The transformation of magnetoacoustic waves into Alfvén waves inside the magnetosphere

A mechanism for the transformation of a magnetoacoustic wave into an Alfvén wave is proposed. During the compression of the magnetosphere by the solar wind the inner edge of the plasma sheet and the contours of B=const move in different ways. In the case of asymmetrical compression, the contours of B=const will cross the inner edge of the plasma sheet. To close the drift currents - that flow in the plasma sheet along the contours of B=const - the appearance of the field-aligned currents is necessary. This appearance corresponds to the generation of the Alfvén wave.     

Interchange instability as a mechanism of magnetotail current disruption at the magnetic storm recovery phase

The theory of relaxation of geomagnetic depression observed during magnetic storm has been proposed based on the knowledge of the interaction between the Dst index and the magnetotail current. It has been indicated that the disruption of the tail current that was enhanced during the storm main phase can be caused by the interchange instability, which develops in the boundary plasma sheet due to the sheet curvature in the near to the Earth region and due to a sharp, directed toward the curvature center, plasma pressure gradient at the boundary between the plasma sheet and the tail lobe. The dispersion equation and expression for the instability growth rate have been obtained. The theoretically predicted characteristic time of storm depression relaxation τ is ～10 h and is in good agreement with the experimental estimate.     

地球远磁尾中的磁场重联——（Ⅱ）局部驱动力的影响

基于可压缩磁流体动力学模型,数值研究了由太阳风引起的局部驱动力对地球远磁尾中磁场重联的影响.结果表明,在远磁尾等离子体片中将发生强迫磁场重联,并形成磁岛和等离子体团.形成磁岛的特征时间很大于流动撕裂模不稳定性引起磁岛非线性饱和的特征时间.磁岛宽度随着磁Reynolds数S的增大而减小,随着尾瓣中等离子体压力与磁压之比值β_∞的降低而减小.认为太阳风引起的局部驱动力对地球远磁尾等离子体片中磁场重联的影响,可能不如流动撕裂模不稳定性那样显著.     

地球远磁尾中的磁场重联——（Ⅱ）局部驱动力的影响

基于可压缩磁流体动力学模型,数值研究了由太阳风引起的局部驱动力对地球远磁尾中磁场重联的影响.结果表明,在远磁尾等离子体片中将发生强迫磁场重联,并形成磁岛和等离子体团.形成磁岛的特征时间很大于流动撕裂模不稳定性引起磁岛非线性饱和的特征时间.磁岛宽度随着磁Reynolds数S的增大而减小,随着尾瓣中等离子体压力与磁压之比值β_∞的降低而减小.认为太阳风引起的局部驱动力对地球远磁尾等离子体片中磁场重联的影响,可能不如流动撕裂模不稳定性那样显著.     

伴随着高速流的磁尾电流片拍动观测研究

磁尾电流片在磁尾动力学过程中起着重要作用.卫星观测表明磁尾电流片经常处于拍动状态.但磁尾电流片拍动的特性和产生机制至今仍然没有被完全弄清楚.本文主要利用欧洲空间局Cluster卫星数据,研究一个伴随高速离子流的电流片拍动事件.该电流片拍动事件具有很强的周期性.拍动的周期约是2 min,磁场振荡幅度约为20 nT.能量电子和离子的通量具有周期性增强和减弱的特征.电流密度X和Y分量也具有周期性的振荡,并且振荡周期与磁场振荡周期一致.通过对粒子流速矢量与电流矢量的分析,发现粒子运动具有涡旋的特征.因此可以推断,该磁尾电流片的拍动不是由磁尾等离子体片高速流产生的,而是与局地等离子体不稳定性有关.     

地球磁尾等离子体片磁洞的统计分析

本文利用THEMIS卫星的磁场数据和等离子体观测数据,统计分析地球磁尾等离子体片区域线性磁洞的发生率、时空尺度、分布特征、和发生率与地磁AE指数的相关性.分析结果表明磁尾等离子体片区域的磁洞的时间尺度为几秒到几十秒,空间尺度小于当地的质子回旋半径.通过磁洞在空间的位置分布和卫星数据在空间的数据采样分布的对比,我们发现线性磁洞在等离子体片内经常发生,然而在磁尾等离子体片中的发生率要小于太阳风中磁洞的发生率.本文最后统计分析了磁洞发生和AE指数的相关性,结果表明磁洞可能与地磁活动有关系.

     

等离子体片边界层内静电离子束流-密度漂移不稳定性

本文研究了地球磁尾等离子体片边界层内由离子束流和等离子体密度梯度联合作用产生的静电不稳定性.模型等离子体由向尾流动的冷离子束流、向地球流动的暖离子束流和背景暖电子组成,等离子体密度是非均匀的,等离子体β（热压强与磁压强之比值）很小,电子等离子体频率与电子退旋频率之比。ω_e/Ω_e》1.结果表明,斜传播的静电快、慢离子束流-密度漂移模能够被激发。     

Formation of accelerated ion flows in Alfvén disturbances of the magnetotail

This paper shows that there exists a mechanism of longitudinal plasma acceleration which is inherent in the process of the process of resonant conversion of a fast magnetosonic wave freely propagating along the magnetic field into an Alfvén wave. This mechanism is caused by the Ampere force arising due to the interaction between the poloidal component of the current of the compressible disturbance and the generated toroidal disturbance. It is shown that plasma acceleration takes place at the stage of increase in the Alfvén wave amplitude and that the accelerated flow retains its velocity when the process of resonant conversion is over. We describe spatiotemporal structures of plasma flows arising with the transformation of fast magnetosonic waves into Alfvén waves. An interpretation of the presence of fast ion flows in the magnetotail as a consequence of the action of the plasma acceleration mechanism considered in this work is proposed.     

Shear flow instabilities in the earth’s plasma sheet region

Shear flow instability arising from the velocity shear between the inner and the outer central plasma sheet regions is studied by treating the plasma as compressible. Based on the linearized MHD equations, dispersion relations for the surface wave modes occurring at the boundary of the inner central plasma sheet (ICPS) and the outer central plasma sheet (OCPS) are derived. The growth rates and the eigenmode frequencies are obtained numerically. Three data sets consisting of parameters relevant to the earth’s magnetotail are considered. The plasma sheet region is found to be stable for constant plasma flows unless M_A>9.6, where M_A is the Alfvén Mach number in the ICPS. However, for a continuously varying flow velocity profile in the ICPS, the instability is excited for M_A\geq1.4. The excited modes have oscillation periods of 2–10 min and 1.5–6 s, and typical transverse wavelengths of 30–100 R_E and 0.5–6 R_E for data sets 1 and 2 (i.e., case of no neutral sheet) respectively. For the data set 3, which corresponds to a neutral sheet at the center of the plasma sheet, the excited oscillations have periods of 2 s-1 min with transverse wavelengths of 0.02–1 R_E.     

等离子体片边界层内静电离子束流-密度漂移不稳定性

本文研究了地球磁尾等离子体片边界层内由离子束流和等离子体密度梯度联合作用产生的静电不稳定性.模型等离子体由向尾流动的冷离子束流、向地球流动的暖离子束流和背景暖电子组成,等离子体密度是非均匀的,等离子体β（热压强与磁压强之比值）很小,电子等离子体频率与电子退旋频率之比。ωe/Ωe》1.结果表明,斜传播的静电快、慢离子束流-密度漂移模能够被激发。     

Large-Scale Variations in Ion Flows in the Central Part and the Boundary of the Plasma Sheet

Geomagnetism and Aeronomy - The results of multisatellite observations in a magnetotail at short distances ( $${\sim }10{{R}_{E}}$$ ) in two close regions inside the plasma sheet of the magnetotail...     

地球磁尾中不同类型磁结构的磁螺度演化特征

在二维三分量MHD数值模拟的基础上 ,对地球磁尾不同类型磁结构的形成作磁螺度分析 .研究表明 ,对于由晨昏电场产生的磁尾驱动重联过程 ,通过系统边界输运的磁螺度通量是引起系统总磁螺度变化的直接原因 .不同的初始磁螺度密度分布和磁螺度通量输运 ,可以引起中性片区域磁螺度密度分布的不同演化 ,从而导致具有不同拓扑位形磁结构的形成 .     

地球磁尾中不同类型磁结构的磁螺度演化特征

在二维三分量MHD数值模拟的基础上 ,对地球磁尾不同类型磁结构的形成作磁螺度分析 .研究表明 ,对于由晨昏电场产生的磁尾驱动重联过程 ,通过系统边界输运的磁螺度通量是引起系统总磁螺度变化的直接原因 .不同的初始磁螺度密度分布和磁螺度通量输运 ,可以引起中性片区域磁螺度密度分布的不同演化 ,从而导致具有不同拓扑位形磁结构的形成 .     

On the remote sensing of plasma sheet from low-altitude spacecraft

The possibility to estimate plasma sheet parameters from low-altitude measurements looks quite attractive, but it critically depends on how isotropic the plasma pressure is in the flux tube. To evaluate the ion pressure anisotropy we compare the values of pressure in the ionospheric and equatorial parts of the field line. Ionospheric values were computed from proton measurements at NOAA low-altitude satellites, they were compared with pressure estimates computed from empirical magnetic field models as well as with average values known from direct plasma sheet measurements. Three different methods of mapping the plasma pressure from plasma sheet to low altitude have been tried; each uses the particle isotropic boundaries observed at low altitudes and/or computed from magnetospheric models. Excluding observations obtained during substorm expansion, from these comparisons we conclude that in the plasma sheet, at geocentric distances 9–20R_E, the pressure estimates in the ionospheric and equatorial parts of the plasma sheet flux tube agree very well, suggesting a good pressure isotropy and thus justifying a possibility to monitor the plasma sheet parameters based on low-altitude measurements. The results also illustrate the usefulness of isotropic boundaries as a label of tail current intensity and as reliable tool for establishing mapping between magnetosphere and ionosphere.     

基于CARISMA地磁台链观测的IPDP事件的统计分析

亚暴期间磁尾等离子体片离子注入内磁层能够激发电磁离子回旋(EMIC)波.对应于这种EMIC波,地面磁力仪可观测到周期逐渐减小的地磁脉动(IPDP).利用GOES卫星数据,地磁指数和加拿大CARISMA地磁台站的数据,我们研究了IPDP事件的产生与亚暴磁尾注入的关系.同时利用CARISMA地磁台链中的MCMU和MSTK两个台站,从2005年4月到2014年5月期间的观测数据,统计分析了亚暴期间的IPDP事件,研究了IPDP事件的出现率关于季节和磁地方时的分布特征.我们总共获得128个两个台站同时观测的IPDP事件.该类事件关于季节分布的发生率,冬季最小,为13.28%,春季最大,为32.81%,结果表明IPDP事件关于季节分布的发生率受到电离层电导率及亚暴发生率的影响.两个台站同时观测到的IPDP事件最大出现率出现在15—18 MLT(磁地方时),结果表明IPDP事件主要由亚暴期间产生的能量离子注入内磁层,西向漂移遇到等离子体层羽状结构(Plume)区的高密度等离子体所激发.     

Poker flat radar observations of the magnetosphere–ionosphere coupling electrodynamics of the earthward penetrating plasma sheet following convection enhancements

The plasma sheet moves earthward (equatorward in the ionosphere) after enhancements in convection, and the electrodynamics of this response is strongly influenced by Region 2 magnetosphere–ionosphere coupling. We have used Poker Flat Advanced Modular Incoherent Scatter Radar (PFISR) observations associated with two relatively abrupt southward turnings of the IMF to provide an initial evaluation of aspects of this response. The observations show that strong westward sub-auroral polarization streams (SAPS) flow regions moved equatorward as the plasma sheet electron precipitation (the diffuse aurora) penetrated equatorward following the IMF southward turnings. Consistent with our identification of these flows as SAPS, concurrent DMSP particle precipitation measurements show the equatorial boundary of ion precipitation equatorward of the electron precipitation boundary and that westward flows lie within the low-conductivity region between the two boundaries where the plasma sheet ion pressure gradient is expected to drive downward R2 currents. Evidence for these downward currents is seen in the DMSP magnetometer observations. Preliminary examination indicates that the SAPS response seen in the examples presented here may be common. However, detailed analysis will be required for many more events to reliably determine if this is the case. If so, it would imply that SAPS are frequently an important aspect of the inner magnetospheric electric field distribution, and that they are critical for understanding the response of the magnetosphere–ionosphere system to enhancements in convection, including understanding the earthward penetration of the plasma sheet. This earthward penetration is critical to geomagnetic disturbance phenomena such as the substorm growth phase and the formation of the stormtime ring current. Additionally, for one example, a prompt electric field response to the IMF southward turnings is seen within the inner plasma sheet.     

TC-1卫星在近地磁尾观测到的持续尾向流、等离子体片变薄和偶极化过程

2004年7月14日TC-1卫星在近地磁尾(-9.3Re,-5.4Re,1.2Re)附近观测到了伴随有持续尾向流的等离体片变薄和偶极化过程.尾向流持续时间为32分钟.偶极化过程中磁场By分量没有明显变化.在偶极化过程发生两分钟之后,地面台站观测到的Pi2脉动.ACE卫星的观测表明行星际磁场有弱的南向行星际磁场（-2nT）,持续时间约55分钟.Imagine卫星在电离层区域没有观测到极光出现.和伴随有极光增亮的亚暴过程相比,南向行星际磁场明显较弱,且持续时间短.TC-1卫星和ACE卫星的联合观测表明尾部释能有大有小,并非达到某值才能发生.但能量小时,不能够引起极光亚暴.其次南向行星际磁场有可能与近地磁尾尾向流有密切关系.     

Solar wind induced processes in the magnetotail

Solar wind/IMF parameters and their variations influence the state and dynamics of the magnetosphere in several different ways, and the plasma sheet plays its own active role to form the magnetotail’s rssponse to external driving. This field is still quite conrtoversial; key unsolved issues are those which probably involve the nonsteady, nonequilibrium and nonlinear character of the system. This paper discusses an interesting development of concepts concerning (1) plasma sheet convection, (2) mechanisms which initiate the substorm onset, and (3) variability of tail dynamics. A remarkable example is a recognition of Bursty Bulk Flows as a basic way for the plasma sheet to sustain the convective transport of plasma, energy and magnetic flux via transient mesoscale dynamic structures. As concerns the substorm onset, the sharp change from the magnetic reconnection (NEL) model to processes in the dipolar-like near tail is now moving into a synthetic stage. Hear non-linear models of different coupled instabilities are actively explored to find a way to excite the tearing mode starting from a singular thin current sheet configuration which seems to form in the near tail prior to breakup. Variable responses to external drivers and the variability of substorms compel a recognition of the magnetotail as a very complicated nonlinear open system which includes a heirarchy of coupled process of different scales. A unique fleet of magnetospheric spacecraft, simultaneously probing different domains and supported by extensive ground observations, global imaging and solar wind monitoring, noe provides a real chance to understand the magnetotail as a global dynamic system.     

Shear flow instabilities in the Earth’s magnetotail

Shear flow instability is studied in the Earth’s magnetotail by treating plasma as compressible. A dispersion relation is derived from the linearized MHD equations using the oscillating boundary conditions at the inner central plasma sheet/outer central plasma sheet (OCPS) interface and OCPS/plasma-sheet boundary layer (PSBL) interface, whereas the surface-mode boundary condition is used at the PSBL/lobe interface. The growth rates and the real frequencies are obtained numerically for near-Earth (\midX\mid\sim10-15 R_E) and far-Earth (\midX\mid\sim100 R_E) magnetotail parameters. The periods and wavelengths of excited modes depend sensitively on the value of plasma-sheet half thickness, L, which is taken as L=5 R_E for quiet time and L=1 R_E for disturbed time. The plasma-sheet region is found to be stable for constant plasma flows unless M_A3>1.25, where M_A3 is the Alfvén Mach number in PSBL. For near-Earth magnetotail, the excited oscillations have periods of 2–20 min (quiet time) and 0.5-4 min (disturbed time) with typical transverse wavelengths of 2–30 R_E and 0.5-6.5 R_E, respectively; whereas for distant magnetotail, the analysis predicts the oscillation periods of \sim8-80 min for quiet periods and 2–16 min for disturbed periods.