Study Of Dynamics Of Energetic Particles Accelerated By CME-Driven Shock Using The IPATH Model

In recent decades,with the development of aviation,deep space exploration becomes the focus of attention of all countries,and the high energy particle radiation environment in near-Earth space has an increasingly significant impact on human aerospace activities.More and more attention has been paid to study the acceleration and transport processes of energetic particles in the interplanetary space,which now is a significant subject of the solar-terrestrial space physics.There are many kinds of energetic particles in the interplanetary space,including solar energetic particles(SEP),galactic cosmic rays(GCR),anomalous cosmic rays(ACR)and so on.Gradual SEP has attracted much attention because of its long duration,high intensity and causing great harm.It is of great practical and scientific significance to study the dynamics of the gradual SEP events.Without considering the variation of solar wind along longitudinal direction,the acceleration of particles in the shock driven by Coronal Mass Ejection(CME)and the transportation of energetic particles in the interplanetary medium are studied by using the two-dimensional improved Particle Acceleration and Transport in the Heliosphere(iPATH)model in the ecliptic plane when the background solar wind reaches its radial steady state.The main research contents and results are as follows.(1)We simulate an ideal fast CME-driven shock in the ecliptic plane,and calculate the shock parameters(compression ratio,velocity,shock angle ?BN)and the maximum particle energy along the shock front.Results show that all of the shock parameters significantly vary with the longitudes.Near nose region,the shock velocity is the fastest,the shock intensity is the strongest,and the particle acceleration efficiency is also the highest.As CME propagates outward,the velocity(strength)of the shock decreases(weakens)gradually.Time intensity profiles and particle spectra at multiple locations and different radial distances,separated in longitudes,are presented.From the perspective of numerical simulation,our results are consistent with the observations of SEPs:the longitudinal extension and the reservoir phenomenon in the decaying phase of SEPs.(2)We simulate a strong and super-fast CME event on July 23,2012.This event was observed by STEREO-A,STEREO-B and ACE satellites together,and its center was oriented towards STEREO-A while the other two satellites were basically unaffected.The proton time intensity profiles at these spacecraft locations are simulated.Comparing the modelled results with the observations,we find that the magnitude and evolution trend of profiles at STEREO-A are close to observations from STEREO-A/HET instrument,and the shock arrival and departure time also coincide well with real observations.The simulated proton fluxes at ACE and STEREO-B are also consist with the observations.But it should be noted that the model can not perfectly reproduce some details of observations,such as the energetic storm phase,which may be attributed to the background solar wind is too idealized and some acceleration processes are not fully understood in the model.(3)Recent detection of superflares on solar-type stars by Kepler mission raised a possibility that they can be associated with energetic CMEs and energetic particles events.Using the iPATH model,we examine the effect of solar-star's rotation speed on the resulting energetic protons in erupting events of CME.The maximum particle energy at the shock front as a function of radial distance r is shown.Our results suggest that within 0.8 AU the maximum particle energy at the shock front increases with the rotation rate of the star.After exceeding 0.8 AU,there is a critical value of shock angle(62°)for different roatation speeds.When shock angle is lagerer than 62°,the corresponding maximum energy curves will drop significantly.One can understand this drop as the following:the total diffusion coefficient ?=??cos2?BN+?? sin2?BN is dominated by ?? when ?BN<62°,and by ?? when ?BN>62°.This is because that cos?BN decreases with increasing ?BN,so the contribution of ?? to the total ?decreases with increasing ?BN,while sin?BN increases with increasing ?BN,so the contribution of ?? to the total ? increases with increasing ?BN.It has to be noted that in iPATH model,?? is decided by ?? through the NLGC theory and ?? is coupled to the excited waves at the shock front which in turn is decided by the injection efficiency that depends on ?BN itself.How ? varies with ?BN is therefore non-linear and is not staightfard to see how ?? or ?? dominates ? at different values of ?BN.Besides,the simulated time intensity profiles and spectra of SEPs also show significant star rortation speeds dependences.For observers at the same location,the faster the star rotates,the slower the time intensity profiles increase,and vice versa.This is because the geometry of the Parker spiral field strongly depends on the rotation rate,therefore the geometry of the shock also strongly depends on the stellar rotation rate.For faster rotation cases,the observers will connect to the shock front by magnetic field at a more westward position than those observes with slower rotation speed.(4)GCR particles reaching the heliosphere can provide massive energetic seed particles for CME shock acceleration and are a greater hazard than SEPs during solar minimum in near-Earth space.Using the latest daily GCR heavy-ions data obtained by ACE/CRIS instrument,the annual average GCR energy spectrum of heavy-ions model constructed by Zhao and Qin(2013)(ZQ13 model)is refined in this paper.The new model can provide accurate monthly average prediction of GCR flux.Similar to ZQ13 model,the new model also contains three key parameters:intensity modulation parameter ?(t),average integral intensity ratio percentage p(t),and energy spectrum shape function ?(t).Applying the lag correlation analysis,the new model considers the optimal lag time between GCR and sunspot number(SSN)in odd and even cycles of solar activity,respectively.In odd cycles,the lagged times of intensity modulation parameters ?(t),p(t),?(t)to SSN are 14,15 and 20 months respectively,and the conspronding values are 6,4 and 10 months in even cycles.Using the historical SSN data,the differential fluxes of carbon and iron elements from 1975 to 1996 are reconstructed,and the modelled results are in good agreement with those early observations by IMP-8 satellite.The differential fluxes of 24 kinds of GCR heavy ions from 1998 to 2018 are also simulated and compared with ACE/CRIS observations.The absolute error of our new model is 14.5%which is significantly better than that of the well-known CREME2009 model(31.4%).