基于试验粒子模拟的电离层人工调制激发的极低频和甚低频波对磁层高能电子的散射效应

电离层调制加热能够有效激发极低频和甚低频(ELF/VLF)波,其中向上传播进入磁层的ELF/VLF波能够与高能电子发生共振相互作用,具有人工沉降高能电子、消除辐射带等潜在实际用途.本文综合运用射线追踪和试验粒子方法模拟电离层人工激发的单频ELF/VLF波在电离层和磁层的传播,以及在外辐射带层与高能电子的共振相互作用过程,通过投掷角和能量散射系数评估人工ELF/VLF波对磁层高能电子的共振散射效应.研究表明,电离层人工ELF/VLF波传播到磁层后呈现高倾斜性,传播所能跨域的空间范围主要取决于加热的纬度位置和调制频率.在内辐射带,与～100 keV到几个MeV高能电子发生一阶共振相互作用的为10 kHz的VLF波段;在外辐射带,为几百Hz到1 kHz的ELF波段.对于L=4.5的外辐射带,试验粒子模拟结果显示,单个粒子在人工ELF波作用下投掷角和能量(α,E)的改变具有随机性,而所有试验粒子平均化的?α2和?E2随时间呈现出近似线性的增大,说明波粒共振散射过程体现出整体性.基于试验粒子模拟得到的共振散射系数表明,幅度为10 pT的人工ELF波可在外辐射带的磁赤道局地对1 MeV电子产生较强的投掷角散射效应,进而影响高能电子的损失、沉降等动力学过程.当人工ELF/VLF波在传播过程中变得高度倾斜,不仅最基本的一阶共振十分重要,高阶共振散射也具有较大效应.这些定量分析结果表明,通过电离层加热激发人工ELF/VLF哨声波来沉降、消除辐射带高能电子具有可行性.

Test particle simulation of resonant interaction between energetic electrons in the magnetosphere and ELF/VLF waves generated by ionospheric modification

Ionospheric modulation can artificially trigger ELF/VLF whistler waves, which can leak into the inner magnetosphere and contribute to resonant interactions with energetic electrons. Combining the ray tracing method and test particle simulations, we investigate the propagation of these artificially generated ELF/VLF waves through the high ionosphere into the inner magnetosphere, and evaluate the subsequent effects of resonant scattering energetic electrons near the heart of the outer radiation belt. The results show that the artificially triggered ELF/VLF waves become highly oblique in the magnetosphere and their spatial extent of L shell and magnetic latitude can be significantly controlled by the initial launch latitude. Corresponding to the principal first-order resonance, the energetic electrons from ～ 100 keV to 3 MeV can resonate with the artificial VLF waves with frequency above 10 kHz in the inner radiation belt, while in the outer radiation belt these hazardous electrons can resonate with ELF waves from ～100 Hz to 1 kHz. At L=4.5 as the focus in this study, the artificial ELF waves can resonate with 1 MeV electron at the harmonics N=-1, 1, 2. In contrast, the Landau resonance rarely occurs for these energetic electrons. The results of test particle simulations indicate that while wave-induced changes in pitch angle and kinetic energy of a single electron are stochastic, the change averaged over all test electrons increases monotonically within the resonance timescale, which implies that resonant scattering is an overall characteristic of energetic electrons under the influence of the artificial whistler waves. Computed resonant scattering rates based on the test particle simulations indicate that aritificial ELF/VLF waves with an observable in situ wave amplitude of ～ 10 pT can drive efficient local pitch angle scattering of energetic electrons at the magnetic equator, thereby contributing considerably to their precipitation loss and magnetospheric electron dynamics. When the waves become highly oblique during the propagation, besides the fundamental first order resonance, higher order resonances can also drive efficient electron scattering. The results support the feasibility of generating artificially ELF/VLF whistler waves for controlled removal of energetic electrons in the Earth radiation belts.
Keywords： wave-particle interactions test particle simulations ELF/VLF whistler waves ionospheric modification