之上)的耀斑电子能谱。结合考虑尾场效应,本文提出了太阳耀斑加速电子从加速区到形成电子事件之间的能谱演化模式,说明了太阳纯电子事件的双幂律电子能谱和太阳质子-电子事件的单幂律电子能谱的形成,认为两类事件的电子能谱差异为耀斑电子日冕传输中不同程度的尾场效应所致,前者尾场效应弱,电子能谱呈双幂律,后者尾场效应较强,电子能谱为单幂律谱。      

The plasma sheet and boundary layers under northward IMF: A multi-point and multi-instrument perspective

During conditions of northward interplanetary magnetic field (IMF), the near-tail plasma sheet is known to become denser and cooler, and is described as the cold-dense plasma sheet (CDPS). While its source is likely the solar wind, the prominent penetration mechanisms are less clear. The two main candidates are solar wind direct capture via double high-latitude reconnection on the dayside and Kelvin–Helmholtz/diffusive processes at the flank magnetopause. This paper presents a case study on the formation of the CDPS utilizing a wide variety of space- and ground-based observations, but primarily from the Double Star and Polar spacecraft on December 5th, 2004. The pertinent observations can be summarized as follows: TC-1 observes quasi-periodic (∼2 min period) cold-dense boundary layer (compared to a hot-tenuous plasma sheet) signatures interspersed with magnetosheath plasma at the dusk flank magnetopause near the dawn-dusk terminator. Analysis of this region suggests the boundary to be Kelvin–Helmholtz unstable and that plasma transport is ongoing across the boundary. At the same time, IMAGE spacecraft and ground based SuperDARN measurements provide evidence of high-latitude reconnection in both hemispheres. The Polar spacecraft, located in the southern hemisphere afternoon sector, sunward of TC-1, observes a persistent boundary layer with no obvious signature of boundary waves. The plasma is of a similar appearance to that observed by TC-1 inside the boundary layer further down the dusk flank, and by TC-2 in the near-Earth magnetotail. We present comparisons of electron phase space distributions between the spacecraft. Although the dayside boundary layer at Polar is most likely formed via double high-altitude reconnection, and is somewhat comparable to the flank boundary layer at Double Star, some differences argue in favour of additional transport that augment solar wind plasma entry into the tail regions.     

A statistical analysis of substorm associated tail activity

Substorm onset timing is a critical issue in magnetotail dynamics research. Solar wind energy is accumulated in the magnetosphere and the configuration of the magnetosphere evolves toward an unstable state during the growth phase. At some point, the expansion phase begins and the stored energy is released through a variety of processes that return the magnetosphere to a lower energy state. In recovery the various processes die away. Unfortunately, the ground and magnetospheric signatures of onset, i.e. energy release, can be seen both in the growth phase prior to onset and in the expansion phase after onset. Some investigators refer to each of these events as a substorm. Tail observations suggest that most substorms have one event that differentiates the behavior of the tail field and plasma. We refer to this time as the “main substorm onset”. Each substorm associated phenomenon is timed independently and then compared with main substorm onsets. ISEE-2 tail observations are used to examine the tail lobe magnetic conditions associated with substorms because ISEE-2 orbit has a high inclination and frequently observes lobe field. Approximately 70 ∼ 75% of tail lobe B_t and B_z change are associated with the main substorm onset. If the satellite is more than 3 Re above (below) the neutral sheet, 86% (57%) of plasma pressure dropouts are associated with substorms. We interpret our results as evidence that the effect of the growth phase is to drive the magnetosphere towards instability. As it approaches global instability local regions become temporarily unstable but are rapidly quenched. Eventually one of these events develops into the global instability that releases most of the stored energy and returns the magnetosphere to a more stable configuration.     

ISEE-1 and 2 observations of field-aligned currents in the distant midnight magnetosphere

Magnetic field measurements obtained in the nightside magnetosphere by the co-orbiting ISEE-1 and 2 spacecraft have been examined for signatures of field-aligned currents (FAC). Such currents are found on the boundary of the plasma sheet both when the plasma sheet is expanding and when it is thinning. Plasma sheet boundary layer current structure and substorm associated dynamics can be determined using the two spacecraft, although for slow traversals of the FAC sheet the spatial/temporal ambiguity is still an issue. We often find evidence for the existence of waves on the plasma sheet boundary, leading to multiple crossings of the FAC sheet. At times the boundary layer FAC sheet orientation is nearly parallel to the X-Z GSM plane, suggesting ‘protrusions’ of plasma sheet into the lobes. The boundary layer current polarity is, as expected, into the ionosphere in the midnight to dawn local time sector, and outward near dusk. Current sheet thicknesses and velocities are essentially independent of plasma sheet expansion or thinning, having typical values of 1500 km and 20–40 km/s respectively. Characteristic boundary layer current densities are about 10 nanoamps per square meter.     

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).     

Current fluctuations in the Earth''s magnetosphere caused by lower-hybrid-drift turbulence

Nonlinear calculations of the anomalous electrical conductivity in the plasma of the earth's plasma mantle, the tail plasma sheet boundary layer and the ionospheric F-region density-trough are presented provided that lower-hybrid-drift turbulence exists. It is shown that in these regions the stabilization of the wave growth is mainly caused by current relaxation. Further, the fluctuations of the electrical currents are estimated via Ampère's and Ohm's laws. It is found that the lower-hybrid-drift turbulence causes maximum anomalous collision frequencies of the order of 10⁻² Hz in the magnetosphere. The maximum current fluctuations are about 3 10⁻⁹ A/m². The theoretical results are in agreement with ISEE and Prognoz-8 measurements.     

Magnetospheric Boundary Layer Structure and Dynamics as Seen From Cluster and Double Star Measurements

In this review, we discuss the structure and dynamics of the magnetospheric Low-Latitude Boundary Layer (LLBL) based on recent results from multi-satellite missions Cluster and Double Star. This boundary layer, adjacent to the magnetopause on the magnetospheric side, usually consists of a mixture of plasma of magnetospheric and magnetosheath origins, and plays an important role in the transfer of mass and energy from the solar wind into the magnetosphere and subsequent magnetospheric dynamics. During southward Interplanetary Magnetic Field (IMF) conditions, this boundary layer is generally considered to be formed as a result of the reconnection process between the IMF and magnetospheric magnetic field lines at the dayside magnetopause, and the structure and plasma properties inside the LLBL can be understood in terms of the time history since the reconnection process. During northward IMF conditions, the LLBL is usually thicker, and has more complex structure and topology. Recent observations confirm that the LLBL observed at the dayside can be formed by single lobe reconnection, dual lobe reconnection, or by sequential dual lobe reconnection, as well as partially by localized cross-field diffusion. The LLBL magnetic topology and plasma signatures inside the different sub-layers formed by these processes are discussed in this review. The role of the Kelvin-Helmholtz instability in the formation of the LLBL at the flank magnetopause is also discussed. Overall, we conclude that the LLBL observed at the flanks can be formed by the combination of processes, (dual) lobe reconnection and plasma mixing due to non-linear Kelvin-Helmholtz waves.