| Shock waves are an important and ubiquitous phenomenon in astrophysics. They can accelerate electrons and ions. The research on magnetohydrodynamics (MHD) shocks is helpful for us to understand various kinds of high energy physical processes. The coronal shock wave is one of the most important shocks in the heliosphere, and it is also the most frequent MHD shock that we can observe in astrophysics. Type Ⅱ radio bursts represent the basic emission feature of coronal shocks. Coronal shocks and type Ⅱ radio bursts are often related with some high energy events in solar eruptions, such as Solar Energetic Particle (SEP) events. However, there are many problems to solve regarding coronal shocks. For example, whether the coronal shocks are generated by flares as a blast-wave or driven by Coronal Mass Ejections (CMEs) as a piston-wave? What is the condition for the generation of type Ⅱ radio bursts? We attempt to answer some of these questions in this paper.Chapter 1 presents a brief introduction on the background of the research topic of the thesis. We introduce the radio emission of the coronal shocks, type Ⅱ radio bursts and the main problems remaining to be solved, the source of the coronal shocks, the condition for the generation of type Ⅱ radio bursts, the physical characteristics of the coronal shocks, and the acceleration and heating of particles by MHD shocks.In Chapter 2, we introduce the main instruments for observations, the source of the data for research and the method for analysis.In Chapter 3, we analyze a short-lived type Ⅱ burst that started at 07:40 UT on 2011 February 28. By carefully checking white-light images, we find that the type Ⅱ radio burst is not accompanied by a CME, but only with a C2.4 class flare and a narrow jet. However, in the extreme-ultraviolet (EUV) images provided by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO), we find a wave-like structure that propagated at a speed of ～600 km s-1 during the burst. The relationship between the type Ⅱ radio burst and the wave-like structure is in particular explored. For this purpose, we first derive the density distribution under the wave by the differential emission measure (DEM) method, which is used to restrict the empirical density model. We then use the restricted density model to invert the speed of the shock that produces the observed frequency drift rate in the dynamic spectrum. The inverted shock speed is similar to the speed of the wave-like structure. This implies that the wave-like structure is most likely a coronal shock that produces the type Ⅱ radio burst. We also examine the evolution of the magnetic field in the flare-associated active region and find continuous flux emergence and cancellation taking place near the flare site. Based on these facts, we propose a new mechanism for the formation of the type Ⅱ radio burst, i.e., the expansion of the strongly-inclined magnetic loops after reconnected with nearby emerging flux acts as a piston to generate the shock wave.In Chapter 4, we focus on the generation condition of type Ⅱ radio bursts. We analyze the physical parameters at the downstream and upstream of the shock partic-ularly. For this prupose, we select a type Ⅱ radio burst that occurred on 2014 January 8, a limb event that was accompanied with a CME. We find a double-layer structure propagating outward in AIA 193 A and 211 A images. The outer layer corresponds to the shock wave while the inner layer to the CME leading edge. As the shock front is like an arc, we fit the shock front with a circle for different moments. We set the line connecting the centre of the solar disk and the original center of the fitted circle as the base line (0° direction), and then select 7 directions from -45° to 45° with an angular separation of 15° direction. We measure the shock speed along the 7 directions in the EUV images, and find that the speed at -15° is the fastest, which is slightly faster than at other directions. We use the differential emission measure (DEM) method to obtain the physical parameters at shock region for different moments during the type Ⅱ radio burst, including the temperature (T), emission measure (EM), temperature ratio (Td/Tu), compression ratio (X) and Alfven Mach number (MA). We compare the compression ratio X to that obtained from band-splitting in radio spectrum, and find that:this type Ⅱ radio burst is generated at a small region of the shock front, and in particular, the source of the type Ⅱ radio burst is located at the 30°-45° direction; the generation of the type Ⅱ radio burst needs other conditions other than the shock speed. For this event, type Ⅱ radio burst is generated at the region where the X and MA values are larger. We use the same method to analyze 7 type Ⅱ radio bursts from January 2013 to December 2014. We find that the value of X and MA of the shock is at range of 1.08-1.36 and 1.06-1.28, respectively. As the compression ratio is an index to indi-cate the strength of the shock, the value of the X and MA can be regarded as a good reference value of the condition of the generation of the type Ⅱ radio bursts.In Chapter 5, we perform a statistical study of type Ⅱ radio bursts from 1997 to 2011. We classify the type Ⅱ radio bursts into three categories:(1) the bursts that only have emission component in metric wavelengths (M events), (2) the bursts that have emission component only in decameter-hectometric (DH), and kilometer (KM) wavelengths (DH events), and (3) the bursts that have emission component in metric, DH, and KM wavelengths (M-KM events). We find that the speed, width, and the proportion of halo CMEs are increased from the first to the third categories of the bursts. This implies that if the shock or the type Ⅱ radio burst is generated or emitted at a higher location, the CME that drives the shock needs to be faster and more energetic. We also get the Waiting Time Distributions (WTDs) of the metric type Ⅱ radio bursts, and compare it with the WTDs of Solar Energetic Particle (SEP). We find that the power-law index and the non-stationary Poisson index of the WTD for the type Ⅱ bursts is similar for to that:for the SEPs. This implies that there is a close relationship between SEPs and the shock that is related to the type Ⅱ radio burst.The conclusions and prospects for future work are given at the end of the thesis. |
