经验和抛物面模式模拟暴时磁场的误差特性研究

利用两种不同的磁场模式,协同GOES10/12、Polar及Geotail的实测资料,本文对3种不同强度磁暴的磁层磁场进行了详细分析,从而检验了两种不同磁场模式的可靠性,给出了模式间的性能对比.我们采用Tsyganenko经验模式及Alexeev抛物面模式,分别模拟了2004年4月、7月和11月的磁暴,3个磁暴过程中最强Dst指数分别达到-110 nT,-150 nT以及-289 nT.结果发现:T01模式对于中等磁暴时磁场模拟准确,而由于建立模式的数据库中强磁暴资料少,模式中不包含亚暴效应,T01对于强磁暴磁场模拟的误差增大,磁场分量远远偏离观测值;A2000模式缺少部分环电流及Ⅱ区场向电流,模式对于近地点Polar轨道上的磁场扰动模拟不足,对强磁暴时亚暴效应的过高模拟使其模拟的磁场分量偏高;T01模式对于中等磁暴时磁场的模拟效果明显好于A2000模式,随着磁层扰动的增强,T01与A2000模式的总体性能相当.     

2001年3月2日磁通量传输事件特性的研究

2001年3月2日11:00 至11:15 UT 期间,Cluster Ⅱ在南半球极尖区晨侧附近磁鞘内探测到3个通量传输事件（简称FTEs）. 本文利用Cluster Ⅱ星簇4颗卫星观测到的磁场和等离子体资料研究了这些通量传输事件的磁场形态和粒子特征. 并利用它们探测到的空间磁场梯度资料由安培定律直接求出星簇所在区域的电流分布. 结果指出：（1）BY占优势的行星际磁场结构在磁层顶的重联可以在极尖区附近发生;（2）FTEs通量管形成初期内外总压差和磁箍缩应力不一定平衡,达到平衡有一发展过程;（3）FTEs通量管截面在L M平面内的线度约为1.89RE;（4）FTEs通量管中等离子体主要沿轴向场方向流动,整个通量管以慢于背景等离子体的速度沿磁层顶向南向尾运动;（5）FTEs通量管中不仅有轴向电流,也存在环向电流. 轴向电流基本沿轴向磁场方向流动. 轴向和环向电流在管内均呈体分布,因而轴向电流产生的环向磁场接近管心时不断减小到零,而环向电流生成的轴向场则不断增大到极值;（6）在通量管的磁鞘部分观测到磁层能量粒子流量的增强,这表明通量管通过磁层顶将磁鞘和磁层内部连通起来了.     

地球磁层的磁场模型

1958年卫星探测发现了磁层,至今已有半个世纪,对磁层电场、电流体系、磁场、粒子分布和等离子体波的探测研究构成了空间物理的重要内容,其结果是各种磁层模型的出现.本文简要综述磁层磁场建模的基本原理、方法和发展历史,对十几种重要的磁层模型的特点、局限性和适用范围进行了对比分析,以Tyganenko模型为例,讨论了磁层模型发展的趋势.     

2001年1月26日高纬磁层顶通量管事件的观测研究--空间电流密度计算及分析

讨论了三种根据Cluster Ⅱ四颗卫星的磁场测量数据计算空间电流的方法及其误差,论证了这几种方法的内在一致性,并得到了完全相同的计算结果. 进而依据Cluster Ⅱ 磁场探测资料,计算了2001年1月26日多重磁通量管和FTE事件中高纬磁层顶边界层和磁鞘区的电流密度. 结果表明,磁通量管内电流密度较大,可达到约10-8A/m2;计算精度较高,结果可靠. 本文还应用最小方差分析法（MVA）,发现电流方向与通量管的轴向基本一致;论证了电流MVA分析在研究通量管性质时的作用,同时提出了电流管的概念.     

磁暴期间太阳风-磁层-电离层间的耦合过程

本文用行星际和地面磁场以及电离层资料,讨论了三次磁暴期间高、中纬电离层电场对太阳风和磁层内变化的响应。 分析表明,当IMF的B_x分量由北向转为南向时,太阳风驱动的磁层对流变化能直接反映出高纬电离层电位的变化。但持续南向的B_x再次增强时,太阳风输入的主要能量耗损于内磁层过程;电离层的响应表现为一个弛豫过程。当B_x由南转北时,环电流的消失对电离层的作用同样有弛豫的特点。此时,驱动电位已撤消,环电流是维持电离层电位的唯一外源。 本文用电路类比及简单模式法结论对上述几种实测情况进行了讨论。     

中低纬度电离层对太阳风状态的响应

本文讨论了行星际磁场B₂分量变化时内磁层和中低纬度电离层的响应.指出B₂变化引起的磁层大尺度对流电场的变化在一定条件下有可能透入内磁层,并沿磁力线映射到中低纬度电离层,在那里产生电场和电流体系,从而使S_q电流体系发生畸变,并在地面磁场中反映出来.数值计算表明,当△B₂<0时,S_q电流体系的焦点向东和向高纬移动,地面磁场会观测到数伽马的变化.这就为中低纬地磁观测诊断磁层和太阳风状态提供了一种可能性.此外,本文还用上述物理过程解释了赤道地区一些高空物理现象,如B₂倒转时电离层漂移速度的变化,赤道磁场异常以及赤道q型偶现E层的消失等等.     

内磁层电流体系的地磁效应

对Ⅱ区场向电流及其伴随的部分环电流和电离层电流组成的内磁层三维电流体系（PRFI电流系）的磁场效应进行了数值计算.这一三维电流体系在中低纬度地面产生的磁场呈现出特殊的纬度分布:X分量几乎不随纬度变化,Y分量随纬度增高近似呈线性变化.这些特征明显不同于对称环电流的磁场分布特征（X∝ cosφ,φ是纬度,Y=0）,也不同于DP2、S_q、L等电流体系的磁场分布特征.利用这一特征我们可以从地磁台子午链观测到的磁场扰动中分离出PRFI电流系的贡献.用1989年3月磁暴的实例检验了上述模型,观测结果与理论结果符合得很好.分析结果还表明,最大的D_st指数并不一定对应着最强的对称环电流.     

地球磁层顶磁场重联的动力学过程

用三维可压缩MHD数值模拟研究了在磁场重联过程中电子压力梯度项的效应研究结果发现在较高等离子体β,较小离子惯性尺度条件下,广义欧姆定理中压力梯度项在重联过程的作用不可忽略.在磁重联过程中,压力梯度项虽然没有明显改变磁场拓扑结构和重联速度,但它使电子和离子速度明显增大.由于在离子惯性尺度下,离子和电子运动解耦,电子是电流的主要载流子,所以场向电流也增大,并导致核心磁场明显增大.考虑到场向电流是磁层电离层耦合的一个重要因素,所以电子压力梯度项的引入加强了行星际磁场南向期间磁层电离层的耦合.电子压力梯度项还在重联区激发了波动,该波动可向重联区外传播.     

地球磁层中哨声导管的物理结构

本文在理论上探讨了地球磁层中哨声导管的物理特征。利用两种能够引导甚低频电磁波的导管模式,求得哨声导管中磁场和电流的分布。当磁场偏离势场或无力场时,将会产生场向电流,该电流与导管的强度和稳定维持密切相关。根据这些结果,我们认为高纬磁层中观测到的较强的场向电流,是高纬地区地面台站频繁接收到哨声及其回波的一个重要原因。     

EISCAT雷达观测到的极区电离层等离子体反常对流

综合分析EISCAT雷达与卫星当地测量数据,并利用磁层磁场模式对磁力线进行追踪,研究了发生在极光椭圆朝极盖边界附近电离层中,一例反常的背离太阳流动的强等离子体对流事件,及相关的太阳风-磁层-电离层耦合过程.结果表明,磁暴期间IMFBz指向南时观测到这一反常高速对流,及其相应的等离子体性态特征,很可能是向阳侧磁层顶磁重联过程在电离层中的印记.     

Dynamics of magnetospheric current systems during magnetic storms of different intensity

The dynamics of the magnetospheric magnetic field during the magnetic storms of different intensity has been studied. The magnetic field variations on the Earth’s surface were calculated using the paraboloid model of the magnetosphere, taking into account the induction currents flowing in the diamagnetically conductive Earth. Dst and its components have been calculated for ten magnetic storms. It has been indicated that relative contributions of magnetospheric sources to Dst change depending on the storm power. For weak and moderate storms, the contribution of the magnetotail current sheet can reach values comparable with the ring current contribution and, sometimes, can even exceed this contribution. For strong storms, the ring current field dominates over the tail current field, the absolute value of which does not exceed 150 nT (also achieved during less intense storms). For storms with minimum Dst exceeding-200 nT, the tail current field is saturated, whereas the ring current can continue developing.     

Control of the current state of geomagnetic activity based on the satellite magnetic measurements in the magnetosphere

The method for controlling the state of the geomagnetic field in real time, based on comparing the up-to-date models of the magnetospheric magnetic field (the Tsyganenko model, the NIIYaF MGU paraboloid model) with this field measured onboard a spacecraft along its trajectory, is presented. The method is based on the possibility of characterizing the state of the geomagnetic field in the near-Earth space in the scope of such models. The specific state is determined, based on the best correspondence of the measurements to one of these three model calculations, by comparing the model calculations of the geomagnetic field along the satellite trajectory, belonging to different classes of the field state (quiet, disturbed, strongly disturbed), with the current experimental magnetic data. The method was tested using the CHAMP satellite data.     

Dynamic geomagnetic field models

A review of modern dynamic models of the Earth’s magnetosphere (the A2000 paraboloid model and Tsyganenko’s T01 model) is presented. For the magnetic storm of January 9–11, 1997, the results of joint calculations of the magnetospheric magnetic field are presented and contributions of the large-scale magnetospheric currents to the D _st variations are analyzed. Both models were shown to be well consistent with measurement data; the contribution of the magnetotail current system to D _st is comparable to the contribution of the ring current. At the same time, the relative dynamics of magnetospheric current systems are different in different models. The differences in the magnetic field variation profiles for various current systems calculated by the A2000 and T01 models are explained by model parameterizations.     

Torus dynamo in the outer rings of galaxies

ABSTRACT

The magnetic fields in the inner parts of some spiral galaxies are understood quite well. Their generation is connected with the dynamo mechanism that is based on the joint action of turbulent diffusion and the α-effect. Usually the galactic dynamo is described with the so-called no-z approximation which takes into account that the galaxy disc is quite thin, with the implication that some spatial derivatives may be replaced by algebraic expressions. Some galaxies have outer rings that are situated at some distance from the galactic centre. The magnetic field can be described there also using the no-z model. As the thickness of such objects is comparable with their width, it is necessary to take into account the z-dependence of the field. We have studied the magnetic field evolution using the no-z approximation and torus dynamo model for the torus with rectangular cross-section in the axisymmetric case.     

A new time-dependent ionosphere–magnetosphere coupling model: Comparison of field-aligned currents against ST5 observations

By using Tsyganenko's model for the magnetosphere's magnetic field, which links two hemispheres of the ionosphere, and adopting a practical boundary condition for the electric potential around the polar cap, we developed a new ionosphere–magnetosphere coupling model based on prairie view dynamo code (PVDC). The new model takes the variations in solar wind and interplanetary magnetic field, as well as the geomagnetic activity, into account. Rather than the previous version of PVDC that is useful only for quiet conditions, the new model enables to calculate the electric potential and currents in the ionosphere and the field-aligned current (FAC) off the ionosphere in quiet and disturbed times. Comparison of the calculated FAC with the measurements of Space Technology 5 (ST5) mission shows a good agreement.     

Current and voltage source induced polarization transients: a comparative consideration

The article discusses the excitation of transient induced polarization responses using current and voltage sources. The first method has found a wide application in induced polarization surveys and—directly or indirectly—in the theory of the induced polarization method. Typically, rectangular current pulses are injected into the earth via grounding electrodes, and decaying induced polarization voltage is measured during the pauses between pulses. In this case, only the secondary field is recorded in the absence of the primary field, which is an important advantage of this method. On the other hand, since the current injected into the ground is fully controlled by the source, this method does not allow studying induced polarization by measuring the current in the transmitter line or associated magnetic field. When energising the earth with voltage pulses, the measured quantity is the transient induced polarization current. In principle, this method allows induced polarization studies to be done by recording the transmitter line current, the associated magnetic field, or its rate of change. The decay of current in a grounded transmitter line depends not only on the induced polarization of the earth but also on the polarization of the grounding electrodes. This problem does not occur when induced polarization transients in the earth are excited inductively. A grounded transmitter line is a mixed‐type source; hence, for a purely inductive excitation of induced polarization transients, one should use an ungrounded loop, which is coupled to the earth solely by electromagnetic induction.     

Selection of parameters for the Saturn’s magnetospheric model based on the pioneer 11 data

The Saturn’s magnetospheric model is described in the paper. This model is based on the existent paraboloid models of the magnetic field of the Earth’s and Jupiter’s magnetospheres. The input parameters of the Saturn’s magnetospheric model are determined from observations using the parameter selection method presented here. The parameters are selected for the Pioneer 11 flight.     

Improved method to derive equivalent current systems from global MHD simulations

Derivation of equivalent current systems(ECS)from a global magnetospheric magnetohydrodynamics(MHD)model is very useful in studying magnetosphere-ionosphere coupling,ground induction effects,and space weather forecast.In this study we introduce an improved method to derive the ECS from a global MHD model,which takes account of the obliqueness of the magnetic field lines.By comparing the ECS derived from this improved method and the previous method,we find that the main characteristics of the ECS derived from the two methods are generally consistent with each other,but the eastward-westward component of the geomagnetic perturbation calculated from the ECS derived from the improved method is much stronger than that from the previous method.We then compare the geomagnetic perturbation as a function of the interplanetary magnetic field(IMF)clock angle calculated from the ECS derived from both methods with the observations.The comparison indicates that the improved method can improve the performance of the simulation.Furthermore,it is found that the incomplete counterbalance of the geomagnetic effect produced by the ionospheric poloidal current and field-aligned current(FAC)contributes to most of the eastward-westward component of geomagnetic perturbation.     

Ring current asymmetry during a magnetic storm

The ring current dynamics during the magnetic storm has been studied in the work. The response of the magnetospheric current systems to the external influence of the solar wind, specifically, resulting in the development of the asymmetric ring current component, has been calculated using the magnetic field paraboloid model. The asymmetric ring current has been considered as a family of spatial current circuits in the Northern and Southern hemispheres, composed of the zones of the partial ring current in the geomagnetic equator plane, which close through the system of field-aligned currents into the ionosphere. The value of the total partial ring current has been estimated by comparing the calculated asymmetry of the magnetospheric magnetic field at the geomagnetic equator with the value of the Asym-H geomagnetic index. The variations in the symmetric and asymmetric components of the ring current magnetic field have been calculated for the magnetic storm of November 6–14, 2004. The contributions of the magnetospheric current systems to the Dst and AU geomagnetic indices have been calculated.     

Simulation of Microwave Emission from the Magnetic Arch with Growing Millimeter Spectrum

A three-dimensional simulation of the microwave emission of nonthermal electrons within a flare magnetic loop that takes into account the influence of the chromosphere has been carried out. The paper investigates the possibility of the generation of a microwave spectrum (observed for some solar flares) with a maximum in the centimeter wavelength range and a positive slope in the millimeter one under the some distribution of the magnetic field strength and the parameters of the anisotropic nonthermal electrons along the loop. By the example of the event on July 5, 2012, it is shown that nonthermal electrons can be responsible not only for the centimeter bell-shaped emission spectrum generated in the coronal part of the loop but also for the increasing millimeter spectrum generated in the chromosphere.