Numerical Simulations On The Eruption Mechanism And Propagation Of Coronal Mass Ejections

As one of the most violent astrophysical phenomena,coronal mass ejections(CMEs)have strong potential space weather effects,such as causing intense geomagnetic storms,influencing high-technical systems of modern society.CMEs,bridging the Sun and the Earth,play an important role in the research on space physics and space weather.In this PhD dissertation,we use magnetohydrodynamics(MHD)numerical simu-lations to study the eruption mechanism and the propagation of CMEs.First,we study the catastrophe of the flux rope system and focus on the energy transformation during the catastrophe.Second,we study the deflection of CMEs in the interplanetary space.Third,we study the deformation of CMEs and its influence on the fits of in situ data.Our research contains the evolution of CMEs in the solar-terrestrial environment.The main contents and results are as follows.1.The catastrophic model of CMEsThe catastrophic model of coronal magnetic flux rope is a very promising scenario to serve as a possible mechanism to trigger the eruption of CMEs.Magnetic energy dur-ing the catastrophe was predominantly studied by the previous catastrophe works as it is generally believed to be the main energy supplier for the solar eruptions.However,the contribution of other types of energies during the catastrophe can't be neglected,espe-cially for the large-scale CMEs in the solar wind background.We use 2.5-dimensional(2.5-D)numerical simulations to study the catastrophe of the coronal flux rope system by considering the solar wind,with emphasis on the transformation of different types of energies during the catastrophe.The flux rope is characterized by its axial and poloidal magnetic fluxes and total mass.As the rope magnetic fluxes increase or the rope mass decreases,a catastrophe can be triggered,leading to an upward eruption of the rope.The magnetic energy of the system and the internal energy of the rope are both released,re-sulting in an increase of the rope kinetic and gravitational energies which are related to the rope eruption.The contribution made by the internal energy is comparable to that made by the magnetic energy.Furthermore,we find that the catastrophe can also be triggered as the flux rope axial magnetic flux decreases.In this case,the magnetic energy of the system provides a very small part of the energy release,or even increase during the catastrophe.Overall,as far as the internal energy is concerned,it can act as the dominant or even the unique energy supplier during the catastrophe.2.The deflection of CMEs in the interplanetary spaceDeflection of CMEs in the interplanetary space,especially in the ecliptic plane,serves as an important factor deciding whether CMEs arrive at the Earth.Several case studies showed many signatures of such deflections,but it has not yet been statistically assessed how significantly the deflected propagation would influence the Earth-arrivals of CMEs.We developed an integrated CME-arrival forecasting(iCAF)system,which assembles the modules of CME detection,3-D parameter derivation,and trajectory re-construction to automatically predict whether or not a CME arrives at the Earth.we find that the success rate of the predictions by iCAF with deflection considered by us-ing a model for CME deflection in the interplanetary space(DIPS)is 19%higher than that without deflection,indicating the importance of this factor for providing a reliable forecasting.Up to now,there is no direct observation showing the CME deflection in the eclip-tic plane.To understand this phenomenon theoretically,we develop a 2.5-D ideal MHD scheme to study the propagation of CMEs travelling with different speeds in the helio-spheric equatorial plane.The simulations confirm the existence of the CME deflection in the interplanetary space,which is related to the difference between the CME speed(vr)and the solar wind speed(vsw)a CME will propagate radially as vr is close to vsw,but eastward or westward when vr is larger or smaller than vsw;the greater the differ-ence,the larger the deflection angle.The deflection can be interpreted as:(1)a slow CME can make the interplanetary magnetic field(IMF)co-rotating with the Sun piled up behind itself,while a fast CME leads to the ahead pileup of the IMF;(2)the magnetic field in the piled-up region will become stronger;(3)the piled magnetic field then drives the corresponding deflection;(4)the speed difference is related to the piled magnetic field strength.Our simulation supports the DIPS model.Furthermore,to test the ef-fectiveness of the usage of the DIPS model in space weather forecasting,we compared the deflection angles which are derived by inputting vr and vsw from the simulation,with those in the simulations.We find that the results can match well with the simula-tions.Overall,our simulations provide a stepstone towards the understanding of CME deflection.3.The deformation of CMEs and the associated in situ data fitsThe deformation of CMEs is important in space weather research as it causes the changes of the CME inner magnetic field profiles and then influences the related space weather effects.We perform the simulations in the meridional plane,and find that the simulated CME with initial circular cross section can be flattened during its propagation by the interaction with the background solar wind.We study the relationship between the CME initial parameters and the flattening.We find that the initial CME condition with more axial magnetic flux and mass,or less poloidal magnetic flux will lead to the severer flattening;but if a CME has large poloidal flux,its deformation will be limited.Magnetic clouds(MCs)are the in situ counterpart of CMEs.The instruments near the Earth(or other planets)could only provide data along 1-D path for MCs.To re-construct the global structures of MCs in two or three dimensional space,various MC models have been developed.We use our simulations to access the influences of the CME deformation on the fitted results by taking two cylindrical flux rope models(lin-ear force free rope and uniform-twisted of nonlinear force free rope,respectively).We compared the fitted parameters of MCs with those in the simulations,and find that the axial magnetic flux of the flattened MC will be underestimated by the fits;the severer flattening the MC,the more underestimation.However,the fitted poloidal magnetic flux is not affected by the MC flattening.Furthermore,we find that although the eval-uation of the goodness of the fits might suggest acceptable fitting results,there still exists differences between MC parameters of the fits and the simulations.This incon-sistency depends both on the fitting models and the trajectory of the spacecraft passing through MCs.This analysis provides important information for properly understanding the fitting results of in situ MCs.