Multi-wavelength And Multi-perspective Studies Of Coronal Mass Ejections And Their Driven Shocks

Coronal Mass Ejections(CMEs)are one of the most fierce explosion phenomena in the solar atmosphere.Eruptions of CMEs usually release a large amount of energy and eject massive magnetized plasma.When the velocities of CMEs exceed the local magnetosonic speeds,shock waves can be driven.CME-driven shocks can further lead to solar energetic particle(SEP)events.CMEs are one of the main causes of geomag-netic storms,and the solar proton events generated by their driven shocks may affect the safety of spacecraft and astronauts.Therefore,researches on CME initiation,shock for-mation and their evolution and propagation in interplanetary space are very important aspects of space weather.Combining observational data from different instruments,we have analyzed CMEs and their driven shocks in different scales and with different behaviors.The observa-tional data mainly come from three satellites including SOHO,SDO and STEREO.First,we analyze a small-scale,short-duration solar eruption(Chapter 2).A CME hot channel first forms and evolves in the low corona,and its fast motion drives the for-mation of a fast-mode shock.Combined with multi-wavelength observations,we in-vestigate their kinematic and thermal properties,and discuss the relationship between the CME hot channel and shock.We then analyze a CME(Chapter 3),associated with jets and driving a shock.For this CME and its driven shock,we perform three-dimensional(3D)reconstructions of these structures to study their evolution.The rela-tionship between two principal radii of curvature of the CME nose is also discussed.In our third work,using the data obtained from the magnetohydrodynamic(MHD)nu-merical simulation,we synthesize white-light(WL)images,and develop the cross-correlation method to calculate the two-dimensional(2D)velocity distribution of the CME.The method is applied to a real observation,and the kinetic energy distribution of the CME is obtained for the first time(Chapter 4).In the fourth work,we analyze a fast CME with a shock,and obtain distributions of the CME density,speed and temperature by combining the observations of the SOHO/LASCO coronagraph and SOHO/UVCS O VI channel and WL channel(Chapter 5).Over the past few years,researchers have extensively studied features of large-scale eruptions in the solar atmosphere,but there is limited knowledge about small-scale CME eruptions accompanied by shock waves.By studying an event on November 4,2015,we have found a small-scale,short-duration event originating from a small region.The impulsive phase of the associated M1.9-class flare was very short(<4 minutes).The kinematic evolution of the CME hot channel reveals some exceptional characteristics,including a very short duration of the main acceleration phase(<2 minutes),a rather high maximal acceleration rate(?50 km s-2),and peak velocity(?1800 km s-1).The fast and impulsive kinematics subsequently results in a piston-driven shock.The expansion and propagation speeds of the CME are less than the shock speeds,and the offset distances between the CME and shock increase with time.The starting fundamental frequency of the type ? radio burst reaches up to?320 MHz.The type ? source is formed at a low height of below 1.1 R? less than?2 minutes after the onset of the main acceleration phase.Through the band-splitting of the type ? burst,we find that the shock compression ratio decreases from 2.2 to 1.3,and the magnetic field strength of the shock upstream region decreases from 13 to 0.5 Gauss at heights of 1.1-2.3 R?.In addition,the CME(?4 × 1030 erg)and flare(?1.6 × 1030 erg)consume similar amounts of magnetic energy.The same conclusion for large-scale eruptions implies that small-and large-scale events possibly share a similar relationship between CMEs and flares.The kinematic particularities of this event are possibly related to the small footpoint-separation distance of the associated magnetic flux rope,as predicted by the Erupting Flux Rope model.Many studies have inferred the corona information of the shock upstream(such as Alfven Mach number)by using a key ratio(?)derived from a shock standoff dis-tance normalized by the radius of curvature of a CME.However,these investigators only consider one radius of curvature of a CME,while the CME always owns a three-dimensional(3D)geometry with two principal radii of curvature in space.In this chap-ter,we have analyzed a CME occurred on August 31,2010.It is associated with a jet and drives a fast shock.The combination of the data from SOHO and STEREO allows us to make 3D reconstructions of the jet,CME and shock in space,and to study the kine-matic features of these structures.Given the almost equal speed between the shock and the CME,and the bow-shock shape of the shock nose,we infer that the nose part of the shock might follow the formation mechanism of a bow shock.With the aid of the mask fitting method,we obtain two principle radii of curvature of the asymmetrical CME and their evolution with time.The magnitude difference of the ratio ? derived from two principal radii of curvature is around four times,inferring that the assumption of one radius of curvature of a CME will result in the high uncertainty in estimations of coro-nal parameters.According to the relationship between the ratio ? and the Alfven Mach number,the coronal plasma parameters have been investigated,including the Alfven Mach number,the Alfvenic speed as well as the coronal magnetic strength.For most CME studies,researchers generally calculate the average speeds of CMEs by tracking bright features(such as CMEs' cores or fronts)in the WL coronagraphic images,and directly regard the average speed of the CME as the CME's overall speed.Nevertheless,CMEs are usually characterized by the presence of significant density inhomogeneities propagating outward with different radial and latitudinal projected speeds,resulting in a complex evolution eventually forming the interplanetary CME.We demonstrate for the first time how coronagraphic image sequences can be analyzed with the cross-correlation technique to derive 2D maps of the almost instantaneous plasma speed distribution within the body of CMEs.The technique is first tested by analyz-ing synthetic WL images through the MHD numerical simulation,and then applied to measure the speed distribution of a real CME occurred on October 28,2010.Results from this work allow us to characterize the distribution and time evolution of kinetic energy inside the CME,as well as the mechanical energy(combined with the kinetic and potential energy)partition between the core and front of the CME.In the future,new generations of coronagraphs will provide CMEs with simultaneous observations in WL and ultraviolet(UV,H I Lya)band-passes,such as Metis on board ESA Solar Or-biter mission as well as the Ly? Solar Telescope(LST)on board the Chinese Advanced Space-based Solar Observatory(ASO-S)mission.The cross-correlation method can be used to measure the speeds of CMEs,constraining the effect of Lya Doppler dimming,which will allow us to further analyze the relevant physical parameters of CMEs in the future.Many previous studies have revealed that CMEs often show different features in different band-passes.Previous works discussed possible diagnostics for observations provided by future multi-channel coronagraphs(such as Metis and LST).These tech-niques will be applied to derive electron densities and temperatures in CME bodies by combining the WL and UV(H I Lya 121.6 nm and others lines)band-passes.We also studied a fast CME with the combination of WL images acquired by SOHO/LASCO coronagraphs,and intensities measured by SOHO/UVCS at 2.45 R? both in the UV(H I Ly? and O ? 103.2 nm lines)and WL channels.This CME generates a shock.Data from UVCS WL channel have been employed to measure the CME position angle with polarization ratio technique for the first time.Plasma electron and effctive tempera-tures of the CME core and void have been estimated combining the UV and WL data.The transit of the CME core(possibly of cooler plasma in the embedded prominence and plasma cooling due to expansion)results in a decrease of the electron temperature down to 105 K.The front is observed as a significant dimming in the Lya intensity,associated with a line broadening due to plasma flows along the line-of-sight.The 2D distribution of plasma speeds within the CME body has been reconstructed from LASCO images and employed to constrain the Doppler dimming of the Lya line and simulate future observations by Metis and LST.In this dissertation,we have analyzed the CMEs and their driven shocks with multi-perspective and multi-wavelength observations obtained from different space and ground instruments.Combining the WL and UV Lya line observations,we have derived the velocity,density and temperature properties of CMEs based on the corresponding methods,and try to provide data analysis tools for the new instruments(such as Metis and LST)in the future.