行星际中间激波的演化

本文采用特征线方法和激波装配法,对磁流体中间激波在行星际空间的演化过程进行数值模拟。主要结论如下:（1） 2→4型中间激波通过向下游发出后向慢压缩波使下游态磁场减幅,通过向上游发出前向快压缩波使上游态磁场增幅,以致2→4型中间激波迅速经导灭激波向慢激波转化;所发出的前向快压缩波经非线性变陡形成快激波。（2）1→3型中间激波首先通过向下游发出前向慢稀疏波而很快变成1→3=4型临界中间激波,并瞬间转变为由前向快激波和前向2→4型中间激波构成的激波系统。其中,2→4型中间激波可在其前导快激波的下游传播较远的距离,有可能为 IAU 附近的飞船观测到,但最终导灭激波转变为慢激波。     

行星际速度增幅扰动的演化

采用二维理想MHD模型，分别在日球赤道面（二维二分量模型）和日球子午面（二维 三分量模型）内研究太阳风中纯速度增幅扰动的演化. 结果表明，该扰动在向外传播的过程 中逐渐演化为双重激波对，即由4个激波组成的激波系统. 该4个激波按离太阳由近及远依次 为后向快激波、后向慢激波、前向慢激波和前向快激波. 双重激波对在子午面内相对扰动源 中心法线基本对称，而在赤道面内则不对称：扰动源中心法线西侧双重激波对结构更为明显 ，所跨经度范围宽于东侧. 初步分析表明，行星际磁场的螺旋结构是产生日球赤道面内双重 激波对结构东西不对称性的主要原因.     

太阳风流界面的成因

在日心距离１ＡＵ处的高速流的前沿部位，经常观测到厚度≈１０^４ｋｍ的流界面（ｓｔｒｅａｍｉｎｔｅｒｆａｃｅ）：跨过它密度陡降，温度陡增，风速上升，气压和磁场几乎连续．本文从日心距离０．３ＡＵ处一典型高速流的方位剖面出发，采用二维定态ＭＨＤ模型，研究它在日球赤道面内随日心距离的演化．结果表明，流界面系高速流前沿非线性演化的产物．它先于前、后向激波形成，在日心距离１ＡＵ处得到充分发展，且作为高速流前沿的特征结构之一，可一直延伸到１ＡＵ以远的外日球层．     

慢激波在快激波下游演化的限制

慢激波的演化受其上游介质性质的制约，在等离子体热压与磁压之比β值和离子、电子温度比Ｔｉ／Ｔｅ大于１的介质中不利于慢波变陡形成慢激波。由飞船ＨｅｌｉｏｓＡ，Ｂ探测资料看出，在日心距０．３-１．０ＡＵ区间只有慢速太阳风流中存在有利于慢激波形成的条件。但理论计算和飞船观测指出，在快激波下游流场中β值和Ｔｉ／Ｔｅ都增大，因而在上述区间不论何种流速的太阳风中当有快激波经过后其下游流场内很难形成慢激波。     

行星际空间慢激波的演化

日心距离０．３ＡＵ以内形成的磁流体慢激波在向行星际空间传播过程中，通过向上游发出快压缩波而不断减弱．所发出的快压缩波经非线性变陡转化为快激波，形成由原慢激波和新生快激波构成的激波系统．强度不断减弱的慢激波将逐渐演变为准切向间断．这可能是在１ＡＵ附近很少观测到慢激波的重要原因．     

行星际中间激波

在行星际空间可能存在由各类激波连接而成的混合激波,其组成部分的连接方式和时间演化遵循“慢激波-导灭激波-中间激波-导生激波-快激波”链式规则.中间激波将作为混合激波的必要组成部分,出现在日球电流片的附近,其阵面凹向太阳.上述结论已初步为观测证实,并对行星际激波的三维特性有着重要影响.     

上游参数空间的激波连接

本文分析各类激波特别是中间激波的特性与上游阿尔文数、激波角和气压-磁压比的关系,阐明各类激波在上游参数空间的连接方式.主要结论是:当上游阿尔文波速大于声速时,慢激波和快激波可通过三种基本方式相互连接;当上游阿尔文波速小于声速时,激波连接只限于慢激波和中间激波之间.这些结论有助于进一步探讨混合激波的结构和演化规津.     

MHD激波能量转换

从ＭＨＤＲａｎｋｉｎｅ－Ｈｕｇｏｎｉｏｔ关系出发，导出激波下游磁能、内能和动能相对上游的增长因子，它们依赖于上游的激波角、等离子体β值和激波强度．对这些因子分析得出：１．由于行星际存在大尺度螺旋磁场，使激波而不同部位能量转换率不同，导致激波面西侧为强磁场区；２．磁能、内能转换与介质流速无关，动能转换与流速呈线性关系；３．在激波驱动过程中，能量通过后向激波输入到前、后向激波间的相互作用区；４．行星际激波能量转换以法向动能为主，随着激波向外传播介质β值不断增大，法向动能增长因子不断减小，致使激波和太阳风介质的相互作用不断减弱．     

MHD激波解的讨论

从Ｒａｎｋｉｎｅ－Ｈｕｇｏｎｉｏｔ关系出发，以激波切向磁场ξ、上游激波角θ_１和等离子体β_１值为参数研究各类激波解的特性及相互关系，阐明各类激波强度随介质β值的变化规律．结果指出：（１）在ξ＜－１的Ⅰ_ｂ型中间激波区和ξ＞４的快激波区存在双解；（２）慢激波可以直接和中间激波连接，但不能和快激波直接连接；（３）各类激波强度（用激波密度比衡量）随β_１值均有变化：Ⅰ_ｂ型中间激波上支随β_１值增加而下降，下支则上升；Ⅰ_ａ型中间激波和慢激波都随β_１值增加而下降；快激波双解区上支随β_１值增加而下降。下支上升；快激波在１＜ξ＜４区间的解随β_１值增加而上升。     

上游参数空间的激波分布

本文从Rankine-Hugoniot关系出发,采用阿尔文数A₁、激波角θ₁和气压-磁压比β₁作为上游参数,导出了参数空间中激波区边界的解析表达式;讨论了各类激波在参数空间的分布范围和相互关系;指出只有当上游背景阿尔文波速超过声速时,才有可能通过中间激波实现快、慢激波之间的空间连接和时间演化.     

Generation and evolution of interplanetary slow shocks

It is well known that most MHD shocks observed within 1 AU are MHD fast shocks. Only a very limited number of MHD slow shocks are observed within 1 AU. In order to understand why there are only a few MHD slow shocks observed within 1 AU, we use a one-dimensional, time-dependent MHD code with an adaptive grid to study the generation and evolution of interplanetary slow shocks (ISS) in the solar wind. Results show that a negative, nearly square-wave perturbation will generate a pair of slow shocks (a forward and a reverse slow shock). In addition, the forward and the reverse slow shocks can pass through each other without destroying their characteristics, but the propagating speeds for both shocks are decreased. A positive, square-wave perturbation will generate both slow and fast shocks. When a forward slow shock (FSS) propagates behind a forward fast shock (FFS), the former experiences a decreasing Mach number. In addition, the FSS always disappears within a distance of 150R_⊙ (where R_⊙ is one solar radius) from the Sun when there is a forward fast shock (with Mach number \geq1.7) propagating in front of the FSS. In all tests that we have performed, we have not discovered that the FSS (or reverse slow shock) evolves into a FFS (or reverse fast shock). Thus, we do not confirm the FSS-FFS evolution as suggested by Whang (1987).     

The superdense plasma sheet in the magnetosphere during high-speed-stream-driven storms: Plasma transport timescales

The superdense plasma sheet in the Earth's magnetosphere is studied via a superposition of multispacecraft data collected during 124 high-speed-stream-driven storms. The storm onsets tend to occur after the passage of the IMF sector reversal and before the passage of the stream interface, and the storms continue on for days during the passage of the high-speed stream. The superdense phase of the plasma sheet is found to be a common feature of high-speed-stream-driven storms, commencing before the onset of the storm and persisting for about 1 day into the storm. A separate phenomenon, the extra-hot phase of the plasma sheet, commences at storm onset and persists for several days during the storm. The superdense plasma sheet originates from the high-density compressed slow and fast solar wind of the corotating interaction region on the leading edge of the high-speed stream. Tracking the motion of this dense plasma into and through the magnetosphere, plasma transport times are estimated. Transport from the nightside of the dipole to the dayside requires about 10 h. The occurrences of both the superdense plasma sheet and the extra-hot plasma sheet have broad implications for the physics of geomagnetic storms.     

Stationary strong magnetohydrodynamic discontinuities and pressure balanced structures in the solar wind stream

Strong disturbances of magnetic clouds in the solar wind stream are considered when solar MHD shock waves from the surrounding plasma collide with these inhomogeneities. The boundaries of the considered plasma inhomogeneities are presented as stationary tangential discontinuities. The collision of solar fast shock waves with the back and front boundaries is studied as a decomposition of an arbitrary discontinuity. It is asserted that secondary waves of rarefaction and reverse shock waves arise depending on the initial conditions. It is pointed out that a change occurs in the configuration of the plasma inhomogeneity under study, which is caused by the incoming perturbation repeatedly observed by spacecrafts.     

Role of the Russell–McPherron effect in the acceleration of relativistic electrons

While it is well known that high fluxes of relativistic electrons in the Earth's radiation belts are associated with high-speed solar wind and its heightened geoeffectiveness, less known is the fact that the Russell–McPherron (R–M) effect strongly controls whether or not a given high-speed stream is geoffective. To test whether it then follows that the R–M effect also strongly controls fluxes of relativistic electrons, we perform a superposed epoch analysis across corotating interaction regions (CIR) keyed on the interfaces between slow and fast wind. A total of 394 stream interfaces were identified in the years 1994–2006. Equinoctial interfaces were separated into four classes based on the R–M effect, that is, whether the solar wind on either side of the interface was either (geo)effective (E) or ineffective (I) depending on season and the polarity of the interplanetary magnetic field (IMF). Four classes of interface identified as II, IE, EI, and EE are possible. The classes IE and EI correspond to CIRs with polarity changes indicating passage through the heliospheric current sheet. To characterize the behavior of solar wind and magnetospheric variables, we produced maps of dynamic cumulative probability distribution functions (cdfs) as a function of time over 10-day intervals centered on the interfaces. These reveal that effective high-speed streams have geomagnetic activity nearly twice as strong as ineffective streams and electron fluxes a factor of 12 higher. In addition they show that an effective low-speed stream increases the flux of relativistic electrons before the interface so that an effective to ineffective transition results in lower fluxes after the interface. We conclude that the R–M effect plays a major role in organizing and sustaining a sequence of physical processes responsible for the acceleration of relativistic electrons.     

Impact of solar wind tangential discontinuities on the Earth’s magnetosphere

The collision of a solar wind tangential discontinuity with the bow shock and magnetopause is considered in the scope of an MHD approximation. Using MHD methods of trial calculations and generalized shock polars, it has been indicated that a fast shock refracted into the magnetosheath originates when density increases across a tangential discontinuity and a fast rarefaction wave is generated when density decreases at this discontinuity. It has been indicated that a shock front shift under the action of collisions with a tangential discontinuity is experimentally observed and a fast bow shock can be transformed into a slow shock. Using a specific event as an example, it has been demonstrated that solar wind tangential discontinuity affects the geomagnetic field behavior.