电离层等效电流体系对行星际激波的响应研究

行星际激波是导致地球磁层-电离层系统发生扰动的重要原因之一,其可以通过对磁层-电离层系统电流体系的改变来影响地磁变化.本文采用全球三维磁流体力学数值模拟方法,分析了行星际激波作用下电离层等效电流体系的即时响应.模拟结果表明,在激波作用下伴随着异常场向电流对的产生,电离层在午前午后出现一对反向的等效电流涡.这对涡旋一边向极侧和夜侧运动,一边经历强度增强和减弱直至消失的过程.激波过后等效电流体系图像逐渐演化为激波下游行星际条件控制的典型图像.这个响应过程与行星际激波强度有关,激波强度越强,则反向的等效电流涡旋强度越大,寿命也就越短.      

第23和24太阳活动周高纬地磁感应电流分布特性

基于高纬度芬兰Mäntsälä地区近两个太阳活动周期（1999—2017年）天然气传输管道的地磁感应电流（GIC，I_GIC）观测数据，统计研究了GIC扰动的分布特征以及强GIC扰动与磁暴和地磁亚暴的相关性.研究发现:95.83%时间段的GIC强度分布在0～1A之间.定义:若某个时间段|I_GIC|_max> 1A，则认为发生GIC扰动；|I_GIC|_max>10A，则认为发生强GIC扰动事件.GIC扰动在磁地方时夜侧附近发生的概率最高，这主要与地磁亚暴发生期间电离层电流最剧烈的变化发生在磁地方时夜侧附近有关；强GIC扰动经常爆发式出现，且都发生在磁暴期间，但大多数磁暴并不伴随强GIC扰动事件发生.磁暴急始驱动的强GIC扰动事件较少，由磁层压缩引起地磁场突然增强驱动的强GIC扰动事件持续时间较短；强GIC扰动事件主要发生在磁暴主相和恢复相，由环电流变化驱动的强GIC扰动事件一般持续时间较长且强度较大.      

场向电流随亚暴位相的变化

利用ISEE1和2卫星测量的磁场数据,计算了电离层中的场向电流。依据每个场向电流事件所伴随的亚暴位相,分别计算了一区和二区场向电流强度、密度及电流片厚度在亚暴成长相、膨胀相和恢复相的平均值及中间值。其结果,从成长相到膨胀相,一区和二区场向电流的强度和密度增加,从膨胀相到恢复相,其值减小。平均说来,一区的电流强度约是二区的1.4倍。电流片厚度的变化在上述期间内与电流强度及密度的变化趋势相反。      

TC-1和Geotail对亚暴过程中近地等离子体片磁扰动的联合观测

以2004年9月28日02:53:20 UT的亚暴为例, 通过TC-1在磁尾约12.5 R_e 和Geotail卫星在近地磁尾等离子体片约8～9 R_e的联合观测, 研究亚暴触发过程中近地磁尾等离子体片中等离子体波动特征. 结果表明, 亚暴触发区是近地磁尾中心等离子体片中较小的一个区域, 在亚暴触发区中低混杂不稳定性在近地磁尾等离子体片中存在, 准垂直传播的低混杂波发生在亚暴触发过程中, 而亚暴触发过程中近地磁尾等离子体片外边界区内的磁场偶极化信号和扰动都非常微弱. 在亚暴触发和亚暴膨胀相过程中出现了多次具有不同特征的磁场偶极化现象.      

夜侧极区电离层对流对行星际激波的响应

利用超级双子极光雷达网（Super Dual Aurora Radar Network,SuperDARN）高频雷达、北半球IMAGE地磁台链以及南极中山站的极光观测数据,研究电离层对流对2012年7月14日一个行星际激波扰动事件的响应.在18:10UT行星际激波到达地球并与磁层相互作用触发地磁急始和磁层亚暴,SuperDARN雷达观测到北半球夜侧极区电离层对流显著增强,观测视野覆盖黄河站的Hankasalmi雷达观测到从激波到达地球至18:33UT,电离层F层出现剧烈扰动,雷达回波数明显增多,并出现局部对流速度反转现象.18:33UT之后,观测到F层出现三块速度高达600m·s-1的逆阳运动不规则体.而与Hankasalmi雷达地磁共轭的南半球Kerguelen雷达探测到的回波主要来自E层,回波数量几乎无变化,但是Kerguelen雷达观测视野内的中山站全天空光学成像仪观测到极光活动显著增强.南北半球夜侧电离层观测结果的差异,主要是由于它们分别处于极夜和极昼.      

反向赤道电集流与磁层扰动

对印度Trivandrum站第21太阳活动周内地磁H分量分析表明,不仅在磁扰日及其随后的静日内,强磁扰对赤道电集流有显着作用,即使在持续静日期间,较弱的磁扰仍然对赤道电离层有很大影响。磁静日昏侧出现反向(西向)电集流是正常现象,弱磁扰是使此反向电流消失的可能机制。      

磁层-电离层电动耦合与中纬地磁指数的变化

本文探讨磁层一电离层耦合过程内中纬地磁指数的变化特点,并与极光电集流和赤道电集流(指数)变化相比较。相关分析和时序叠加分析均表明,高、中、低纬地磁指数变化可归结为磁层一电离层电动耦合的统一物理图象。有R事件的磁暴主相初期和无R事件的磁扰期内,赤道电集流和中纬地磁指数的变化各不相同。这再次证明,耦合分析中将磁层源扰动的直接穿透作用与经电离层内动力过程的效应二者加以区分和综合研究是很重要的。      

磁层－电离层电动耦合与中纬地磁指数的变化

本文探讨磁层一电离层耦合过程内中纬地磁指数的变化特点，并与极光电集流和赤道电集流（指数）变化相比较．相关分析和时序叠加分析均表明，高、中、低纬地磁指数变化可归结为磁层一电离层电动耦合的统一物理图象．有Ｒ事件的磁暴主相初期和无Ｒ事件的磁扰期内，赤道电集流和中纬地磁指数的变化各不相同．这再次证明，耦合分析中将磁层源扰动的直接穿透作用与经电离层内动力过程的效应二者加以区分和综合研究是很重要的．     

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     

亚暴起始位置和膨胀相持续时间对行星际磁场Bz分量的响应

磁层亚暴是太阳风–磁层–电离层耦合过程中的重要爆发性事件,其特性受太阳风参数的影响很大。本文利用对IMAGE卫星在2000 － 2005年观测到的4193个亚暴起始事件,统计研究了在不同的行星际磁场（IMF）B_z 条件下亚暴起始位置和膨胀相持续时间。结果表明,南向IMF发生的亚暴比北向IMF下发生的亚暴要多。南向IMF条件下亚暴AE指数最大值的平均值基本上>600 nT,并有随南向IMF持续时间增大而增大的趋势。北向IMF条件下亚暴AE指数最大值的平均值基本上<500 nT,并有随北向IMF持续时间增大而减小的趋势。亚暴的起始磁纬度基本上位于65° － 70°之间。当南向IMF或北向IMF的持续时间增大,超过80 min时,北半球的亚暴起始磁纬度会降低。亚暴起始磁地方时大部分位于22:15 － 23:15 MLT之间。但整体分布比较分散,显示不出特别清晰的随IMF B_z持续时间变化的趋势。相比于南向的IMF,北向IMF期间发生亚暴的平均膨胀相持续时间增大了将近10 min,表明南向IMF期间,亚暴强度虽然较大,但其膨胀相持续时间较短,亚暴能量释放和耗散的速度更快。      

The solar wind control of the equatorial ionosphere dynamics during geomagnetic storms

The interplanetary magnetic field, geomagnetic variations, virtual ionosphere height h′F, and the critical frequency foF2 data during the geomagnetic storms are studied to demonstrate relationships between these phenomena. We study 5-min ionospheric variations using the first Western Pacific Ionosphere Campaign (1998–1999) observations, 5-min interplanetary magnetic field (IMF) and 5-min auroral electrojets data during a moderate geomagnetic storm. These data allowed us to demonstrate that the auroral and the equatorial ionospheric phenomena are developed practically simultaneously. Hourly average of the ionospheric foF2 and h′F variations at near equatorial stations during a similar storm show the same behavior. We suppose this is due to interaction between electric fields of the auroral and the equatorial ionosphere during geomagnetic storms. It is shown that the low-latitude ionosphere dynamics during these moderate storms was defined by the southward direction of the B_z-component of the interplanetary magnetic field. A southward IMF produces the Region I and Region II field-aligned currents (FAC) and polar electrojet current systems. We assume that the short-term ionospheric variations during geomagnetic storms can be explained mainly by the electric field of the FAC. The electric fields of the field-aligned currents can penetrate throughout the mid-latitude ionosphere to the equator and may serve as a coupling agent between the auroral and the equatorial ionosphere.     

SAMBO-GEOS : Electric field measurements in the disturbed ionosphere and magnetosphere

Measurements of the electric field in the ionosphere and the equatorial plane during the pre-onset and actives phases of a substorm (March 4, 1979) are compared. Correlations and disagrements between the measurements are considered. The preliminary conclusion is reached that the model of electrojet polarisation proposed by CORONITI and KENNEL (1972) could possibly explain part of our observations.     

极光西向涌浪的二维模式(线性部分)

极光西向涌浪(WTS)被定义为磁层亚暴开始的重要标志, 它是伴随带电粒子沉降过程在极光带电离层出现的特定的极光运动形态.本文给出了极光西向涌浪的二维动力学模式, 亚暴开始, 伴随粒子沉降在电离层产生离子密度梯度(电导率梯度), 在背景电场中激发出低频漂移波, 它的传播给出极光西向涌浪运动的主要特性.      

Case study of September 24–26, 1998 magnetic storm

We compute global magnetospheric parameters based upon solar wind data obtained from the WIND spacecraft upstream. Using the paraboloid magnetospheric model, calculations of the dynamic global magnetospheric current systems have been made. The solar wind dynamic pressure, the interplanetary magnetic field, the strength of the tail current, and the ring current control the polar cap and auroral oval size and location during the magnetic storm. The model calculations demonstrate that the polar cap and the auroral oval areas are mainly controlled by the tail current. The substorm onset at 0630 UT on September 25, 1998 happened near the minimum in the main phase field depression. The substorm expansion onset time is also marked by a sudden enhancement in the solar wind dynamic pressure and an enhancement in the tail current. The magnetic signatures of these two effects cancel each other, which explains why the D_st profile shows no strong time variation during the substorm. Evidence for the substorm expansion includes not only the signature in the AL index but also the strong asymmetry of the low latitude magnetic disturbances (substorm positive bay signature). Model calculations were checked by comparison with the GOES 8 and 10 magnetic field measurements.     

Magnetospheric electric field variations caused by storm-time shock fronts

On January 20, 2005 there was an X 7.1 solar flare at 0636 UT with an accompanied halo coronal mass ejection (CME). The resultant interplanetary shock impacted earth ∼36 h later. Near earth, the Advanced Composition Explorer (ACE) spacecraft observed two impulses with a staircase structure in density and pressure. The estimated earth-arrival times of these impulses were 1713 UT and 1845 UT on January 21, 2005. Three MINIature Spectrometer (MINIS) balloons were aloft on January 21st; one in the northern polar stratosphere and two in the southern polar stratosphere. MeV relativistic electron precipitation (REP) observed by all three balloons is coincident (<3 min) with the impulse arrivals and magnetospheric compression observed by both GOES 10 and 12. Balloon electric field data from the southern hemisphere show no signs of the impulse electric field directly reaching the ionosphere. Enhancement of the balloon-observed convection electric field by as much as 40 mV/m in less than 20 min during this time period is consistent with typical substorm growth. Precipitation-induced ionospheric conductivity enhancements are suggested to be (a) the result of both shock arrival and substorm activity and (b) the cause of rapid (<6 min) decreases in the observed electric field (by as much as 40 mV/m). There is poor agreement between peak cross polar cap potential in the northern hemisphere calculated from Super Dual Auroral Radar Network (SuperDARN) echoes and horizontal electric field at the MINIS balloon locations in the southern hemisphere. Possible reasons for this poor agreement include (a) a true lack of north–south conjugacy between measurement sites, (b) an invalid comparison between global (SuperDARN radar) and local (MINIS balloon) measurements and/or (c) radar absorption resulting from precipitation-induced D-region ionosphere density enhancements.     

Auroral Electrojet Event Associated With Magnetospheric Substorms

The auroral electrojet index is an important index in monitoring and predicting substorms. A substorms usually includes auroral breakup, auroral electrojet event marked by AE increase, energetic particle injection at geosynchronous orbit, mid-low latitude Pi2, etc. However the question whether an auroral electrojet event corresponds to a substorm remains unanswered. Using the auroral electrojet index in 2004, we analyzed five auroral electrojet events and studied their relation with substorms. The results show that there are three kinds of auroral electrojet events: (1) simultaneous rapid increase of westward auroral electrojet and eastward auroral electrojet; (2) rapid increase of westward auroral electrojet and almost unchangeable eastward auroral electrojet; (3) rapid increase of eastward auroral electrojet and almost unchangeable westward auroral electrojet. Most of auroral electrojet events correspond to substorms. However a few auroral electrojet events are not accompanied by substorms. This situation most often occurs for the auroral electrojet event in which eastward auroral electrojet dominates.      

Real-time global MHD simulation of the solar wind interaction with the earth’s magnetosphere

We have developed a real-time global MHD (magnetohydrodynamics) simulation of the solar wind interaction with the earth’s magnetosphere. By adopting the real-time solar wind parameters and interplanetary magnetic field (IMF) observed routinely by the ACE (Advanced Composition Explorer) spacecraft, responses of the magnetosphere are calculated with MHD code. The simulation is carried out routinely on the super computer system at National Institute of Information and Communications Technology (NICT), Japan. The visualized images of the magnetic field lines around the earth, pressure distribution on the meridian plane, and the conductivity of the polar ionosphere, can be referred to on the web site (http://www2.nict.go.jp/y/y223/simulation/realtime/).The results show that various magnetospheric activities are almost reproduced qualitatively. They also give us information how geomagnetic disturbances develop in the magnetosphere in relation with the ionosphere. From the viewpoint of space weather, the real-time simulation helps us to understand the whole image in the current condition of the magnetosphere. To evaluate the simulation results, we compare the AE indices derived from the simulation and observations. The simulation and observation agree well for quiet days and isolated substorm cases in general.     

Multisatellite investigations of substorm onsets

Substorm onsets, identified by Pi2 pulsations observed on the AFGL Magnetometer Network, have been studied using ISEE 1 electric and magnetic field data and GOES 2 and GOES 3 magnetic field data. The relative positions of the spacecraft with respect to the substorm current system were determined from the Pi2 polarizations. One onset occurred when ISEE 1 and GOES 2 were on the same field line but in opposite hemispheres. During this onset ISEE 1 and GOES 2 observed magnetic signatures which appear to be due to conjugate field-aligned-currents flowing out of the western edge of the westward electrojets. A broadband burst of wave noise was seen in the ISEE 1 electric field at the same time as field-aligned-currents were observed. These may be current driven ion cyclotron waves. A three minute perturbation in the electric field data prior to the initial substorm onset indicates that there was an azimuthal westward flow of plasma starting ～ 1 min before the substorm onset.