Multi-wavelength and Multi-perspective Studies of Coronal Mass Ejections and Their Driven Shocks

Abstract

Coronal Mass Ejections (CMEs) are one of the most fierce explosion phenomena in the solar atmosphere. CMEs usually release a large amount of energy and eject massive magnetized plasma. CME-driven shocks can further lead to solar energetic particle (SEP) events and affect the safety of spacecraft and astronauts. Therefore, researches on CME initiation, shock formation and the evolution and propagation of CMEs in interplanetary space are essential aspects of space weather. Combining observational data from different instruments, we have analyzed four CMEs and their driven shocks at different scales and with other behaviors. The observational data mainly come from three satellites, including SOHO (Solar and Heliospheric Observatory), SDO (Solar Dynamics Observatory) and STEREO (Solar Terrestrial Relations Observatory).

First, we analyze a small-scale, short-duration solar eruption occurred on November 4, 2015. The impulsive phase of the associated M1.9-class flare is very short ($<$4 min). The kinematic evolution of the CME hot channel reveals some exceptional characteristics, including a very short duration of the main acceleration phase ($<2$ min), a rather high maximal acceleration rate ($\sim50 ~\rm km \cdot s^{-2}$), and peak velocity ($\sim1800 ~\rm km \cdot s^{-1}$). The fast and impulsive motion subsequently results in a piston-driven shock. The starting fundamental frequency of the type II radio burst reaches up to $\sim$320 MHz. The type II source formed less than $\sim$2 min after the onset of the main acceleration phase. Through the band-splitting of the type II burst, we estimate the shock strength and the magnetic field strength of the shock upstream and so on. Besides, the CME ($\sim4 \times 10^{30}$ erg) and flare ($\sim1.6 \times 10^{30}$ erg) consume similar amounts of magnetic energy, implying that small- and large-scale events possibly share a similar relationship between CMEs and flares. The kinematic characteristics of this event are perhaps related to the small footpoint-separation distance of the associated magnetic flux rope, as predicted by the Erupting Flux Rope model.

Then, we analyze a CME associated with jets on August 31, 2010. The CME nose drives a shock. For this CME and its driven shock, we perform three-dimensional (3D) reconstructions of these structures to study their kinematic features. Given the almost equal speed between the shock and 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 principal radii of curvature of the asymmetrical CME and their evolution with time. We find that the maximal radius of curvature (ROC) is 2 to 4 times the minimal ROC of the CME, inferring that the assumption of one radius of curvature of a CME will result in the high uncertainty in estimations of coronal parameters. Based on the relationship between the ratio $\delta$ and the Alfv{\'e}n Mach number, coronal plasma parameters have been investigated, including the Alfv{\'e}nic speed and the coronal magnetic strength.

Using the data obtained from the magnetohydrodynamics (MHD) numerical simulation, we synthesize white-light (WL) images and develop the cross-correlation method to calculate the two-dimensional (2D) velocity distribution of the CME firstly. The technique is first tested by analyzing 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. The results of 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, such as Metis onboard the Solar Orbiter mission and the Ly$\alpha$ Solar Telescope (LST) onboard the Chinese Advanced Space-based Solar Observatory mission, will observe CMEs simultaneously in WL and ultraviolet (UV, H I Ly$\alpha$) band-passes. The cross-correlation method can be used to measure the speed of CME and constrain the effect of Ly$\alpha$ Doppler dimming, so that we can further analyze the relevant physical parameters of CMEs in the future.

We study a fast CME with the combination of WL images acquired by SOHO/LASCO (the Large Angle Spectroscopic Coronagraph), and intensities measured by SOHO/UVCS (The UV Coronagraph Spectrometer) at 2.45 $R_{\odot}$ both in UV (H I Ly$\alpha$ 121.6 nm lines and O VI 103.2 nm lines) and WL channels. This CME generates a shock. Data from the UVCS WL channel have been employed to measure the CME position angle with a polarization-ratio technique for the first time. Electron temperature and effective kinetic temperatures of the plasma at the CME core and cavity have been estimated combining the UV and WL data. The transit of the CME core results in a decrease of the electron temperature down to $10^{5}$ K. The CME front is observed as a significant dimming in the Ly$\alpha$ intensity. The 2D distribution of plasma speed within the CME body has been reconstructed from LASCO images and employed to constrain the Doppler dimming of the Ly$\alpha$ 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 WL and UV Ly$\alpha$ 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.