地球弓激波的三维模拟

利用磁流体动力学(MHD)全球模拟结果,根据弓激波的跃变特性确定出弓激波位置,建立了一个新的综合考虑了快磁声马赫数、太阳风动压、行星际磁场强度以及磁层顶曲率半径的弓激波三维位型模型.将新模型与以往模型的模拟结果进行比较发现,新的弓激波全球模型结果可靠,解决了部分现有模型不能描述弓激波三维位型的问题.研究结果表明,在行星际磁场北向时,随着快磁声马赫数的增大,弓激波日下点距离减小,但是在行星际磁场南向时,快磁声马赫数的变化对弓激波日下点距离影响不大;弓激波位型在赤道面与子午面上存在明显的不对称性,而且随着行星际磁场的转向,这种非对称性也会发生相应改变;行星际磁场南向,B_z值较小时,子午面内弓激波位型已经不是简单的抛物线,出现了明显的类似于极尖区磁层顶的凹陷变化区.      

Bow shock: Power aspects

It is clear that the primary energy source for magnetospheric processes is the solar wind, but the process of energy transfer from the solar wind into the magnetosphere, or rather, to convecting magnetospheric plasma, appears to be rather complicated. Bow shock is a powerful transformer of the solar wind kinetic energy into the gas dynamic and electromagnetic energy. A jump of the magnetic field tangential component at front crossing means that the front carries an electric current. The solar wind kinetic energy partly transforms to gas kinetic and electromagnetic energy during its passage through the bow shock front. The transition layer (magnetosheath) can use part of this energy for accelerating of plasma, but can conversely spend part its kinetic energy on the electric power generation, which afterwards may be used by the magnetosphere. Thereby, transition layer can be both consumer (sink) and generator (source) of electric power depending upon special conditions. The direction of the current behind the bow shock front depends on the sign of the IMF Bz-component. It is this electric current which sets convection of plasma in motion.     

远磁尾磁层顶位形的偏转

利用WIND和ARTEMIS卫星观测数据,分析远磁尾磁层顶对行星际和太阳风变化的响应,尤其是偏离日地连线的太阳风速度改变对远磁尾磁层顶的影响.研究发现在2011年9月13日的事件中,P2卫星观测到高速且高密度的磁鞘流.利用最小变量法进行分析发现,磁层顶沿着偏离日地连线的太阳风速度方向发生偏转.根据相似三角形定理,推断出本次事件中磁层顶在y方向和z方向上的偏转幅度分别达到10R_e和6R_e.P1和P2卫星的相对位置也证实了这一观点.因此,偏离日地连线的太阳风速度对远磁尾磁层顶的位形影响很大.研究结果可为建立包含太阳风速度v_y和v_z效应的磁层顶模型提供观测证据.     

Comparison of the magnetic field before the subsolar magnetopause with the magnetic field in the solar wind before the bow shock

Crossings of the magnetopause near the subsolar point are analyzed using data of THEMIS mission. Variations of the magnetic field near magnetopause measured by one of THEMIS satellites are studied and compared with simultaneous measurements in the solar wind by another THEMIS satellite. The time delay of the solar wind arrival at the subsolar point of the magnetopause is taken into account. 30 and 90 s averaging of the magnetic field in the magnetosheath is produced. The results of averaging are compared with the results of measurements in the solar wind before the bow shock and foreshock. It is shown, that _Bx$B_x$ component of the magnetic field near magnetopause is near to zero, which supports the possibility to consider the magnetopause as the tangential discontinuity. Comparatively good correlation of _By$B_y$ component in the solar wind and near the magnetopause is observed. The correlation of _Bz$B_z$ component near the magnetopause and IMF is practically absent, the sign of the _Bz$B_z$ near the subsolar point does not coincide with the sign of IMF _Bz$B_z$ in ∼$\sim$30% cases.     

Universal scaling laws for fully-developed magnetic field turbulence near and far upstream of the Earth’s bow shock

We analyze the multifractal scaling of the modulus of the interplanetary magnetic field near and far upstream of the Earth’s bow shock, measured by Cluster and ACE, respectively, from 1 to 3 February 2002. The maximum order of the structure function is carefully estimated for each time series using two different techniques, to ensure the validity of our high-order statistics. The first technique consists of plotting the integrand of the pth order structure function, and the second technique is a quantitative method which relies on the power-law scaling of the extreme events. We compare the scaling exponents computed from the structure functions of magnetic field differences with the predictions obtained by the She–Lévêque model of intermittency in anisotropic magnetohydrodynamic turbulence. Our results show a good agreement between the model and the observations near and far upstream of the Earth’s bow shock, rendering support for the modelling of universal scaling laws based on the Kolmogorov phenomenology in the presence of sheet-like dissipative structures.     

On the modular approach of a 3D model of the system magnetosheath–magnetosphere

This article presents some preliminary features of a new self consistent model of the system magnetosheath–magnetosphere, recently developed in the Institute of Mechanics, Sofia, Bulgaria. The flow in the magnetosheath is governed by 3D ideal gas-dynamic equations. The positions and the shapes of the bow shock and the magnetopause are calculated iteratively as a part of the solution. These surfaces are essentially three-dimensional (generally non-axially-symmetric). The self-consistency between the regions is ensured via the boundary conditions. The magnetopause cusp indentations are formed, influencing essentially the magnetosheath flow. Prediction of the position and the shape of the bow shock for different values of the sonic Mach number are derived. Distribution of some flow parameters in the magnetosheath is presented. 3D numerical finite element model, calculating the field due to the magnetopause currents for an arbitrary magnetopause geometry, is used in the magnetosphere. The fields due to the current systems inside the magnetosphere(cross-tail current, ring current, and Birkeland current) are taken from the Tsyganenko empirical model. The magnetopause surface is calculated from the requirement the outside gas-dynamic pressure to be balanced by the magnetic pressure inside. The magnetosphere model can be viewed as an improved version of the empirical model but with more realistic magnetopause form and shielding field. Not a final but a beta version is used in this approach. The final model version as well the model details will be presented elsewhere.     

偶极倾角对弓激波日下点距离和尾部张角的影响

通过对IMP 8,Geotail,Magion 4和Cluster 1四颗卫星弓激波穿越数据的统计及拟合分析,定量研究了偶极倾角对弓激波日下点距离和尾部张角的影响.结果表明:弓激波日下点距离随偶极倾角绝对值的增大而增大,且偶极倾角为负值时比其为正值时日下点距离增大的幅度更大;弓激波尾部张角随偶极倾角绝对值增大而减小;当偶极倾角由负变为正的时候,弓激波向地球一侧移动,同时尾部张角增大.研究结果为进一步建立包含偶极倾角效应的弓激波模型奠定了基础.     

A Theoretic Interpretation of Movement of the Cusp Equatorward Boundary

A preliminary model is proposed to describe quantitatively the position and movement of cusp equatorward boundary. This integrated model, consisting of an empirical model of the magnetopause and a compressed dipolar model of Open/Closed field line, connects quantitatively the solar wind conditions, subsolar magnetopause and cusp equatorward boundary. It is shown that the increasing solar wind dynamic pressure and the increasing southward Interplanetary Magnetic Field (IMF) component drive the magnetopause to move inward and the cusp equatorward. This model is adopted to interpret quantitatively the cusp movement of August 14, 2001 observed by Cluster. The results show that the subsolar magnetopause moved earthward from 10.7 He to 9.0 Re during the period of 002300-002800 UT, and correspondingly the cusp equatorward boundary shifted equatorward. The observations of Cluster C1 and C4 show the cusp equatorward boundary that Cluster Cl and C4 were crossing during same interval moved equatorward by 4.6°. The cusp equatorward boundary velocity computed in the theoretical model (10.7km/s) is in good agreement with the observed value (9.4km/s) calculated from the data of CIS of Cluster C4 and C1.     

太阳风中小尺度磁通量管边界重联的统计研究

通过对WIND卫星1995—2005年的数据,利用程序筛选及人工识别两种不同方法确定的小尺度磁通量管进行比较,发现程序筛选法中41%的小尺度磁通量管有边界重联现象,与人工识别法确定的小尺度磁通量管的统计结果接近;通过人工识别和程序筛选两种方法确定的小尺度磁通量管的边界重联特征,包括磁场剪切角、磁场强度以及重联耗散区的持续时间等,也具有相同的统计趋势.结果表明,两种方法确定的小尺度磁通量管在重联特性上没有本质区别,因此采用这两种方法得到的数据作为样本来统计小尺度磁通量管前后边界重联事件.本文共确定了71个重联事件,统计结果显示有50个(70%)重联耗散区磁场的减小超过20%,47个(66%)磁场剪切角大于90°;多数重联事件的磁场剪切角大于90°,表明小尺度磁通量管边界中主要发生的是反平行重联.将小尺度磁通量管的前后边界重联分开进行统计,结果显示其前后边界重联的特征是相似的,与磁云前后边界存在差异的性质不同,这意味着太阳风中的小尺度磁通量管并不具有磁云这种大尺度磁通量管的膨胀特征.     

弓激波模型的对比

使用Cluster卫星的弓激波穿越数据,比较了Peredo弓激波模型、Merka弓激波模型、Chao弓激波模型和Lu弓激波模型在极端太阳风条件、偶极倾角较大和平静太阳风条件下的预测精度.结果表明:Peredo模型在极端太阳风条件和平静太阳风条件下的预测误差均较大;Merka模型在极端太阳风条件下的预测误差较大;Chao模型可以较为准确地描述平静太阳风条件下的弓激波位型,但不能准确描述偶极倾角较大时的弓激波位型;Lu模型可以同时准确描述极端太阳风条件和平静太阳风条件下的弓激波位型.     

Spatial variation of the supersonic thermal plasma flow downstream of the termination shock

In this paper we start from the most recently observed fact that the solar wind plasma after passage over the termination shock is still supersonic with a Mach number of about 2. To explain this unexpected phenomenon and to predict the evolution of properties of the downstream plasma flow we here consider a two-fluid proton plasma with pick-up protons as a separate suprathermal, second proton fluid. We then formulate a self-consistent system of hydrodynamical conservation equations coupling the two fluids by dynamical and thermodynamical coupling terms and taking into account the effects of newly incorporated protons due to charge exchange with the H-atoms in the heliosheath. This then allows us to predict that in the most probable case the solar wind protons will become subsonic over a distance of about 30 AU downstream of the shock. As we can also show, it may, however, happen that the plasma mixture later again reconverts to a supersonic signature and has to undergo a second shock before meeting the heliopause.     

Modeling of the outer heliosphere with the realistic solar cycle

Time-dependent kinetic-continuum model of the solar wind interaction with the two-component local interstellar cloud (LIC) has been developed recently [Izmodenov, V., Malama, Y.G., Ruderman, M.S. Solar cycle influence on the interaction of the solar wind with local interstellar cloud. Astron. Astrophys. 429, 1069–1080, 2005a.]. Here, we adopted this model to the realistic solar cycle, when the solar wind parameters at the Earth’s orbit are taken from space data. This paper focuses on the results related to the termination shock (TS) excursion with the solar cycle that may help to understand Voyager 1 data obtained at and after the crossing of the termination shock and to predict the time of the TS crossing by Voyager 2.     

Challenges to cosmic ray modeling: From beyond the solar wind termination shock

Voyager 1 crossed the solar wind termination shock on December 16, 2004 at a distance of 94 AU from the Sun, to become the first spacecraft to explore the termination shock region and to enter the heliosheath, the final heliospheric frontier. By the end of 2006, Voyager 1 will be at ∼101 AU, with Voyager 2 at ∼81 AU and still approaching the termination shock. Both spacecraft have been observing the modulation of galactic and anomalous cosmic rays since their launch in 1977. The recent observations close to or inside the heliosheath have provided several interesting ‘surprises’ with subsequent theoretical and modeling challenges. Examples are: what does the modulation of galactic cosmic rays amount to in this region?; how do the anomalous cosmic rays get accelerated and modulated?; why are there ‘breaks’ in the power-law slopes of the spectra of accelerated particles? Several numerical models have been applied to most of these topics over the years and comprehensive global predictions have been made the past decade, thought to be based on reasonable assumptions about the termination shock and the heliosheath. Examples of these predictions and assumptions are concisely discussed within the context of the main observed features of cosmic rays in the vicinity of the termination shock, ending with a discussion of some of the issues and challenges to cosmic ray modeling in particular.     

太阳风中的动力论Alfven激波

本文指出因动力论Alfven波的存在非线性作用，从而能引起波连续陡化;这种情况下考虑频散效应，理论上应该形成中间激波.针对这一物理思想，在二元流体模型下，引进离子声反常阻尼，数值解出中间激波的结构.     

Two-velocity structure observed in the inner solar wind

Measurements of the motion of plasma density inhomogeneities in the inner solar wind are presented. The speeds were estimated using a cross-correlation analysis of radio frequency fluctuations of the Galileo spacecraft measured simultaneously at widely spaced ground stations. The radial projections of the correlation baselines on the pattern plane were of the order of several thousand kilometers. For cross-correlation functions calculated with comparatively short averaging times, we find that a pronounced two-velocity configuration is occasionally observed over the range of heliocentric distances 20 R < R < 40 R. The typical mean speed for such observations is about 300–400 km/s and the difference between the two predominant speeds is about 150–200 km/s. These results may indicate that the density fluctuations are associated with slow magnetosonic waves propagating in opposite directions at the local speed of sound in the reference frame moving with the mean solar wind speed. Quite reasonable estimates of the solar wind speed and speed of sound are obtained from this model. Another possible explanation of the two-velocity structures is that two independent solar wind streams are present simultaneously along different segments of the radio ray path.     

A Review of Density Holes Upstream of Earth's Bow Shock

Larmor size transient structures with density depletions as large as 99% of ambient solar wind density levels occur commonly upstream of Earth＇s collisionless bow shock. These ＂density holes＂ have a me...     

一个基于观测的太阳风背景模型

建立由太阳光球磁场和日冕偏振亮度等观测约束的单流体太阳风模型,包括日冕和太阳风的等离子体密度、速度和磁场,温度还有待于以后处理.这里采用高山观测台(HAO)MKⅢ的日冕偏振亮度(pB)在1.36Rs上的观测概图,根据Guhathakurta在1996年发展的日冕电子密度反演模型确定日冕的电子密度分布.同时采用Wilcox太阳观测台(WSO)的光球磁场视向分量的观测概图作为底部边界,根据Zhao等在1994年发展的水平电流-电流片(HCCS)模型得到全球磁场.Phillips在1995年及McComas在2003年分别用Ulysses第一次和第二次跨极飞行的观测发现,归一化到1 AU的太阳风动量流密度除了在10°～30°的纬度范围内略低以外几乎不变.根据这一结论,结合已经得到的密度数据,就可以得到日冕和太阳风的速度.将上面的模型应用于1918卡林顿自转周稳态太阳风的研究,结果与太阳活动极小期的观测基本相符,但是与观测相比较低速高密度区偏大,因此密度模型还有待改进.