哨声波非线性物理的数值模拟和卫星观测研究

Whistler-mode wave is a very common plasma wave in the Earth’s magnetosphere. Both theoretical and observation results have shown that whistler waves can not only scatter lower-energy (~10s keV) electrons into the loss cone leading to precipitation into the atmosphere, but also accelerate seed electrons to highly relativistic energies (~MeV) through cyclotron resonances. Based on plasma linear theory and quasi-linear diffusion theory, the study on whistler waves has achieved greatly. However, with the improvement of satellite data resolution, whistler waves exhibit more and more interesting properties and structures, which are not predicted by the linear or quasi-linear theory. Recent studies demonstrate that the generation of chorus waves (rising or falling) and multiband whistler waves, and the formation of TDS, etc., are directly related to nonlinear physics of whistler waves. Therefore, studying nonlinear physics of whistler waves has great significance for both expansion of plasma theory and practical application in space physics. Based on satellite data, Gao et al. first proposed a generation mechanism for multiband chorus waves known as lower band cascade, which is involved the nonlinear wave-wave coupling process. In support of this project, with both data analysis and particle simulations, we try to investigate the observational properties and generation mechanism of multiband whistler waves, the electron energization by generated higher-frequency waves, parametric instabilities of whistler waves, and the formation of TDS in the Earth’s magnetosphere, and reveal potential physics of the nonlinear evolution of whistler waves.

哨声波是地球磁层中的一种非常普遍的等离子体波动。准线性结果表明，它不仅能通过散射造成较低能量电子在大气层的沉降，而且能通过共振加速热电子到相对论能量。利用线性和准线性理论，前人对哨声波的研究获得了丰硕的成果。然而，随着卫星数据分辨率的大幅提升，哨声波呈现出越来越多的线性和准线性理论无法解释的观测现象和特征。近期研究表明，合声波（升调或降调）和多波段哨声波的产生，TDS的形成等，都与哨声波的非线性物理有直接关系。因此哨声波的非线性研究对空间理论扩展和实际应用都很有意义。通过分析观测数据，申请人首次提出低频波段串级机制来解释多波段哨声波的产生，其物理本质为非线性波模耦合。在本项目资助下，申请人拟结合粒子模拟和卫星资料分析两种研究手段，进一步对多波段哨声波的观测特征、形成机制和高频波对磁层电子的作用，以及哨声波的参量不稳定性和TDS的形成等方向展开深入研究，揭示哨声波非线性演化的物理本质。

哨声波是地球磁层中非常普遍的一种等离子体波动，对地球辐射带演化起着极为重要的作用。项目负责人早期的研究指出，在地球磁层中哨声波会引起非线性物理过程，从而改变哨声波的波谱形态和演化过程。然而，哨声波涉及的非线性物理过程的具体形式以及对哨声波所产生的影响仍需进一步研究。利用理论推导、粒子模拟和卫星资料分析，本项目提出并证实了多波段合声波的形成机制，即低频波串级。理论预言并发现了地球磁层中存在两种类型的多波段合声波，并建立了其全球分布模型。低频波串级是合声波功率间隙的重要物理机制之一，另一种机制是由于束流电子的有效阻尼。多波段合声波在近太阳行星际也被观测到。哨声波还能够发生衰变不稳定性，法向角越大越容易发生。哨声波能够有效束缚在密度增强管中，并且会堆积在大尺度导管的外边界。该项目的研究结果表明地球磁层中哨声波存在多种非线性物理过程，在哨声波波谱形态、传播演化中起到了重要的作用。