• ISSN 2097-1893
  • CN 10-1855/P

火星感应磁层边界观测研究进展

Review of observations on the boundary of the Martian induced magnetosphere

  • 摘要: 关于太阳风-火星相互作用边界的探测研究几乎贯穿了整个火星空间探测的历史. 太阳风-火星相互作用区中间总是存在一个边界,即火星感应磁层边界. 过去几十年,受火星探测器所携带仪器种类和精度的限制,火星感应磁层边界一直基于单一或部分仪器探测数据来定义,导致其具有多种多样的名称,如,离子成分边界、感应磁层边界、磁堆积边界等. 本文沿着Phobos-2、MGS、MEX以及MAVEN等一系列火星探测器的探测历程,简单梳理了火星感应磁层边界观测研究的发展. 总结了具有不同名称的边界的物理特征和定义标准及其之间的位置关系. 最新研究基于MAVEN探测器的等离子体和磁场综合探测数据,研究证实了火星感应磁层边界为磁流体动力学间断面,其物理性质与地球磁层顶类似. 磁场的堆积位于边界间断面的上游,其性质更类似于地球磁层顶上游的等离子体耗尽层. 相关工作进一步厘清了太阳风-火星相互作用边界的结构和物理性质. 最早对太阳风-火星相互作用边界的认识来自于Phobos-2探测器对火星等离子体边界的报道. 太阳风等离子体流在火星电离层上方几百公里处停滞,形成阻碍太阳风的边界,当时被称为质子边界. 在该边界附近,同样观测到了将上游太阳风质子和下游行星氧离子分隔开的边界,称为离子成分边界. 同时,还观测到了穿出边界时强电场波动消失的特征,称作行星顶. 这三种边界可能重合. 由于当时对火星磁场研究的不足,还无法确定该边界是否由行星内禀磁场的障碍产生. 随着MEX和MAVEN探测器提供了对等离子体观测的更精确数据,离子成分边界有了更清晰的定义,即边界将其上下游分别由太阳风质子和火星重离子(如O+和O2+)主导的等离子体分隔开来,上游的太阳风质子流在边界处停滞. Phobos-2探测器上携带的电磁场仪器,在太阳风-火星相互作用区探测到了电磁场波动减弱和磁场强度增强的信号,该信号所形成的边界称为磁堆积边界. MGS探测器首次提供了清晰的火星全球磁场图像,也为磁堆积边界的研究提供了大量数据,并给出了该边界位置的经验模型. MGS探测器证实,火星不存在全球内禀磁场,因此在磁堆积边界处,行星际磁场受到火星电离层的阻碍,发生堆积,表现为磁场强度的增强和磁场波动的减弱. 在此前研究的基础上,利用该特性基于MAVEN探测器的观测数据,定量地给出了磁堆积边界的判定条件. 早期因为缺少磁场和等离子体的综合观测数据,压力平衡边界的概念和模拟研究早于对其的观测研究. 太阳风在火星弓激波处减速加热,形成了火星磁鞘. 来自上游磁鞘的动压、热压和磁压与下游感应磁层中的热压和磁压达到平衡,形成了压力平衡边界. 基于MAVEN探测器提供的磁场和等离子体的综合观测数据,对压力平衡边界进行了相关的统计研究,并给出了其统计平均位置. 以上三种定义的边界的位置会受到太阳风动压、太阳EUV、火星剩余磁场等因素的影响. 然而,三者之间的相对位置尚未有定论,且相互之间的物理联系也没有解释. 基于MAVEN探测器的等离子体和磁场综合探测数据的最新研究显示,等离子体成分和流速变化所标记离子成分边界上,存在平行于边界方向的磁场的突变(方向或大小),形成了边界间断面(包括切向间断面和旋转间断面),与地球磁层顶性质类似. 而边界上游存在磁场堆积,在磁堆积区域,等离子体成分为太阳风质子主导,且质子能谱与火星磁鞘中的质子能谱类似,质子流速伴随磁堆积减小但并没有停滞. 因此,火星磁堆积区域是火星磁鞘的一部分,其性质类似于地球磁层顶上游的等离子体耗尽层. 该工作给出了太阳风-火星相互作用区域的边界结构和物理特性.

     

    Abstract: The boundary of the Mars-solar wind interaction has been investigated throughout the history of Mars space exploration. There is always a boundary between the solar wind and Mars, namely the boundary of the Martian induced magnetosphere. In the past few decades, due to the limitations of the types and accuracy of instruments on board Mars probes, the definition of the boundary of the Martian induced magnetosphere has been based on single or partial instrument data, resulting in different boundary names, such as the ion composition boundary, the induced magnetosphere boundary, and the magnetic pile-up boundary. This article briefly reviews the development of the observational studies on the boundary of the Martian induced magnetosphere along the exploration history of a series of Mars probes such as Phobos-2, MGS, MEX, and MAVEN, and summarizes the physical characteristics and definition criteria of boundaries with different names, as well as their positional relationships. Based on the comprehensive detection data of plasma and magnetic field from MAVEN, it is confirmed that the boundary of the Martian induced magnetosphere is a magnetohydrodynamic discontinuity, and its physical properties are similar to those of the Earth's magnetopause. The pile-up process of the magnetic fields occurs upstream of the discontinuity, and its properties resemble those of the plasma depletion layer upstream of the Earth's magnetopause. The relevant work clarified the structure and physical properties of the Mars-solar wind interaction boundary. The earliest knowledge of the Mars-solar wind interaction boundary came from the reporting on the plasma boundary by Phobos-2. The solar wind flow was blocked several hundred kilometers above the Martian ionosphere, forming a boundary known as the Protonopause. Near the Protonopause, another boundary termed the ion composition boundary (ICB), was observed that separates upstream solar wind protons from downstream planetary oxygen ions. Meanwhile, a boundary, referred to as the Planetpause, characterized by the disappearance of strong electric field fluctuations was also observed. These three boundaries were believed to overlap. Lacking sufficient research on the Martian magnetic field at that time, it was not yet possible to determine whether the boundary was formed due to the solar wind blocked by the planet's intrinsic magnetic field. With more accurate plasma data on the MEX and MAVEN spacecraft, the definition of ICB has been clearer, that is, the boundary separates the plasma dominated by upstream solar wind protons and downstream Martian heavy ions (such as O+and O2+), and the upstream solar wind proton flow ceases at the boundary. Phobos-2 spacecraft, equipped with electromagnetic field instruments, detected the attenuation of electromagnetic field fluctuations and enhancement of the magnetic field strength in the Mars-solar wind interaction region, which are the signals of the boundary called the magnetic pile-up boundary (MPB). The MGS probe drew a comprehensive Martian global magnetic field map for the first time and also provided sufficient data for the extensive investigation of MPB. With these data, an empirical model of MPB was given. The MGS probe confirmed no global intrinsic magnetic field on Mars. Thus, the interplanetary magnetic field obstructed by the Martian ionosphere was piled up at MPB, resulting in an increase of magnetic field strength and a decrease in magnetic field fluctuations. Based on these features, the criterion of MPB was quantified using the MAVEN data. Without comprehensive data on magnetic fields and plasma around Mars previously, the Martian pressure balance boundary was in the concept and the investigations on it were also limited to the simulations. The solar wind slows down and heats up as crossing the Martian bow shock, forming the Martian magnetosheath. The dynamic pressure, thermal pressure, and magnetic pressure in the upstream magnetosheath were balanced with the thermal pressure and magnetic pressure in the downstream induced magnetosphere, forming the pressure balance boundary. Based on the comprehensive data of magnetic field and plasma on MAVEN spacecraft, statistical studies were conducted on the pressure balance boundary and provided its statistical average location. The locations of the boundaries defined above will be affected by many factors such as solar wind dynamic pressure, solar EUV, and the crustal magnetic field on Mars. However, their positional relationship is still under debate, and their physical relationship has not been explained. The latest research using the comprehensive data of plasma and magnetic field on MAVEN spacecraft shows that there is a sudden change in the magnetic field component (the orientation or magnitude) parallel to the boundary plane on the ICB, characterized by the separation of different plasma compositions and the ceasing of solar wind flow. Thus, a MHD discontinuity (including the tangential and rotational discontinuity), namely the boundary, forms with properties similar to those of the Earth's magnetopause. There are piled-up magnetic fields upstream of the boundary. In the pile-up region, the plasma is dominated by solar wind protons, and the proton energy spectrum is similar to that in the Martian magnetosheath. The proton flow slows down with the accumulation of the magnetic field and ceases until the magnetopause. Therefore, the Martian magnetic pile-up region is part of the Martian magnetosheath, with properties similar to the plasma depletion layer upstream of the Earth's magnetopause. This work clarifies the boundary structures and physical characteristics in the Mars-solar wind interaction region.

     

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