The interactions between ULF waves and cold charged particles in the Earth's magnetosphere
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摘要: 太阳风—磁层耦合过程会产生各种等离子体波，其中超低频波的频率最低（1 mHz～1 Hz）、波长最长（与内磁层磁力线长度相当）、能量密度最大. 超低频波在磁层粒子加速、物质输运和能量转化中起着重要作用. 以往的研究主要关注超低频波的全球性传播和分布特征以及这些波动与磁层能量粒子（辐射带电子和环电流离子）的相互作用过程. 最近几年人们逐渐发现超低频波与低能粒子之间相互作用对磁层动力学过程会产生重要影响. 本文主要综述了以下几个方面的研究进展，包括：（1）多卫星联合观测、多地面台站联合观测、辐射带能量电子的“回旋镖形”投掷角色散特征都证明存在局域性的超低频波，理论和观测表明这类超低频波的存在跟等离子体层羽状结构有关；（2）多卫星观测证明存在等离子体层顶表面波，并且发现“锯齿形”极光的形成跟等离子体层顶表面波有着密切联系；（3）理论和卫星观测表明超低频波可以通过漂移—弹跳共振加速等离子体层低能电子，通过 E × B 调制作用实现对低能离子成分的区分. 最后，本文还总结了超低频波与低能粒子相互作用对磁层动力学过程造成的可能影响，并展望了本研究方向亟待解决的问题.Abstract: In the solar wind-magnetosphere coupling processes, many kinds of plasma waves can be excited in the Earth's magnetosphere including ULF waves, hiss waves, chorus waves, etc. Among these waves, ULF waves are featured by the lowest wave frequency (1 mHz～1 Hz), the longest wavelength (comparable with the magnetic field line in the inner magnetosphere) and the highest wave power density. ULF waves can propagate along the geomagnetic field lines into the ionosphere and cause the joule heating of the ionosphere. Besides, after excited at the magnetopause, they can propagate earthward across the geomagnetic field lines and generate the standing Alfvén waves (poloidal mode and toroidal mode) via the field line resonance. The electric field component of the poloidal mode standing is aligned with the drift direction of charged particles trapped in the magnetosphere, allowing a strong interaction between ULF waves and particles. Therefore, ULF waves play a crucial role in the particle energization, mass transportation and energy transfer within the magnetosphere. Previous studies focus on the global propagation and distribution of ULF waves, and their interactions with energetic particles such as radiation belt electrons and ring current ions. Recent studies within 5 years shed new light on the localized ULF waves and the interactions between ULF waves and cold plasmaspheric particles, which are reviewed in this paper. The existence of the localized ULF waves has been verified in different ways including multi-spacecraft observations, coordinated ground station measurements and "boomerang-shaped" pitch angle features of resonant radiation belt electrons. Studies suggested that the localized ULF waves are probably associated with the plasmaspheric plume structure, which can generate the Alfvén speed barriers. Multi-spacecraft observations have demonstrated that there are plasmaspause surface waves, which modulate particle populations and different waves around the plasmapause, and cause the formation of the sawtooth aurora. The acceleration of cold electrons by ULF waves via drift-bounce resonance has been found based on the observations of electron's pitch angle features and energy spectrum, theoretical analyses and statistical studies. Cold ions can be energized and modulated by ULF waves via E × B , which can be used to distinguish ion species and study ions with energies out of the scope of instrument. In the final, we summarize the possible roles of the ULF wave-cold particle interactions in the dynamics of magnetosphere and list some important issues for future studies.
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Key words:
- ULF waves /
- plasmasphere /
- plume /
- drift-bounce resonance /
- cold particles /
- surface wave
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图 1 （a）2015年9月10日00～16 UT期间，地球磁层中Van Allen Probes卫星、GOES系列卫星以及MMS卫星的轨道（实线）及观测到超低频波的区域（虚线）（修改自Ren et al., 2017b）；（b）相同纬度不同经度处地磁台站对超低频波的联合观测（修改自Li et al., 2017）
Figure 1. (a) Spacecraft trajectories of two Van Allen Probes, two GOES satellites (Goes 13 and 15) and MMS in the equatorial plane of GSM coordinate during the time interval of 00～16 UT on 10 September 2017 (modified from Ren et al., 2017b); (b) Magnetic field measurements from ground stations in the similar magnetic latitude but different magnetic longitudes (modified from Li et al., 2017)
图 2 “回旋镖形”投掷角色散特征的观测和理论解释：（a）Van Allen Probe B卫星在晨侧观测的“回旋镖形”投掷角色散特征；（b）假设源区在昏侧通过理论估算得到的不同投掷角的能量电子到达Probe B卫星的时间；（c）“回旋镖形”投掷角色散特征形成的解释（修改自 Hao et al., 2017）
Figure 2. Observation and theoretical explanation of the boomerang-shaped pitch angle distributions: (a) The boomerang-shaped pitch angle distributions of 466.8 keV electrons observed by Van Allen Probe B in the dawnside; (b) The estimated arrival time of electrons at different pitch angles assuming that the source is located in the duskside; (c) The schematic illustration of the pitch angle evolution of drift resonant electrons when interacting with localized ULF waves (modified from Hao et al., 2017)
图 3 Van Allen Probes卫星观测到的30个“回旋镖形”事件和9个“竖条形”事件的源区分布特征，及其与等离子体层羽状结构之间的关系. 其中，空心圆和实心圆分布代表“回旋镖形”事件和 “竖条形”事件，黑色和红色分布代表源区位于等离子体层羽状结构内和结构外（修改自Zhao et al., 2021）
Figure 3. The magnetic local time (MLT) distributions of the origins of 30 "boomerang-shaped" events and 9 straight events, and their relationship with the plasmaspheric plume (modified from Zhao et al., 2021). The hollow and solid cycles indicate the "boomerang-shaped" and straight events, respectively, and the black and red colors represents that events occur inside and outside the plasmaspheric plume, respectively
图 4 等离子体层顶表面波的联合观测及相关的“锯齿形”极光. （a）等离子体层结构示意图，彩色实线代表卫星轨道，白色实线代表从极光观测得到的极光边界；（b～c）DMSP F17卫星对北半球和南半球的极光观测；（d～f）Van Allen Probe A卫星对高杂波、低能电子以及低能离子的观测；（g～i）ERG卫星对高杂波、低能电子以及低能离子的观测（修改自He et al., 2020）
Figure 4. Coordinated observations of plasmaspause surface waves and their related sawtooth aurora. (a) A schematic of plasmasphere, the white line indicates the auroral boundary, the colored lines represent the spacecraft trajectories of Van Allen Probes and ERG; (b～c) Sawtooth aurora observed by DMSP F17 in the northern and southern hemispheres; (d～f) Upper hybrid waves, cold electrons and ions observed by Van Allen Probe A; (g～i) The same format as (d～f) except for ERG (modified from He et al., 2020)
图 5 2015年9月10日Van Allen Probe B观测到的超低频波和低能电子. （a～b）Van Allen Probe B卫星连续两个轨道观测到的极向模磁场的小波谱图；（c～d）Van Allen Probe B卫星连续两个轨道观测到的低能电子的能谱和投掷角谱分布图，PA：投掷角（修改自Ren et al., 2017b）
Figure 5. ULF waves and low-energy electrons observed by Van Allen Probe B on 10 September 2015. (a～b) The wavelet spectra of poloidal mode magnetic field in two consecutive orbits; (c～d) Energy spectra and pitch angle distributions of low-energy electrons in two consecutive orbits (modified from Ren et al., 2017b)
图 6 Van Allen Probes卫星在2012年11月至2018年11月期间观测到的超低频波与等离子体层低能电子相互作用事件的发生率的全球分布情况（修改自Ren et al., 2019b）
Figure 6. The distribution of the normalized occurrence rates of the events associated with interactions between ULF waves and cold plasmaspheric electrons observed by Van Allen Probes during the time interval from September 2012 to September 2018 (modified from Ren et al., 2019b)
图 7 低能离子对超低频波的响应. （a）E×B漂移速度；（b）从FPI仪器探测获得的离子速度；（c）从FPI仪器探测获得的离子密度；（d）FPI仪器对离子能谱的观测；（e）HPCA仪器对He+和O+能谱的观测；（f）13:01～13:03 UT期间FPI探测的离子能谱；（g）13:02:07 UT时刻的离子能谱，竖虚线代表E×B造成的能量变化（修改自Liu et al., 2019）
Figure 7. The responses of cold ions to ULF waves. (a) E×B drift velocities; (b) Ion bulk velocities obtained from the FPI instrument; (c) Plasma density obtained from the FPI instrument; (d) Ions spectrum from the FPI instrument; (e) He+ and O+ spectra from the HPCA instrument; (f) Ions spectrum from the FPI instrument during the time interval of 13:01～13:03 UT; (g) Ions spectrum at 13:02:07 UT, where the vertical dashed lines indicate the energy change caused by E×B (modified from Liu et al., 2019)
图 8 超低频波与低能粒子之间的相互影响及对磁层动力学过程的影响（修改自Ren et al., 2019a）
Figure 8. Interactions between ULF waves and cold charged particles and their possible roles in the dynamics of the magnetosphere (modified from Ren et al., 2019a)
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