【作者】 姚丽；

【导师】 刘振兴；

【作者基本信息】 中国科学院研究生院（空间科学与应用研究中心） ， 空间物理， 2009， 博士

【摘要】 本论文主要基于TC-1卫星,并辅助Cluster及其它探测卫星的观测数据研究了磁层对行星际激波的响应现象及物理机制,并且深入研究了一些磁尾动力学过程:近地磁尾磁场偶极化现象以及磁层亚暴膨胀相“锋面”触发过程,共包括三个方面的内容.其主要研究成果如下:1.利用TC-1卫星,并辅助其它磁层观测卫星GOES,LANL,Cluster及地磁台站的观测分析了磁层对行星际激波的响应,主要是磁尾等离子片及地球同步轨道区域的响应.首先我们讨论了2004年11月7日两例行星际激波触发的磁尾等离子体片振动增强事件.这两例行星际激波对应的行星际条件为弱南向或者北向.当激波作用于磁层时,等离子体片温度、数密度均突然增加,并且离子流流速突然增加,增强的等离子体流持续一段时间.其中最显著的现象是等离子体准周期振动显著增强,增强的等离子体流与局地磁场近似垂直.这种激波触发的等离子体片准周期对流振动增强的现象迄今为止还没有人报道过.我们推断激波触发的等离子片扰动很有可能是由激波在传播过程中引起磁鞘等离子体动压增强从而对磁尾对称压缩引起的.以前的研究表明太阳风动压增强的磁层响应与行星际磁场方向有关,在相关研究中,为了去除行星际磁场变化带来的影响,一般只考察稳定的行星际磁场条件下动压增强的效应.但是P_dy和B_Z同时发生突变的综合效应有待于进一步研究.我们分别分析了磁场方向发生南向偏转和北向偏转情况下动压脉冲结构作用于磁层时引起的磁层各区域不同的响应.2004年7月22日,WIND飞船探测到一个典型的行星际激波,激波前为持续较长时间的微弱南向磁场,越过激波面,磁场发生南向偏转.当激波作用于磁层时位于磁尾等离子体片不同位置的TC-1卫星和Cluster卫星都观测到等离子体对流迅速增强.Cluster上搭载的电场探测仪器可以直接观测到晨昏电场的增加.位于磁尾等离子体片以及地球同步轨道不同位置的卫星观测到的磁场变化则不同:TC-1观测到磁场大小几乎不变但磁场仰角变小,而离赤道较远的Cluster卫星则观测到磁场显著增强;位于午夜侧附近的GOES-10卫星观测到磁场强度突然增加,磁场仰角变小;位于晨侧的GOES-12卫星的观测则表现出简单的磁场压缩,即磁场强度及各分量都显著增加.另外,分布在各个磁地方时的LANL卫星观测的高能质子和高能电子通量都脉冲增强,在向阳面粒子通量的变化比夜侧明显,位于午夜侧的粒子通量响应则最弱.以上磁层响应是由动压脉冲结构以及磁场南向偏转共同作用的结果.2004年11月9日,WIND飞船也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分钟的弱南向磁场,越过激波面,磁场发生北向偏转.在此强动压脉冲结构作用下,磁层被压缩至一个很小的区域.激波作用于磁层时除了与稳定太阳风条件下动压脉冲结构作用于磁层相似触发尾瓣SI现象外,地球同步轨道区域磁场和高能粒子响应则呈现出新的观测现象,主要有:（1）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、质子通量显著增强,但是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.不同种类的高能粒子响应时序不同是一种新的观测现象.（2）根据向阳面正午两侧的GOES-10和GOES-12卫星的观测我们推断当激波作用于磁层时,日下点附近局部磁层顶电流的减弱,减弱的磁层顶电流同增强I区电流相连.这些现象是磁场北向偏转和动压脉冲结构共同作用的结果.2.我们首先以2004年10月12日的一个磁场偶极化为例来研究近地磁尾磁场偶极化过程中磁场和等离子体特征,研究发现:在偶极化前一分钟,有较强的（VX <-100 km/s）持续时间超过3 min的尾向流发生.尾向流期间,磁场线被拉伸.当尾向流逐渐减弱并有地向流出现时磁倾角急剧上升,即发生了磁场偶极化.考察磁场和等离子体速度矢量之间夹角θBV发现,尾向流期间,θBV的平均值约为150?,也就是该尾向流在垂直于磁场方向有较大的分量,分析发现,正是在该垂直速度的作用下,磁场线在尾向流期间被尾向拉伸,磁场表现出亚暴膨胀相特征.当尾向流结束,在地向流和磁张力的作用下,磁场线向地球一侧收缩,磁倾角增加,磁场偶极化发生.其次,我们利用TC-1于2004–2006年期间在近地磁尾的观测资料,共认证了59例磁场偶极化事例,应用时序迭加法,进行统计分析发现:（1）磁场偶极化前约10分钟都观测到磁场BZ分量的逐渐增加,即发生磁通量堆积;（2）磁场偶极化开始前后分别观测到持续的尾向流和地向流,它们对应的磁场位型分别是非偶极型和偶极形磁场.磁场偶极化前,热离子数密度、温度逐渐下降,磁场逐渐增强;而磁场偶极化开始后,热离子数密度、温度迅速上升,磁场迅速减弱.这说明偶极化开始前后,能量在磁能和等离子体热能之间发生了一定程度的转化;（3）从等离子体片边界层过渡到低纬尾瓣区,偶极化前尾向流的温度由明显的各向异性过渡到各向同性.以上个例和统计研究表明,尾向流在磁场偶极化的过程中,对改变磁场线的位型有着重要的作用,因此,尾向流在亚暴膨胀相触发过程中可能有着重要的作用,它可能对亚暴膨胀相前后能量的储存和释放过程有着一定的影响.3.亚暴膨胀相“锋面”触发模型是刘振兴在2006年36th北京COSPAR大会上首次正式提出的.我们以2004年10月1日的亚暴为例,分析亚暴膨胀相“锋面”触发过程.观测表明,在此次亚暴期间有较强的尾向流出现.在尾向流的增强相,磁能增加,热能减少,而在尾向流的减弱相,磁能减少,而热能迅速增加;由于极光点亮就发生在这一阶段,同时TC-1所在的磁尾区域在电离层的足点和亚暴极光点亮的位置非常接近,因此,我们推断,这一阶段的能量转化和释放过程和亚暴膨胀相触发过程有着密切的联系.更多还原

【Abstract】 In this dissertation, observational study on the near-Earth magnetotail dynamic pro-cesses are made mainly based on TC-1 spacecraft observations, and the observations ofCluster and other scientific satellites, with emphasis on the magnetospheric responses tointerplanetary shocks, near-Earth magnetotail magnetic dipolarization as well as the sub-storm expansion phase“front”triggering process.Two interplanetary shocks are examined to determine the responses of the magneticfield and plasma in the plasma sheet upon the shock impacts by using TC-1 observationaldata. The two shocks are observed by WIND on November 7, 2004. Prior to and afterthe shock, the IMF is either weakly southward or northward. The responses of the plasmasheet to the two shocks are intense and much similar. When the shock interacts with themagnetosphere, the magnetic field impulsively increases 1～2 min after the geomagneticfield sudden impulse （SI） judged from the Sym-H index change, and the magnetic field lineis stretched. On the other hand, all of the ion density, the ion temperature, and the velocityof ion flow in the plasma sheet increase. Interestingly, quasi-periodical oscillations of theion flow are suddenly enhanced, and the plasma flow is basically perpendicular to the localmagnetic field. The responses of the magnetic field and the plasma are nearly simultaneous.The responses in the plasma sheet are probably caused by the lateral compression due tothe dynamic pressure enhancement downstream the shock when the shock propagatesantisunward in the magnetosheath. As far as we know, the quasi-periodical convectionoscillations enhancement directly induced by shocks have never been reported in previousstudies.Magnetospheric responses to the solar wind dynamic pressure pulses are intensivelydependent on the direction of IMF. In previous studies, to remove the effects of the vari-ations of IMF, the effects of dynamic pressure enhancements are usually investigated onstable IMF condition. The joint effects of simultaneous abrupt changes of the two param-eters Pdy and BZ are significant and urgent to be further investigated. We studied thedi?erent responses of the magnetosphere to the solar wind dynamic pressure pulse on theconditions of IMF southward turning and northward turning respectively. On July 22,2004, the WIND spacecraft detected a typical interplanetary shock. There was sustainingweak southward magnetic field in the preshock region and the southward field was suddenlyenhanced across the shock front （i.e., southward turning）. When the shock impinged onthe magnetosphere, the magnetospheric plasma convection was abruptly enhanced in the central plasma sheet, which was directly observed by both the TC-1 and Cluster space-craft located in di?erent regions. Simultaneously, the Cluster spacecraft observed thatthe dawn-to-dusk electric field was abruptly enhanced. The variations of the magneticfield observed by TC-1, Cluster, GOES-10 and GOES-12 that were distributed in di?erentregions in the plasma sheet and at the geosynchronous orbit are obviously distinct. TC-1observations showed that the magnetic intensity kept almost unchanged and the elevationangle decreased, but the Cluster spacecraft, which was also in the plasma sheet and wasfurther from the equator, observed that the magnetic field was obviously enhanced. Si-multaneously, GOES-12 located near the midnight observed that the magnetic intensitysharply increased and the elevation angle decreased, but GOES-10 located in the dawnside observed that the magnetic field was merely compressed with its three componentsall sharply increasing. Furthermore, the energetic proton and electron ?uxes at nearly allchannels observed by five LANL satellites located at di?erent magnetic local times （MLTs）all showed impulsive enhancements due to the compression of the shock. The responsesof the energetic particles are much evident on the dayside than those on the nightside.Especially the responses near the midnight were rather weak. In this paper, the possiblereasonable physical explanation to above observations is also discussed. All the shock-induced responses are the joint effects of the solar wind dynamic pressure pulse and themagnetic field southward turning.On November 9, 2004,the WIND spacecraft also detected a typical interplanetaryshock. Before the shock, there is sustaining weak southward magnetic field lasting about50 min. Across the shock front, the magnetic field turns northward. The magnetosphereis compressed to a rather small region upon the impacts of the DPP. During the impinge-ment of the shock, the DPP-induced lobe SI phenomena which is similar to that on steadyIMF condition are observed. The responses of the magnetic field and the energetic particle?ux at the geosynchronous orbit show new characteristics, which are: （1） two-mode dis-turbances of the energetic particle at the geosynchronous orbit are triggered, i.e. particle?uxes enhancement due to the compression near dawn and dusk and dispersionless particleinjection similar to substorm. The disturbances propagate from dayside to nightside. Theenergetic particle ?uxes on the dayside first increase, and then after about 1 min, the par-ticle ?uxes near dawn and dusk on the nightside begin to increase. Finally, dispersionlessparticle injection is seen near the midnight. Furthermore, near the noon the responses ofelectron are prior to those of proton, contrarily, near the midnight the responses of protonare prior to those of electron. （2） Based on the GOES-10 and GOES-12 observations ontwo sides of the noon, it is inferred that while the shock impinged on the magnetosphere,the local magnetopause current weakened on the subsolar location, which then was con- nected with the Region I field-aligned current. All the shock-induced responses were thejoint effects of the solar wind dynamic pressure pulse and the magnetic field northwardturning.A magnetic field dipolarization event on Oct. 12, 2004 is investigated as an exampleto study the magnetic field and plasma characteristics during the dipolarization. Priorto the dipolarization, tail-ward flow （VX < -100 km/s） lasting over 3 min was observed.During the tail-ward flow, magnetic field was stretched tail-ward. When the tail-ward flowweakened and Earth-ward flow appeared, the magnetic elevation angle increased sharply,that is, dipolarization occurred. By examining the angle between the magnetic field andthe ion velocityθBV, it is found that the average value ofθBV was～150? during the inter-val of the tail-ward flow, which indicates that there was a considerable velocity componentof the tail-ward flow perpendicular to the magnetic field. The magnetic field was stretchedtail-ward led by plasma flow with perpendicular velocity during the tail-ward flow, andthe magnetic field was characterized by that in the substorm growing phase. Magneticfield was contracted Earth-ward due to the joint effects of the Earth-ward flow and mag-netic tension when the tail-ward flow ended, simultaneously the magnetic elevation angleincreased sharply which indicated the onset of dipolarization. By using the TC-1 observa-tions during 2004–2006 at near-Earth magnetotail, we identified 59 dipolarization events.The statistical results by applying superposed epoch analysis （SEA） are as follows: （1）Continuous enhancement of BZ, i.e., magnetic ?ux pile-up, startes about 10 min beforethe dipolarization onset; （2） Evident continuous tailward flow and Earth-ward flow areseen before and after the dipolarization onset. The tailward and earthward flow are asso-ciated with tail-like and dipole-like magnetic field respectively. Meanwhile, the density andthe temperature of the hot ion gradually decrease during the interval of tailward flow andsharply increase during the earthward flow. However, the magnetic field strength ascendsgradually and descends sharply before and after the dipolarization onset respectively. Thisindicates that energy is transform between magnetic energy and plasma thermal energybefore and after the dipolarization. Thermal energy is transformed to magnetic energyduring the tailward flow and the magnetic energy is transformed to thermal and dynamicenergy during the earthward flow to some extent; （3）The hot ion temperature during thetailward flow just before the dipolarization onset becomes more isotropy from the plasmasheet boundary layer to the lobe region. The above results indicate that tail-ward flowplays an important role in changing the configuration of the magnetic field during the thedipolarization processes. So, there may be some considerable effects of tail-ward flow ontriggering substorm expansion phase, it probably a?ects the process of energy deposit andrelease before and after the substorm expansion phase onset. The substorm expansion phase“front”triggering model was first formally proposedby ZhenXing Liu on 36^th COSPAR convoked in beijing in july, 2006. We analyze thesubstorm expansion phase triggering process occurred on Oct. 1, 2004 based on TC-1observations. During this substorm the tail-ward flow was observed. In the tail-ward flowenhancement phase, the magnetic energy increase, and the thermal energy decrease; onthe other hand, in the tail-ward flow weakening phase, the magnetic energy decrease, andthe thermal energy increase. Since auroral breakup occurred just at the period of thetail-ward flow weakening phase, and the footprint of magnetotail region where TC-1 waslocated was just very near the location where auroral breakup occurred, we infer that theenergy changing is in close relation with the substorm expansion phase triggering.更多还原