Subjects: Geosciences >> Space Physics submitted time 2016-05-13
Abstract： Solar energetic particle (SEP) can destroy the spacecraft, and has great influence on human activities. Thus, it is of importance to forecast SEP events. SEP events can be divided by three general categories: impulsive SEP events, gradual SEP events and mixed SEP events. Impulsive SEP events, with characteristics of low intensity, short duration, and enhanced He-3, are correlated to solar flares. Gradual SEP events which become the main simulation branch of current research are related to coronal mass ejections (CMEs) and they usually have high flux in protons and last longer. Mixed SEP events maintain complex features of the former two kinds of events. Several theories and numerical models are applied to simulate gradual SEP events. Each model makes some assumptions to simplify the complex acceleration and transportation processes. No research to date has combined accurate shock evolution, particles injection and acceleration, with their interplanetary transport. In order to develop the physics-based SEP prediction model and improve the ability to forecast SEP events, it is necessary to self-consistently combine the magnetohydrodynamic (MHD) model that describes CME-driven shock propagation with particle model that simulates the acceleration and transportation processes of particles. In this paper, firstly, we review current models, which have been used to describe gradual SEP events and their applications. Generally, there are two major approaches to model gradual SEP events: some studies include the acceleration mechanisms of SEPs induced by CME-driven shocks, while others assume a fixed particle injecting source at the shock. In addition, some researchers also consider the effects of perpendicular diffusion on SEPs propagation in three-dimensional interplanetary magnetic fields. These models can partially reproduce the observed properties for SEP events. Then, we make a brief review of numerical MHD simulation models, such as Space Weather Modeling Frame (SWMF), CORonal and HELiospheric (CORHEL) model, Solar InterPlanetary-Conservative Element Solution Element (SIP-CESE) model, COronal INterplanetary (COIN) model. All these models can be used to workout the propagation parameters of CME and CME-driven shocks, which are expected to provide inputs to the particle model. Finally, some discussions of the future work about how to combine MHD and particle models are presented.
Peer Review Status:
Awaiting Review
Subjects: Geosciences >> Space Physics submitted time 2016-04-22
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.
Peer Review Status:Awaiting Review
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