• ISSN 2096-8957
  • CN 10-1702/P

水星磁层观测研究

钟俊

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水星磁层观测研究

Observational research on Mercury's magnetosphere

    Corresponding author: Zhong Jun, j.zhong@mail.iggcas.ac.cn
  • CLC number: P352

  • 摘要: 水星磁层无辐射带、电离层、等离子层,大气层明显消失,只有微弱的外逸层. 由于磁层尺度小,行星内核感应效应较为明显. 行星空间环境显著区别于地球. “信使”号卫星对水星磁层的观测研究丰富了对水星空间环境的认识和理解. 本文主要从磁层尺度及变化性、磁场重联及磁通量绳的形成、典型磁层动力学活动过程、磁层行星重离子时空变化、极端太阳事件下磁层响应特征等方面对水星磁层“信使”号观测研究进展进行简要总结. 并对BepiColombo卫星探测进行相关问题研究展望.
  • 图 1  太阳风与水星相互作用示意图

    Figure 1.  The primary features of solar wind-Mercury interaction

    图 2  水星磁尾磁重联区观测.(a)~(c)为穿越磁尾电流片的概况;(d)~(i)为穿越电流片中心重联扩散区的详细数据.(a)、(b)为磁场强度及在MSM坐标系下三分量;(c)离子能谱数据;(d)~(g)磁场强度及在局地电流片LMN坐标系下三分量;(h)NS仪器探测高能电子(>20~40 keV)计数率. 根据BN磁场结构及变化特征,离子扩散区可以划分为5个不同特征的时间段:T1~T5,分别对应于右示意图(修改自Zhong et al., 2018

    Figure 2.  MESSENGER observations of an active reconnection site in Mercury's magnetotail. Panels (a)~(c) show an overview of the current sheet crossing. Panels (d)~(i) show a subset of the data near the diffusion region. (a),(b) The magnetic field magnitude and its three components in the LMN coordinates. (c) Spectrogram of the ion differential energy flux. (d)~(g) The magnetic field magnitude and its three components. (h) Count rate of energetic electrons detected from the NS instrument with 1 s resolution in its burst mode. The diffusion region crossing is divided into five short subintervals, T1~T5. Right: Schematic of the rapidly evolving reconnection process in Mercury's magnetotail (modified from Zhong et al., 2018)

    图 3  水星空间大尺度磁通量绳结构形成过程示意图.(a)众多离子尺度磁通量绳相互作用、多步骤合并形成FTEs;(b)近磁尾和远磁尾重联形成等离子体团结构;(c)极端太阳风条件下磁尾电流片撕裂模不稳定性形成多重联线及离子尺度磁岛链,众多磁岛合并形成大尺度磁通量绳,并周期性释放典型的水星磁层能量输入、输出过程(修改自Zhong et al., 2019, 2020a, 2020c

    Figure 3.  Schematic of macroscale flux rope structures formation in Mercury's space. (a) Macroscale FTEs at Mercury's dayside magnetopause; (b) Giant plasmoid formed and trapped between two widely separated reconnection sites in Mercury's magnetotail; (c) Multiple X-line reconnection in Mercury's magnetotail. (top) Formation of ion-scale flux ropes and the occurrence of multiple X-line reconnection in the elongated tail current sheet. (bottom) Formation of a large-scale flux rope through the interaction and coalescence of many of ion-scale flux ropes and their tailward ejection(modified from Zhong et al., 2019, 2020a, 2020c

    图 4  (a)水星磁层亚暴膨胀相期间阿尔芬波及压缩波的形成示意图(修改自Sun et al., 2015a). (b)水星磁尾亚暴电流楔形成的直接观测结果示意图(修改自Poh et al., 2017a)

    Figure 4.  (a) A schematic to illustrate the Alfvénic and compressional waves generated during the substorm expansion phase in Mercury's magnetotail (modified from Sun et al., 2015a). (b) Schematic illustrations of asymmetries in Mercury's current sheet. Left: the formation of a substorm current wedge in the near-Mercury region. Right: current sheet structure in the postmidnight and premidnight views (modified from Poh et al., 2017a)

    图 5  不同磁场大小下水星磁层顶K-H波多尺度特征. 夜侧磁层顶K-H波的频率和局地Na+回旋频率接近,由于磁场大小的不同,频率可以小于(a)、等于(b)或大于(c)日测K-H波频率(修改自Gershman et al., 2015

    Figure 5.  Illustration of K-H wave growth along Mercury's magnetopause for increasing magnetic field given a constant vortex speed. Toward the tail, where the Na+ is expected to dominate the plasma mass density, the observed frequency of K-H waves (blue spacecraft) matches that of the Na+ gyrofrequency, which can be (a) less than, (b) equal to, or (c) greater than that observed on the dayside (red spacecraft) (modified from Gershman et al., 2015)

    图 6  (a)极尖区观测到的Na+形成机制.(左)太阳风离子溅射和光电离;(右)磁层顶附近或太阳风区域外逸层中性原子电离,被太阳风“拾起”进入极尖区和磁层(修改自Raines et al., 2014).(b)THEMIS遥感探测外逸层典型Na分布模式.(上)日侧低纬单峰模式;(下)中纬地区双峰模式(修改自Mangano et al., 2015

    Figure 6.  (a) Two possible sources for Na+ ions in the cusp: (left) Na+ ions are generated in the cusp, both by solar wind impact and photoionization, and are accelerated by processes there. (right) Neutral Na atoms are ionized near the magnetopause and swept into the cusp (modified from Raines et al., 2014). (b) Examples of the 8 recurrent Na emission patterns identified in the Mercury's exosphere. Top: equatorial Peak. Bottom: two peaks in middle latitude (modified from Mangano et al., 2015)

    图 7  左:少数极端情况下卫星轨道未穿越向阳侧磁层事例.(a)~(d)磁场强度及其三分量;(e)磁场天顶角(红)和方位角(蓝);(f)质子能谱,磁场在(g)X-Z 平面和(h)X-Y平面的投影,以及弓激波(BS)和磁层顶(MP)观测位置与平均磁层顶模型(虚曲线)的比较(修改自Zhong et al., 2015a). 右:向阳侧磁层消失事件观测结果示意图(修改自Slavin et al., 2019

    Figure 7.  Left: Example of MESSENGER missed the dayside magnetosphere. (a)~(d) Magnetic field intensity and its three components; (e) magnetic field zenith and azimuthal angles; (f) spectrogram of proton flux, and the MESSENGER orbit and the vector plots of the magnetic field in the (g) noon-midnight and (h) equatorial planes relative to Mercury's surface (circle) and the average magnetopause from the model (dashed linesh) (modified from Zhong et al., 2015a) . Right: Illustration of the primary features of the disappearing dayside magnetosphere events (modified from Slavin et al., 2019)

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  • [1] 李坤崔峻魏勇 . 空间电场的原位测量. 地球与行星物理论评, doi: 10.16738/j.dqyxx.2021-013
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  • 收稿日期:  2021-04-09
  • 网络出版日期:  2021-05-07

水星磁层观测研究

摘要: 水星磁层无辐射带、电离层、等离子层,大气层明显消失,只有微弱的外逸层. 由于磁层尺度小,行星内核感应效应较为明显. 行星空间环境显著区别于地球. “信使”号卫星对水星磁层的观测研究丰富了对水星空间环境的认识和理解. 本文主要从磁层尺度及变化性、磁场重联及磁通量绳的形成、典型磁层动力学活动过程、磁层行星重离子时空变化、极端太阳事件下磁层响应特征等方面对水星磁层“信使”号观测研究进展进行简要总结. 并对BepiColombo卫星探测进行相关问题研究展望.

English Abstract

    • 人类对水星空间环境的认识起始于20世纪70年代“水手10”号(Mariner 10)探测计划. 该飞行器于1974年和1975年两次穿越水星—太阳风相互作用区,发现了水星具有类似地球的内禀偶极磁场(Ness et al., 1974). 虽然磁场大小仅为地球的1%,但仍能把太阳风阻挡在水星表面以上,形成类似地球的磁层结构. 限于探测数据量,接下来几十年,对水星空间物理的认知主要停留于理论研究. 本世纪初,美国国家航空航天局“信使”号(MESSENGER)于2011年3月入轨,成为首颗环绕水星探测卫星,为系统研究水星空间环境提供了充分的数据基础,掀起了水星空间物理观测研究的热潮.

      “信使”号持续约4年的数据观测,丰富了对水星空间环境的认识和理解. 水星的磁偶极矩(195±10 nT)×RM3(1RM=2 440 km,水星半径),约为地球的千分之五,偶极子轴几乎与自转轴平行(夹角小于3º),而偶极中心向北偏移484±11 km(Anderson et al., 2011). 水星轨道处强太阳风驱动和弱行星磁场的相互作用,形成太阳系中尺度最小且动力学特征最活跃的行星磁层. 尽管水星磁层结构和地球类似,具有明显的磁层顶边界层、极尖区、磁尾等离子体片及尾瓣等结构,但其空间尺度及动力学过程时间尺度比地球小一个数量级(Slavin et al., 2008; Slavin et al., 2009; Rong et al., 2018). 辐射带、电离层、等离子层及大气层等明显消失,只有微弱的外逸层,行星内核感应效应较为明显,构成“太阳风—磁层—外逸层/星体”的耦合系统(Milillo et al., 2005). 由此导致行星空间环境显著区别于地球. 研究水星磁层动力学过程及空间环境特征是认识太阳风向弱磁层空间传输能量和物质一般规律的重要途径,同时对理解地球空间、认知电离层和磁层尺度对空间环境影响等都具有重要意义. 本文主要从以下五个方面对水星磁层“信使”号观测研究进展进行简要总结,并对欧空局—日本航空局联合的BepiColombo卫星探测进行相关问题研究展望.

    • 磁层顶是磁层的外边界,其位置与形态表征着磁层空间的尺度. 通常水星磁场在日下点附近足以阻挡太阳风至星体表面以上,约0.5RMWinslow et al., 2013). 通过对磁层顶观测数据的重构显示水星弱偶极磁层形态具有显著的三维特征,包括,极尖区深度凹陷:中心凹陷深度~0.64RM,展宽覆盖大部分日侧区域,导致日侧磁层顶在东西方向拉伸;磁尾晨昏收缩:南北尺度大于东西尺度,在磁尾1.5RM 处其尺度分别为2.6RM和2.2RM;以及闭合磁尾等(Zhong et al., 2015a). 从三维磁层顶模型推断,水星弱偶极磁场并不能像当前地球一样有效屏蔽太阳风粒子的直接轰击. 即使在正常太阳风条件下,太阳风仍可在南半球中纬区域与水星直接相互作用(图1). 这一由极尖区深度凹陷及内禀偶极磁场中心偏离(Anderson et al., 2011)导致的直接相互作用区并未在过去理论研究中被注意到. 由于水星轨道偏心率大(离日距离0.31~0.47AU),水星磁层尺度对太阳风轨道周期变化响应显著. 当水星从远日点运行到近日点,磁层特征尺度(日下点距离、侧翼距离、磁尾半径等)全球性收缩15%,日侧磁层空间缩小近40%(Zhong et al., 2020b). 在近日点附近,日下点磁层顶被压缩至0.2RM 高度以内的概率为2.5%,即可能会出现太阳风粒子在日下点附近直接作用于星体表层(Zhong et al., 2015b).

      图  1  太阳风与水星相互作用示意图

      Figure 1.  The primary features of solar wind-Mercury interaction

      相比地球,水星磁层更易受行星际磁场(IMF)的控制. 其中IMF锥角对磁层尺度的影响和轨道效应相当,IMF准平行情况下磁层全球性膨胀,而准垂直情况下收缩小于平均尺度(Zhong et al., 2020b). 与地球不同的是,统计上IMF南北向对向阳侧磁层尺度影响不明显. 由于水星磁层尺度小,行星内核感应场阻止磁层对太阳风条件变化的响应显著. Zhong等(2015b)基于观测数据获得磁层顶与离日距离关系,利用太阳风动压随离日距离的变化规律,并与内核感应模型(Glassmeier et al., 2007)进行比较,发现日下点距离观测值大于压力平衡观测计算结果,尤其在远日点观测结果接近内核感应模型,为水星内核感应电流效应存在提供了观测证据. Johnson等(2016)利用近日点和远日点附近卫星穿越水星磁层的磁场数据,通过球谐函数拟合获得总偶极距大小在近日点比远日点增加近5%,进一步证实了水星内核感应效应.

    • 类似地球,磁场重联是驱动水星磁层动力学过程的重要机制. 水星轨道处强IMF和高阿尔芬速度导致磁重联比地球空间更加高效(DiBraccio et al., 2013). 由于“信使”号等离子体仪器探测的限制,对水星空间磁重联区的直接观测研究仍较少. Zhong等(2018)在磁尾发现重联扩散区的观测证据——霍尔磁场四极结构,并报道了水星磁尾磁重联的快速演化过程. 在35 s的重联区观测时间内,磁重联位于等离子体片磁场区域发展到尾瓣区域磁场,伴随着1~3 s尺度的时序尾向传输的重联锋面和次级磁岛结构;相对稳态的时间段内重联率平均值接近0.2,是地球空间观测平均值的2倍,呈现强驱动快速脉冲式重联特征(图2). 在该事件中,中子质谱仪探测器探测到直接与非稳态重联相关的相对论电子爆发性现象,证实了在小尺度磁层空间磁重联也可以直接产生相对论电子,为早期“水手10”号水星磁层高能电子探测线索提供了一种起源证据.

      图  2  水星磁尾磁重联区观测.(a)~(c)为穿越磁尾电流片的概况;(d)~(i)为穿越电流片中心重联扩散区的详细数据.(a)、(b)为磁场强度及在MSM坐标系下三分量;(c)离子能谱数据;(d)~(g)磁场强度及在局地电流片LMN坐标系下三分量;(h)NS仪器探测高能电子(>20~40 keV)计数率. 根据BN磁场结构及变化特征,离子扩散区可以划分为5个不同特征的时间段:T1~T5,分别对应于右示意图(修改自Zhong et al., 2018

      Figure 2.  MESSENGER observations of an active reconnection site in Mercury's magnetotail. Panels (a)~(c) show an overview of the current sheet crossing. Panels (d)~(i) show a subset of the data near the diffusion region. (a),(b) The magnetic field magnitude and its three components in the LMN coordinates. (c) Spectrogram of the ion differential energy flux. (d)~(g) The magnetic field magnitude and its three components. (h) Count rate of energetic electrons detected from the NS instrument with 1 s resolution in its burst mode. The diffusion region crossing is divided into five short subintervals, T1~T5. Right: Schematic of the rapidly evolving reconnection process in Mercury's magnetotail (modified from Zhong et al., 2018)

      在极端太阳风条件下,水星磁层磁重联的表现特征有所不同. 分析2011年11月23日行星际日冕物质抛射事件(ICME)平稳驱动水星磁尾过程的典型事例,发现磁尾多X线重联主导并控制磁层的整体动力学过程(Zhong et al., 2020a). 在该事件中,多X线重联发生在不稳定的离子扩散区,观测到霍尔磁场结构、时序穿越重联线导致的霍尔磁场扰动、以及周期性离子尺度磁通量绳的产生(图3c). 估算出的重联率为0.1,重联时间尺度~0.1 s,与多X线重联理论(Fu and Lee, 1986)预测结果一致. 同时发现众多离子尺度通量绳合并形成超大磁通量绳结构,将磁尾能量周期性释放,释放周期与Dungey循环时间(Slavin et al., 2010a; Imber and Slavin, 2017)相当.

      图  3  水星空间大尺度磁通量绳结构形成过程示意图.(a)众多离子尺度磁通量绳相互作用、多步骤合并形成FTEs;(b)近磁尾和远磁尾重联形成等离子体团结构;(c)极端太阳风条件下磁尾电流片撕裂模不稳定性形成多重联线及离子尺度磁岛链,众多磁岛合并形成大尺度磁通量绳,并周期性释放典型的水星磁层能量输入、输出过程(修改自Zhong et al., 2019, 2020a, 2020c

      Figure 3.  Schematic of macroscale flux rope structures formation in Mercury's space. (a) Macroscale FTEs at Mercury's dayside magnetopause; (b) Giant plasmoid formed and trapped between two widely separated reconnection sites in Mercury's magnetotail; (c) Multiple X-line reconnection in Mercury's magnetotail. (top) Formation of ion-scale flux ropes and the occurrence of multiple X-line reconnection in the elongated tail current sheet. (bottom) Formation of a large-scale flux rope through the interaction and coalescence of many of ion-scale flux ropes and their tailward ejection(modified from Zhong et al., 2019, 2020a, 2020c

    • 磁通量绳或磁岛是磁重联的重要产物. “信使”号探测显示水星磁层充满大量的磁通量绳结构. 在磁层顶,磁通量绳一般称作通量传输事件(FTEs). 水星FTEs同样倾向更多地发生在行星际磁场南向期间(Leyser et al., 2017),观测持续时间约1 s或更短、发生周期在8~10 s左右,通常被称作为“FTE showers”(Slavin et al., 2012; Slavin et al., 2014; Sun et al., 2020b). 其空间尺度约为300~400 km,和离子回旋半径相当,与磁层相对尺度和地球FTEs相似,一般被认为是多X线重联产生(Lee and Fu, 1985). 高频率产生的FTEs对水星磁层等离子体和能量输运起着重要的作用. 磁鞘等离子体沿着FTEs进入极区,形成极尖区等离子体精细结构,向下延伸至更低的高度,甚至可能会出现在星体表层附近(Slavin et al., 2014; Poh et al., 2016).

      在邻近的磁鞘区经常观测到大尺度FTEs,观测持续时间可以达到几秒,对应空间尺度0.5~−1RMSlavin et al., 2010b; Imber et al., 2014). Imber等(2014)考虑快速的产生率,估算出这些大尺度FTEs携带的磁通可以贡献驱动水星磁层亚暴所需磁通的~30%,而这一数值在地球空间通常小于2%. Fear等(2019)同时考虑磁层顶爆发性磁重联产生的开放磁通,发现这一磁通贡献率被低估5倍,认为FTEs携带的磁通量足以驱动水星磁层亚暴过程. 通过行星磁层尺度类比,这些大尺度FTEs对应于地球磁层10~15地球半径的空间尺度,现有的FTEs理论模型很难解释其形成. 通过分析水星磁层顶边界层观测到的处于形成和演化阶段大尺度FTEs事件,Zhong等(2020c)提出大尺度FTEs形成机理,即磁层顶电流片通过强烈压缩,发生撕裂模不稳定性产生众多离子尺度通量绳,这些离子尺度通量绳在磁层顶传输过程中相互作用、多步骤合并从而产生宏观大尺度结构(图3a). 该过程区别于地球多X线磁重联形成FTEs的观测特征(Zhong et al., 2013),反映了不同行星磁层空间尺度导致磁重联及磁通量绳的形成与演化过程的差异性.

      在水星磁尾同样观测到大量离子尺度通量绳结构(DiBraccio et al., 2015). 磁尾磁通量绳的产生是间歇性的,在大部分等离子体片穿越过程中并未观测到磁通量绳结构,而在重联活跃期,其产生率为5 min−1,观测率是地球磁尾的200倍(Smith et al., 2017). Sun等(2016)通过统计分析发现这些磁通量绳具有明显的晨昏不对称分布,晨侧观测率大于昏侧,表明水星近磁尾磁重联更易在晨侧区域发生,而地球近磁尾磁重联更易在昏侧区域发生. 利用非无力场模型对磁通量绳进行观测拟合,Zhao等(2019)发现一部分磁通量绳呈现扁平结构,尺度较平均值大,携带更多的磁通,推测这些磁通量绳尚处于形成演化阶段. 除了磁通量绳结构以外,水星磁尾还可以形成南北尺度大于水星直径、观测持续时间接近Dungey循环时间尺度的等离子体团(Plasmoid)结构(Zhong et al., 2019)(图3b). 这些等离子体团可能是由近磁尾和远磁尾重联产生,近磁尾高密度等离子体及大量行星重离子的存在对其形成可能起着重要的作用.

    • 为解释水星磁尾急剧的动力学过程,Siscoe等(1975)最早把地球亚暴的概念应用到水星. 而Luhmann等(1998)基于数值模拟提出观点认为,太阳风通过磁尾快速磁重联瞬时驱动水星弱磁层同样可以导致类似的急剧动力学过程. 由于没有电离层,水星是否存在类似地球的亚暴过程尚存争议. Slavin等(2010a)基于“信使”号卫星观测到尾瓣区磁场周期性增强和衰减,提出类似地球亚暴期间的“装—卸载”过程,支持了Siscoe提出的水星亚暴观点. 统计分析表明“装—卸载”过程持续时间可以从几十秒到几分钟,平均3 min左右;尾瓣的磁通甚至可以增强至磁层总磁通的40%,是地球的4倍左右(Imber and Slavin, 2017). Sun等(2015b)发现磁层亚暴期间水星近磁尾磁场结构演化过程类似地球,但时间尺度短,膨胀相和增长相的持续时间均大约为1 min,远短于地球0.5~1小时的持续时间. 在水星亚暴膨胀相期间,等离子体片边界层附近观测到具有显著阿尔芬波特征的磁场扰动,随着等离子体片的变厚,在等离子体片中心观测到一系列的压缩波(Sun et al., 2015a)(图4a). 结合阿尔芬波和压缩波的周期以及波动的空间分布特征,Sun等(2015a)推测这些波动是由行星向运动的高速流加速形成的,提出了水星亚暴膨胀相期间类Pi2波动的形成机制.

      图  4  (a)水星磁层亚暴膨胀相期间阿尔芬波及压缩波的形成示意图(修改自Sun et al., 2015a). (b)水星磁尾亚暴电流楔形成的直接观测结果示意图(修改自Poh et al., 2017a)

      Figure 4.  (a) A schematic to illustrate the Alfvénic and compressional waves generated during the substorm expansion phase in Mercury's magnetotail (modified from Sun et al., 2015a). (b) Schematic illustrations of asymmetries in Mercury's current sheet. Left: the formation of a substorm current wedge in the near-Mercury region. Right: current sheet structure in the postmidnight and premidnight views (modified from Poh et al., 2017a)

      亚暴偶极化过程对近地的能量、粒子通量传输过程中扮演着重要角色. 通过对磁尾典型活跃期和平静期等离子体片特征比较,证实了水星近磁尾亚暴偶极化过程可以有效地加速和加热质子,偶极化过程较多的发生在晨侧区域是导致质子温度和超热质子分布的晨昏不对称的原因(Sun et al., 2017). 偶极化过程同样与高能电子的加速与注入具有明显的相关性(Dewey et al., 2017). 通过引入Kappa分布函数对等离子体片质子进行拟合,比较水星和地球磁层亚暴过程中近磁尾质子的变化特征,发现在偶极化之后质子变得更加热而稀疏;在水星磁层中,Kappa数值在整个亚暴过程中的变化显著大于地球,表明质子加速机制在两个磁层中不同(Sun et al., 2018). 偶极化相联系的高速流减速和磁通量堆积发生位于夜侧星体以上~900 km高度,一部分偶极化事件甚至可以延伸至向阳侧磁层或夜侧星体表层(Dewey et al., 2020). Poh等(2017a)报道了水星磁尾亚暴电流楔形成的直接观测(图4b),利用线性电流模型估算出由磁通堆积卫星穿越亚暴电流楔的电位~9 kV,获得的水星壳层电导率与基于场向一区电流估算结果(Anderson et al., 2014)一致,推断水星表面风化层的导电性足以使得电流在行星内核闭合.

    • 磁场震荡或拍动普遍存在于行星磁尾,但形成机制有所区别(Rong et al., 2015b; Poh et al., 2017b; Gao et al., 2018). Zhang等(2020)利用自主开发的单点卫星磁场分析方法(Rong et al., 2015a)诊断了水星电流片拍动类型及传播方向,通过统计水星磁尾磁场振荡事件发现电流片磁场的振荡周期约为8~20 s,远小于地球磁尾10~20 min的振荡周期;磁场的振荡幅度在磁尾两侧翼较大;振荡形成波动的传播方向指示振荡源在磁尾两侧翼处. 结果表明,太阳风会通过水星磁尾侧翼处的某种动力学活动快速激发磁场振荡,激发形成的磁场波动以阻尼波的形式向中心处传播(Zhang et al., 2020). 通过最小方差分析方法对水星磁尾电流片拍动波进行统计分析,Poh等(2020)发现大幅度震荡的拍动波是沿越尾方向传播的扭结型波;拍动主要出现在昏侧,和磁尾电流片Na+密度以及磁层顶K-H波动的晨昏分布一致. Poh等(2020)认为除了外部太阳风和内部等离子体驱动机制以外,磁尾BZ梯度引起的磁双梯度不稳定性是水星磁尾拍动的重要机制.

    • 水星磁层顶K-H波与地球存在显著的区别. 水星K-H波主要发生在IMF北向期间,波幅可以达到100 nT以上,周期为10~20 s;和地球不同,水星K-H波发生在日下点磁层顶附近至昏测(Sundberg et al., 2012). 由于水星磁层尺度小,离子有限回旋半径效应对形成晨昏不对称分布起着重要的作用(Paral and Rankin, 2013). 向阳侧磁层顶K-H波可以激发内磁层同频率的ULF波动(Liljeblad et al., 2016; Liljeblad and Karlsson, 2017)及环形磁力线共振(James et al., 2019),从而影响着水星全球性磁层能量传输活动. 和日侧不同,夜侧磁层顶K-H波的频率和局地Na+回旋频率接近,波动可以延伸至磁鞘和高纬磁层(Sundberg et al., 2012; Gershman et al., 2015). 与向阳侧尾向传播来的K-H波发生合并、破裂以及生成次级K-H波,从而形成从磁流体尺度到动力学尺度的多尺度K-H波特征(Gershman et al., 2015)(图5).

      图  5  不同磁场大小下水星磁层顶K-H波多尺度特征. 夜侧磁层顶K-H波的频率和局地Na+回旋频率接近,由于磁场大小的不同,频率可以小于(a)、等于(b)或大于(c)日测K-H波频率(修改自Gershman et al., 2015

      Figure 5.  Illustration of K-H wave growth along Mercury's magnetopause for increasing magnetic field given a constant vortex speed. Toward the tail, where the Na+ is expected to dominate the plasma mass density, the observed frequency of K-H waves (blue spacecraft) matches that of the Na+ gyrofrequency, which can be (a) less than, (b) equal to, or (c) greater than that observed on the dayside (red spacecraft) (modified from Gershman et al., 2015)

    • 对于地球,磁层阻挡住了绝大部分太阳风等离子体,只有在极隙区太阳风才能直接接触到电离层. 太阳风是经磁层间接地向地球电离层的离子传递能量和动量,并造成行星离子的逃逸. 而水星弱偶极磁场并不能像当前地球一样有效屏蔽太阳风粒子. 太阳风粒子在水星极尖区注入导致在行星表面发生离子溅射、电荷交换等物理化学反应,从而改变风化层的物理化学特性、外逸层环境、乃至整个行星空间环境的变化,也是水星空间重离子的主要起源(Wurz et al., 2019). 水星空间重离子成分有Na+、O+、K+、Ca+、Mg+等,其中Na+为主要成分,观测平均密度可达到太阳风H+成分的10%(Gershman et al., 2014),尤其在极端太阳风环境下这一比例会更高(Winslow et al., 2020). 因此,Na+时空变化是研究水星系统耦合尤其是太阳风和水星直接相互作用的很好指示器.

      “信使”号探测显示磁层Na+的空间分布与极尖区太阳风粒子注入具有明显的相关性(Zurbuchen et al., 2011). 同时,磁层顶附近或太阳风区域外逸层中性原子电离,被太阳风“拾起”通过磁层顶磁重联进入极尖区和磁层(Raines et al., 2014)(图6a). 地面遥感探测显示外逸层Na通常呈现日侧中纬地区的双峰模式,偶尔出现低纬单峰模式(例如ICME期间)(图6b),结合卫星观测表明其空间分布与IMF方向与大小存在相关性,被认为与重联导致全球性拓扑结构改变引起(Mangano et al., 2015; Orsini et al., 2018). 外逸层及重离子通量具有小时量级的短周期变化特征(Mangano et al., 2013; Massetti et al., 2017),也具有很好的水星轨道长周期性变化(Jasinski et al., 2021; Milillo et al., 2021). 行星离子通过磁层对流,进入等离子体片,一部分向向阳侧输运、漂移形成在磁尾晨昏不对称性分布(Delcourt, 2013; Raines et al., 2013; Gershman et al., 2014). 其中一部分重离子,以及磁尾高能质子,可以在夜侧中低纬区域沉降行星表层,发生再次离子溅射(Ip, 1993; Delcourt et al., 2003; Zhao et al., 2020). 由于“信使”号探测仪器缺乏对低能行星重离子的探测,行星重离子在磁层中的真实通量可能被严重低估. James等(2019)通过分析磁层磁力线共振频率,利用幂律模型以指数作为沿磁力线的等离子体质量密度剖面的自由参数,基于水星平均磁层磁场模型(Korth et al., 2017),推测近水星空间等离子体主要成分为Na+,估算出其密度和上游太阳风等离子体密度接近. 若属实,这些低能行星重离子将对内磁层动力学过程起着重要的影响,可能会改变当前对水星空间电流体系的认识(Exner et al., 2020).

      图  6  (a)极尖区观测到的Na+形成机制.(左)太阳风离子溅射和光电离;(右)磁层顶附近或太阳风区域外逸层中性原子电离,被太阳风“拾起”进入极尖区和磁层(修改自Raines et al., 2014).(b)THEMIS遥感探测外逸层典型Na分布模式.(上)日侧低纬单峰模式;(下)中纬地区双峰模式(修改自Mangano et al., 2015

      Figure 6.  (a) Two possible sources for Na+ ions in the cusp: (left) Na+ ions are generated in the cusp, both by solar wind impact and photoionization, and are accelerated by processes there. (right) Neutral Na atoms are ionized near the magnetopause and swept into the cusp (modified from Raines et al., 2014). (b) Examples of the 8 recurrent Na emission patterns identified in the Mercury's exosphere. Top: equatorial Peak. Bottom: two peaks in middle latitude (modified from Mangano et al., 2015)

    • 在极端太阳风事件下,水星磁层受到更强的太阳风驱动,可能会导致水星空间环境的根本性变化. 关注点主要集中在向阳侧磁层是否存在以及磁层动力学过程新特征两个方面. Slavin等(2014)首次对ICME和太阳风高速流(HSS)驱动下水星向阳侧磁层结构进行详细分析. 在ICME事件中,磁层顶外形成低等离子体β值(等离子体热压和磁压之比)的等离子体耗尽层结构,导致高效的磁重联率,伴随着准周期性FTEs和极尖区等离子体精细结构的产生. 在HSS事件中,由于高β值的磁鞘,尽管磁层顶电流片两侧磁场接近反平行,重联率却较低0.03~0.1. 基于磁层顶旋转对称平均模型假设,推测磁层顶日下点距离为1.03~1.13RM,太阳风动压可以达到正常情况下的5倍. 基于相同的辨别方法,Jia等(2019)分析了8个向阳侧磁层强压缩事件,结合星体内部和磁层耦合的全球MHD模型(Jia et al., 2015),发现在低磁剪切条件下,内核感应效应强于重联的“剥蚀”效应,而在高磁剪切条件下“剥蚀”效应起主导作用. Winslow等(2017)统计分析了卫星在轨期间的69个ICME驱动磁层事例,发现磁层顶日下点距离相对正常情况减小~15%,极尖区向赤道和地方时上的展宽,同时等离子体压强增强2倍以上,导致粒子注入星体的平均通量增加一个数量级.

      在少数极端情况下,向阳侧磁层在一个卫星轨道周期内并未被观测到,结合低高度弓激波观测(图7a~h),Zhong等(2015a)推测这类事件中向阳侧磁层可能会消失. Slavin等(2019)分析了多个类似的事件,发现这类事件主要发生在强太阳风动压(约140~290 nPa)和行星际磁场南向(Bz约−100~400 nT)期间,日下点弓激波通常位于离水星表面~1 200 km,可能是太阳风直接轰击水星表层被吸收所致;同时,大量的FTEs在高纬被观测到(图7i). Winslow等(2020)结合STEREO-A卫星观测详细分析了其中一个事例,认为向阳侧磁层消失并不是磁场重联导致,极端的太阳风动压足以将向阳侧磁层顶压缩至星体表面附近. 同时该事例显示从低纬到高纬Na+密度显著增大,证实ICME期间外逸层产生变强. 结合地面观测,Orsini等(2018)发现水星外逸层Na+通量及分布与ICME具有明显的相关性.

      图  7  左:少数极端情况下卫星轨道未穿越向阳侧磁层事例.(a)~(d)磁场强度及其三分量;(e)磁场天顶角(红)和方位角(蓝);(f)质子能谱,磁场在(g)X-Z 平面和(h)X-Y平面的投影,以及弓激波(BS)和磁层顶(MP)观测位置与平均磁层顶模型(虚曲线)的比较(修改自Zhong et al., 2015a). 右:向阳侧磁层消失事件观测结果示意图(修改自Slavin et al., 2019

      Figure 7.  Left: Example of MESSENGER missed the dayside magnetosphere. (a)~(d) Magnetic field intensity and its three components; (e) magnetic field zenith and azimuthal angles; (f) spectrogram of proton flux, and the MESSENGER orbit and the vector plots of the magnetic field in the (g) noon-midnight and (h) equatorial planes relative to Mercury's surface (circle) and the average magnetopause from the model (dashed linesh) (modified from Zhong et al., 2015a) . Right: Illustration of the primary features of the disappearing dayside magnetosphere events (modified from Slavin et al., 2019)

      不同的极端事例下,磁尾动力学过程也有所区别. 在ICME驱动下,磁层对流却反常呈现准稳态,磁尾动力学过程由多X线重联主导,多磁岛合并形成超大磁通量绳结构,进行磁尾能量周期性释放;该过程区别于类似地球亚暴相联系的“装—卸载”过程(Zhong et al., 2020a). Sun等(2020a)对比该ICME事件,发现在太阳风高速流期间,电流片呈现准周期性高频率的偶极化锋面结构. 水星磁尾动力学过程对极端太阳风条件的响应有待进一步多事例详细研究.

    • 基于“信使”号卫星大量在轨数据,水星磁层观测研究已取得重要进展. 本文仅从以上五个方面进行简要总结,相关水星磁层物理问题的深入研究有待于欧空局和日本航空局联合的BepiColombo的高质量、高精度、多仪器的双星联合探测. BepiColombo双星将于2021年开始飞越水星,2025年底正式入轨,提供更为全面和丰富的磁场、等离子体(包括行星重离子)、中性原子等多种空间探测数据. 其中,水星行星轨道器(MPO)主要用于探测水星的表面和内部结构,水星磁层轨道器(Mio)主要用于探测水星磁场及其与太阳风的相互作用,其科学目标之一就是水星空间环境及多圈层耦合问题(Milillo et al., 2020; Mangano et al., 2021). 有望在以下相关研究问题上获得重要的进展和突破:(1)太阳风与水星的作用方式,尤其在极端太阳活动期间向阳侧磁层的响应状态;(2)磁层能量输运的一般规律,除了时间尺度外,水星磁层亚暴是否和地球存在本质区别;(3)磁层活动相关的电流体系的认知,场向电流如何在水星表层或附近闭合;(4)水星空间等离子体环境特征,磁层空间粒子的来源与加速机制;(5)行星重离子对水星磁层动力学过程的影响等.

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