Effects Of The Solar Wind Background On The Numerical Simulation Of The Solar Energetic Particle(SEP) Transportation

The transportion and acceleration processes of SEPs are promising issues in space weather studies,and SEPs could cause the destructive damages to spacecraft systems and constrain human activities in the space.The transport processes of SEPs in the inner heliosphere are modulated by ambient solar wind velocity and magnetic field.Previous simulation works often simplify the solar wind velocity as radial and constant,and treat the magnetic field as Parker spiral.In order to fully understand the effect of realistic underlying background conditions on the particles' transport and acceleration process,a realistic background solar wind and magnetic field is required.In order to investigate the SEPs in the realistic solar wind velocity and magnetic field based on a numerical solution of the focused transport equation,we change the fixed solar wind speed into spatial-dependent speed profile based on Parker's theory,replace the Parker magnetic field with another Parker-like magnetic field based on in situ data at 1 AU,and add solar wind velocity and magnetic field,which are generated by the 3D high-resolution MHD model into the SEP model.By changing the fixed solar wind speed into spatial-dependent speed profile based on Parker's theory,our results show that the omnidirectional flux decreases faster than that of fixed solar wind speed in the decay phase under fast solar wind speed assumption,but both peak fluxes remain the same.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,diffuse are predominantly important than adiabatic cooling in initial phase.The peak flux changes a little.Furthermore,slow solar wind speed has less impact on the time profiles of SEP flux.When replacing the Parker magnetic field with another Parker-like magnetic field based on 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 different,multi-peak occur,anisotropy also has some differences.We think it results from the magnetic field polarity,which affects the pitch angle.When a particle jumps into the magnetic field line of opposite polarity,it will move towards opposite direction with the former pitch angle ?.Hence,it is necessary to modify the pitch angle ? with regions of different magnetic polarity.The flux has a similar trend as that of Parker magnetic field when the pitch angle is modified.We conclude that the pitch angle ? should be modified when using the in situ data of magnetic field.The MHD modeling provides a 3D interplanetary structure,which is needed for SEP transport simulations.Therefore,the combination of the two models is crucial to such purpose.After the brief introduction of the MHD model and SEP model used,the combination of these two models is described in some details to establish the MHD-SEP model for investigating the transport process of SEPs.With the MHDSEP model,we study the influence of interplanetary structures,such as compression regions,on SEP time intensity profiles.Our results show that:(1)In compression regions,one peak of the omnidirectional flux occurs in the decay phase.We find that there exists a compression region in the extraordinary flux enhancement region.The magnetic focusing effect becomes weaker in this region,and particles may be mirrored back and observed.(2)LBgenerally describes the strength of magnetic focusing effect.In MHD conditions,LBvaries between positive and negative values,which are different from the only positive values in the Parker magnetic field.Moreover,a small(LB)-1region generally corresponds to a compressed region,which could affect the transport process of SEP.(3)A compression region also has an influence on pitch-angle diffusion.Because the parallel mean free path ? becomes smaller in compression regions and the omnidirectional flux correspondingly decays more slowly.But,the change of pitch-angle diffusion is not responsible for the special flux enhancement in the decay phase.(4)We simulate a SEP event that was observed by STEREO A on March 21,2011 with our MHD-SEP model.There are obvious flux enhancements at different energies in the decay phase,which are caused by the compression region.Our simulation results could qualitatively reproduce the pattern of the flux enhancements during the decay phase for this event.