Effects of the solar wind background field on the numerical simulation of the Solar Energetic Particle(SEP) transportation
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摘要: 太阳高能粒子(SEP)事件是一类重要的空间天气灾害性事件,如能准确预报SEP事件,人们便可以采取必要的防护措施,保障卫星、星载设备以及航天员的安全,尽可能地降低经济损失.因此,其数值预报研究在空间天气预报研究中占有很重要的地位.SEP事件中的高能粒子在不同的时间尺度内被耀斑过程或者CME驱动的激波加速,并且在被扰动后的行星际太阳风中传输,这些过程都紧紧依赖于太阳风背景场.因此获取更加接近物理真实的太阳风背景场是模拟SEP事件的重要部分,也是提高SEP物理模式的关键因素之一.我们目前的工作基于张明等发展的SEP在行星际空间传播的模型,尝试将Parker太阳风速度解及WIND飞船观测的磁场实时数据融入模型中,研究不同的太阳风速度以及真实磁场分布对SEP在行星际空间中传播的影响.通过求解聚焦传输方程,我们的模拟结果表明:(1)快太阳风条件下,绝热冷却效应项发挥了更大的作用,使粒子能量衰减的更快,而慢太阳风对粒子的通量变化没有显著影响;(2)加入观测的磁场数据时,粒子的全向通量剖面发生了比较明显的变化,具体表现在:通量峰值推迟到达、出现多峰结构、各向异性也发生一些改变.分析表明真实磁场的极性对粒子在行星际空间中传播有着重要的影响.Abstract: Solar energetic particles(SEPs) pose one of the most serious hazards to spacecraft systems and constrain human activities in space. Thus, it is of importance to forecast SEP events. Several theories and numerical models are applied to simulate SEP events. Each model makes some assumptions to simplify the complex acceleration and transportation processes within such events. In general, SEP will interact with ambient solar wind and background magnetic field during transportation. It is recognized that interplanetary transport effects must be taken into account at any analysis of SEP propagation. In the previous simulation, it always assumed Parker magnetic field and fixed solar wind speed as the input parameters. However, these assumptions are too simple when compared with the real conditions. In order to get better results, it is necessary to use more accurate background conditions. Recently, we change the fixed solar wind speed into spatial-dependent speed profile based on Parker's theory, and replace the Parker magnetic field with another Parker-like magnetic field based on in situ data at 1 AU. By solving the focused transport equation with simulation of time-backward stochastic processes method, our results show that:(1) Under fast solar wind speed assumption, it is clear that the omnidirectional flux decreases faster than that for the situation with slow solar wind speed in the decay phase. We suggest that it is due to the adiabatic cooling effect. Fast solar wind speed has a significant effect on the adiabatic cooling, which leads the SEPs to lose energy more quickly during transportation. However, slow solar wind speed has less impact on the time profiles of SEP flux and anisotropy. We also compare the time profiles of SEP event observed at different observatories and energies, the results remain the same as previous;(2) When applying in situ data of magnetic field observed by WIND during different Carrington Rotations, the omnidirectional flux time profiles vary greatly, and the main results are as followings:the peak flux appears to be delayed, multi-peak occur, anisotropy also has some differences.We think it results from the magnetic field polarity, which affects the pitch angle, and, furthermore, modulates the momentum. The characteristics are similar in solar minimum and solar maximum, while the peaks seem to be more when solar activity is active. We conclude that the real magnetic field polarity may exert a significant influence during the propagation of SEP. In the future, we will try to use the real-time background conditions which obtain from MHD models in our simulations, in order to make a thorough study of the SEP propagation.
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Key words:
- Solar energetic particle /
- Magnetic field /
- Solar wind /
- Interplanetary transport
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