Forced Extrapolation Of The Global Coronal Magnetic Field And Dynamics Of A Prominence-horn Structure

The coronal magnetic field is a critical element of solar physics.Yet,it is d-ifficult to directly measure the vector magnetic field in the corona because of less plasma density,weak magnetic field,and high temperature there.At present,the 3D magnetic fields in the corona are generally obtained by extrapolation methods and numerical simulation.Potential field extrapolation is the simplest method developed so far.Yet,current and magnetic helicity in magnetic field have been neglected.Many numerical algorithms have been developed under the framework of none linear force-free field extrapolation(NLFFF),which includes the effect of twisted fields.Yet,the magnetic force is still not taken into account.Magnetohy-drodynamic(MHD)numerical simulation based on photospheric magnetogram can produce self-consistent results with interaction between magnetic field and plasma.But additional plasma parameters and much more computational re-sources are needed.We present a method of forced-field(i.e,non-force-free field:NFFF)ex-trapolation of the global magnetic field in the corona,on the basis of single-layer vector magnetogram,by extending an extrapolation technique of local magnetic field first developed by Hu et al.(2008).The forced coronal magnetic field is described by a system with the minimum dissipation rate(MDR)which is ap-propriate for the corona as a forced and open system.The obtained solution of the magnetic field can be decomposed into three components including one potential field and two linear force-free fields(LFFF).Starting from the given single-layer vector magnetogram,the bottom boundary condition for each com-ponent is determined with an iterative method to achieve a minimum difference of the transverse component between the extrapolated field and the original mag-netogram.The final extrapolated forced field is given by the sum of the three component fields with the obtained bottom boundaries.The method is tested with an analytic magnetohydrostatic(MHS)solution.It is shown that the ex-trapolated forced field is highly consistent with the MHS solution at least from the solar disk to the heliocentric distance of 1.5 solar radii.For instance,the normalized and mean vector errors(En',Em')are as high as?97%and 95%,re-spectively.Further comparisons between magnetic strength,force and field line distributions indicate that the MHS solution has been successfully reconstructed.Formation,stability,and evolution of prominences are the main research topics of solar physics.They are highly dynamic structures with flows and os-cillations.The oscillations provide an approach to obtain information about physical conditions of the prominence.Coronal cavity is the region of the no-tably reduced emission observed from solar limb.The structure generally gets involved in magnetic flux rope that shapes the cavity ellipse-like.There are of-ten some bright U-shape structures,called as horns,at the bottom of the cavity.Physical connections among and formation mechanisms of various components of the prominence-horn cavity system remain elusive.In this study,we present observations of such a system,focusing on a sec-tion of the prominence that rises and separates gradually from the main body.This forms a configuration sufficiently simple to yield elues regarding the above issues.It is characterized by embedding horns,oscillations,and a gradual dis-appearance of the separated material.The prominence-horn structure exhibits a large-amplitude longitudinal oscillation with a period of?150 minutes and an amplitude of?30 Mm along the trajectory defined by the concave horn structure.The horns also experience a simultaneous transverse oscillation with a,much smaller amplitude(?3 Mn)and a shorter period(?10-15 minutes),likely representative of a global mode of the large-scale magnetic structure.The gradual disappearance of the structure indicates that the horn,an observational manifestation of the field-aligned transition region separating the cool and dense prominence from the hot and tenuous corona,is formed due to the heating and diluting process of the central prominence mass;most previous studies suggested that it is the opposite process,i.e.,the cooling and condensation of coronal plas-mas,that formed the horn.This study also demonstrates how the prominence transports magnetic flux to the upper corona,a process essential for the gradual build-up of pre-eruption magnetic energy.In summary,the thesis mainly includes two main parts,a novel method of forced extrapolation of the global coronal magnet ic field and an observational study of dynamics of a prominence-horn structure.It is generally suggested that large scale prominent structures are supported by magnetic forces,and promi-nence are intrinsically associated with forced magnetic field in the corona.There-fore,the two topics of this thesis have close association.In future,it is necessary to study the physics of the large-scale prominence structure with the method of forced field extrapolation of coronal magnetic field based on photospheric vector magnetogram.Furthermore,the plasma flow in prominence and horn structure have significant scientific values to investigate the large-scale configurations of magnetic field.Thus,studies combing the prominence dynamics and magnetic extrapolation are important to further our understanding of prominence forma-tion and dynamics as well as the topology of the coronal magnetic field.