地球辐射带动态变化和辐射带粒子快速加速研究
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摘要
通过SAMPEX卫星的观测,定量地研究了在CME和CIR磁暴期间1.5~6.0 MeV"杀手"电子的通量分布的变化。发现外辐射带的内、外边界都可以被随着L壳指数衰减的函数很好地拟合出来。另外,该报告根据这一指数衰减函数和由此得到的动态的外辐射带内、外边界改进了RBC指数的计算,并由此得到,CME磁暴有可能比CIR磁暴产生更多的相对论电子。辐射带物理模型STEERB基于三维的Fokker-Planck方程实现,包含局地波粒相互作用、径向扩散和绝热输运等物理过程。由于数值格式的限制,以往的辐射带模型均没有引入局地波粒相互作用相关的交叉扩散项。STEERB模型的对比实验显示,交叉扩散项的忽略能够导致电子通量被高估5倍甚至几个数量级。这个结果说明,交叉扩散项对于辐射带电子通量的准确评估具有重要意义。以往的辐射带物理模型常常采用固定的偶极磁场,忽略了背景磁场变化引起的绝热过程。STEERB模型则采用了时变的背景磁场,同时引入绝热和非绝热过程。对比实验结果显示,绝热输运过程能够显著地影响辐射带电子通量的演化。行星际激波与磁层的相互作用能够在内磁层激发ULF波;激发的极性模ULF波会造成"杀手"电子的快速加速过程。极向模和环向模ULF波对漂移-共振加速的作用在不同L值区域有所不同。环向模ULF波对能量电子的加速在L值较大的区域(外磁层)较为重要,而在L值较小的区域(内磁层),极向模ULF波则对能量电子的加速起主要作用。
We have quantitatively studied the radiation belt electrons' variations. It is found that the boundaries determined by fitting an exponential to the flux as a function of L shell obtained in this study agree with the observed outer and inner boundaries of the outer radiation belt. Furthermore, we have constructed the Radiation Belt Content(RBC) index by integrating the number density of electrons between those inner and outer boundaries. According to the ratio of the maximum RBC index during the recovery phase to the prestorm average RBC index, we conclude that CME-driven storms produce more relativistic electrons than CIR-driven storms in the entire outer radiation belt, although the relativistic electron fluxes during CIR-related storms are much higher than those during CME-related storms at geosynchronous orbit. The physical radiation belt model STEERB is based on the three-dimensional Fokker-Planck equation and includes the physical processes of local wave-particle interactions, radial diffusion, and adiabatic transport. The physical radiation belt model STEERB is based on the three-dimensional Fokker-Planck equation and includes the physical processes of local waveparticle interactions, radial diffusion, and adiabatic transport. The numerical experiments of STEERB have shown that the energetic electron fluxes can be overestimated by a factor of 5 or even several orders(depending on the pitch angle) if the cross diffusion term is ignored. This implies that the cross diffusion term is indispensable for the evaluation of radiation belt electron fluxes. Formal radiation belt models often adopt dipole magnetic field; the time varying Hilmer-Voigt geomagnetic field was adopted by the STEERB model, which self-consistently included the adiabatic transport process. The test simulations clearly indicate that the adiabatic process can significantly affect the evolution of radiation belt electrons. The interactions between interplanetary shocks and magnetosphere can excite ULF waves in the inner magnetosphere; the excited polodial mode ULF wave can cause the fast acceleration of "killer electrons". The acceleration mechanism of energetic electrons by poloidal and toroidal mode ULF wave is different at different L shells. The acceleration of energetic electrons by the toroidal mode ULF waves becomes important in the region with a larger L shell; in smaller L shell regions, the poloidal mode ULF becomes responsible for the acceleration of energetic electrons.
We have quantitatively studied the radiation belt electrons' variations. It is found that the boundaries determined by fitting an exponential to the flux as a function of L shell obtained in this study agree with the observed outer and inner boundaries of the outer radiation belt. Furthermore, we have constructed the Radiation Belt Content(RBC) index by integrating the number density of electrons between those inner and outer boundaries. According to the ratio of the maximum RBC index during the recovery phase to the prestorm average RBC index, we conclude that CME-driven storms produce more relativistic electrons than CIR-driven storms in the entire outer radiation belt, although the relativistic electron fluxes during CIR-related storms are much higher than those during CME-related storms at geosynchronous orbit. The physical radiation belt model STEERB is based on the three-dimensional Fokker-Planck equation and includes the physical processes of local wave-particle interactions, radial diffusion, and adiabatic transport. The physical radiation belt model STEERB is based on the three-dimensional Fokker-Planck equation and includes the physical processes of local waveparticle interactions, radial diffusion, and adiabatic transport. The numerical experiments of STEERB have shown that the energetic electron fluxes can be overestimated by a factor of 5 or even several orders(depending on the pitch angle) if the cross diffusion term is ignored. This implies that the cross diffusion term is indispensable for the evaluation of radiation belt electron fluxes. Formal radiation belt models often adopt dipole magnetic field; the time varying Hilmer-Voigt geomagnetic field was adopted by the STEERB model, which self-consistently included the adiabatic transport process. The test simulations clearly indicate that the adiabatic process can significantly affect the evolution of radiation belt electrons. The interactions between interplanetary shocks and magnetosphere can excite ULF waves in the inner magnetosphere; the excited polodial mode ULF wave can cause the fast acceleration of "killer electrons". The acceleration mechanism of energetic electrons by poloidal and toroidal mode ULF wave is different at different L shells. The acceleration of energetic electrons by the toroidal mode ULF waves becomes important in the region with a larger L shell; in smaller L shell regions, the poloidal mode ULF becomes responsible for the acceleration of energetic electrons.
引文