太阳活动高年银河宇宙线调制研究

Galactic cosmic rays (GCRs) are one of the most important sources of space radiation and the major concern for space weather research. While transporting in the heliosphere, GCRs are modulated by solar activities. During solar cycle 24, high precision GCR energy spectra have been observed by AMS-02, and it will provide new opportunities for us to study the diffusion and drift effects during periods of high solar activity. In this proposal, we will develop a time-dependent and physical based diffusion coefficients model, and it will make use of the observations of Voyager spacecraft, the latest diffusion theory as well as the results of turbulence transport model. To solve the difficulty in GCR modulation during solar maximum, which is caused by the ambiguous solar magnetic polarity, we assume the variation of solar magnetic polarity is stochastic during the periods of solar magnetic polarity reversal. The above approach is then introduced to our GCR modulation model, which is based on numerically solving the stochastic differential equations, to develop a new data-driven GCR modulation model. The numerical model will be used to simulate the proton and electron energy spectra observed by AMS-02, and to study the GCR modulation processes during the periods of high solar activity.

银河宇宙线（GCR）是空间粒子辐射的重要来源之一，也是空间天气研究的重要关注对象。GCR在日球层中传播时受到太阳活动的调制，在第24太阳活动周，AMS-02仪器观测了跨越太阳活动极大期的高精度GCR能谱，为研究GCR在太阳活动高年的调制机制提供了新机遇。本项目计划利用Voyager飞船的观测数据、最新的扩散理论和磁场湍动传播模型的结果，建立时间依赖的、基于物理的扩散系数模型；并在太阳活动极大期，采用随机方法描述太阳磁场极性翻转的过程，解决由太阳磁场极性的不确定性带来的GCR调制难题；将上述子模型引入到自主开发的基于随机微分方法的GCR数值调制模式中，建立新的数据驱动GCR调制模型。利用该模型计算太阳活动高年的质子和电子能谱，并通过与AMS-02的观测对比，深入探究太阳活动高年GCR调制的扩散和漂移效应，预期将为建立基于物理的GCR数值预报模式奠定基础。

在太阳活动高年，太阳磁场极性发生翻转，频繁的太阳活动使得日球层的结构更加复杂。如何在数值模型中处理这些问题，并准确模拟此时期银河宇宙线能谱变化是银河宇宙线调制研究中的难题。本项目针对太阳活动高年宇宙线调制的两个关键物理机制，即扩散和漂移效应，开展了以下研究：1）构建了太阳磁场极性变化的随机模型，其效果相当于减弱了太阳活动高年的漂移效应，自洽解决了太阳磁场极性突变引起数值模拟结果突变的问题；2）对于扩散效应，拓展了非线性平行和垂直扩散理论（NLGCE-F）的适用范围。根据Ulysses、Voyager等仪器的观测数据，探究了磁场湍动相关长度和磁场湍动能量的时空变化特征。发现太阳活动高年的较大的磁场和湍流强度减弱了宇宙线的扩散效应，是造成太阳活动高年宇宙线强度较低的主要原因。3）结合以上，建立了从基本原理出发的宇宙线调制模型，模型结果与多种宇宙线成分（H、He、Li ... Ni）的观测结果吻合。本项目的结果有助于建立基于物理的GCR数值预报模式。