合声波引起的外辐射带相对论电子投掷角分布快速演化
doi: 10.11728/cjss2025.05.2024-0187 cstr: 32142.14.cjss.2024-0187
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岳佳旭 男, 2001年1月出生于河北省秦皇岛市, 现为长沙理工大学物理与电子科学学院硕士研究生, 主要研究方向为辐射带波–粒相互作用等. E-mail: yuejx170514@163.com -
杨昶 男, 1977年7月出生于湖南省永州市, 现为长沙理工大学特聘教授, 硕士生导师, 主要研究方向为地球磁层带电粒子的分布和演化、辐射带波–粒相互作用等. E-mail: yangchang@163.com
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Rapid Evolution of the Relativistic Electron Pitch Angle Distributions Caused by Chorus in the Earth’s Outer Radiation Belt
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摘要: 投掷角是带电粒子运动方向与背景磁场的夹角, 研究电子投掷角分布对于理解地球辐射带动力学演化具有重要意义. 本文利用范艾伦探针数据, 对2016年9月7日15:19－15:49 UT期间发生的一次外辐射带相对论电子投掷角分布演化事例进行了详细分析. 卫星在此期间运行于远地点附近, 轨道速度较慢, 空间位置变化很小, 基本保持在L≈5.8, MLT≈2, Mlat≈1.7° 附近, 因此可以忽略位置变化对观测结果的影响. 卫星数据显示相对论电子的投掷角分布在30 min内从蝴蝶型分布逐步转变为平顶型分布, 且该区域存在强的哨声模合声波. 数值模拟结果表明哨声模合声波对高能电子的扩散作用是导致该事件中电子投掷角分布转变的主要物理机制. 本文研究进一步证明了合声波对于辐射带演化的重要意义.Abstract: The pitch angle, defined as the angle between a charged particle’s velocity vector and the ambient magnetic field, is a key parameter that governs the particle’s motion within the magnetic field. In Earth’s outer radiation belt, energetic electrons display diverse Pitch Angle Distribution (PAD) patterns. These patterns are influenced by various factors and frequently undergo changes, typically occurring over timescales ranging from several hours to several days. Investigating electron PAD variations and uncovering the underlying physical mechanisms are of significant importance for understanding the dynamic evolution of the Earth’s outer radiation belt. This paper utilizes Van Allen Probe-B data to conduct a detailed analysis of the evolution of relativistic electron PADs in the outer radiation belt during an event that occurred from 15:19 UT to 15:49 UT on 7 September 2016. During this period, the satellite was operating near its apogee, with a slow orbital speed and minimal changes in spatial position, remaining approximately at the location L≈5.8, MLT≈2 and Mlat≈1.7°. As a result, the impact of positional changes on the observational results can be considered negligible. Satellite observations revealed that relativistic (Ek ≥ 1 MeV) electron PADs transitioned from butterfly patterns to flat-top patterns during this period, within a timescale of only 30 minutes, which is significantly shorter than previously reported cases. Concurrently, intense whistler-mode chorus waves were detected in this region. Based on observational data, we calculated the chorus-driven diffusion coefficients of relativistic electrons. We then simulated the evolution of electron PADs by solving a Fokker-Planck equation. The simulation results indicate that the diffusion driven by whistler-mode chorus waves is the primary physical mechanism responsible for the transformation of the electron PADs during this event. The research presented in this paper further demonstrates the significant role of chorus waves in the evolution of the radiation belts.
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图 1 地磁活动指数SYM-H (a), 2016年9月7日由Probe-B上的EMFISIS测量的磁场谱(b)和电场谱(c). (b)(c)中的点划线、虚线和白色实线分别表示电子回旋频率1.0 fce, 0.5 fce和0.1 fce
Figure 1. Geomagnetic activity index SYM-H (a). The magnetic field spectrum (b) and electric field spectrum (c), where the dot-dashed, dashed and solid white lines in both panels represent 1.0 fce, 0.5 fce and 0.1 fce respectively
图 2 2016年9月7日15:19－15:49 UT期间由Probe-B测量的1.0～2.6 MeV电子微分通量 (实验数据进行了时间尺度为2 min的滑动平均处理)
Figure 2. Differential flux of 1.0～2.6 MeV electrons measured by Probe-B during 15:19－15:49 UT on 7 September 2016 (The experimental data has been processed using a 2-minute moving average)
图 3 2016年9月7日15:19－15:49 UT期间观测波磁场谱的高斯拟合 (绿色点表示观测到的波谱, 红色圆圈表示这个频率区间内波谱密度的平均值, 蓝线表示高斯拟合结果)
Figure 3. Gaussian fitting of the magnetic field spectrum of the observed waves during 15:19－15:39 UT on 7 September 2016 (The green dots represent the observed wave spectra, the red circles denote the averaged spectra over each period and the blue lines show the fitting results)
图 4 弹跳平均投掷角(a)、动量(b)和交叉扩散系数(c)的计算结果
Figure 4. Calculation results of the bounce-averaged pitch angle (a), momentum (b) and cross diffusion coefficients (c)
图 5 观测与模拟得到的微分通量j的比较. (a) 为初始状态, (b)～(d)为每10 min后的观测结果 与模拟结果, 实线表示计算得到的演化结果(实际观测数据已在2 min时间尺度内进行了滑动平均处理)
Figure 5. Comparisons between the observed (dots) and simulated (lines) differential flux j. (a) Initial state, (b)～(d) comparison between the observed and simulated differential flux j at 10 min intervals. The green, blue, purple, and red circles represent the observed electron flux values at 1.0, 1.8, 2.1, and 2.6 MeV, respectively, while the solid lines show the calculated evolution results (The observed data have been smoothed using a 2-minute time window)
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