Systematic Study Of Solar Radio Bursts

Solar flare,as one of the most violent bursts in the solar atmosphere,is a hot topic in the solar physics research.Magnetic reconnection has been considered as the trigger of releasing non-potential magnetic energies and solar flares.A significant fraction of these non-potential magnetic energies will be transferred to energetic electrons and ions.These non-thermal particles,in turn,will enhance the radio and X-ray emissions from the Sun.Radio and X-ray emissions,hence,carry rich dynamic information of energetic electrons in solar flares.In this thesis,we study the characteristics of solar flares in the radio and X-ray wavelengths.In chapter 1,we introduce the research background of this thesis.In section 1.1,we introduce the structure of the Sun and various solar activity in the solar atmosphere.Some commonly used radio instruments are introduced in section 1.2.An accurate cali-bration of a radio instrument is the basis to correctly capture the radio information from the Sun.There are many calibration methods for radio instruments.In this thesis,we will introduce the relative and the nonlinear calibration methods in detail.In addition,we will also give detailed descriptions of the calibration and the imaging methods of the radio instruments Solar Broadband Radio Spectrometer(SBRS)and Mingantu Ul-trawide Spectral Radioheliograph(MUSER)in China.Section 1.3 introduces the basic radiation mechanisms and radiation transfer processes in plasmas.Gyrosynchrotron and bremsstrahlung radiation mechanisms are highlighted,since they are frequently applied to the radio and X-ray emissions from solar flares.Additionally,we explain how the radio and X-ray radiation are generated with thermal and non-thermal electrons,and present the relation between the radio,X-ray radiation spectra and the electron energy distribution.Radiation mechanism is a bridge to understand the dynamic processes of solar flares via remote observations of the emission they produced.Section 1.4 de-scribes the relation between the radio,X-ray emissions and energetic electrons from the observational point of view.Via 2D radio imaging,we can accurately locate the sites where electrons are energized.While temporal relation between the radio and X-ray lightcurves provides the electron propagation information.And spectra calculated with 2D radio and X-ray imagings can provide us the dominant radiation mechanism in different locations of solar flares.Furthermore,we locate the coronal sources and current sheets of solar flares via time evolution of the radio and X-ray sources.A stan-dard model for solar flare including radio radiation,X-rays,and energetic electrons is constructed by multi-band observations.Chapter 2,based on the analyses of the Mengcheng Solar Radio Spectrometer(McSRS)observations of an M2.9 class solar flare occurred on 2015 August 27,we find that the classical calibration method does not give satisfactory results,due to elec-tronic noises of the instrument.By using the X-ray and radio data of Geostation-ary Operational Environmental Satellites(GOES),and Nobeyama Radio Polarimeter(NoRP)/Nobeyama Radioheliograph(NoRH),and combined with the relevant radia-tion mechanisms,we improved the calibration method for the McSRS.Compared to the classical one,the improved calibration method provides better results,which are consistent with the NoRP/NoRH observations and reveal the typical evolution of the radio spectrogram for this M2.9 class solar flare.Chapter 3,we further analysed the M2.9 class solar flare on August 27,2015 with multi-channel observation data.We found that the impulse and gradual phases of the radio radiations from this solar flare were produced in different source areas.Further-more,radio emissions of these two phases were dominated by different radiation mech-anisms,i.e.,synchrotron radiation by double thermal electron species(bremsstrahlung)leaded to the impulsive-(gradual-)phase radio radiations.To explain the spectrum of the gradual phase,Different Emission Measure(DEM)method is used.We found that cool plasmas play an important role and contribute more to the radio radiations of the gradual component than hotter plasmas.Chapter 4,since short timescale flux variations are closely related to the energy release processes of magnetic reconnection during solar flares,radio light curves at 1,2,3.75,9.4,and 17 GHz of 209 flares observed by the Nobeyama Radio Polarimeter from 2000 to 2010 were analyzed with a running smooth technique.We found that the impulsive component(with a variation timescale shorter than 1 second)at 1 GHz of most flares peaks at a few tens of solar flux unit and lasts for about 1 minute.While the impulsive component at 2 GHz peaks at a lower flux level and has a shorter period while at the three high frequency channels,the occurrence frequency of flares increases with the decrease of the flux density up to the noise level of the corresponding background.The gradual components of these emissions,however,have similar duration and peak flux density distributions.We also derived the power spectra on different timescales.A normalized wavelet method was used to confirm features on short timescales.With a time resolution of 0.1 second,more than?60%of these radio light curves show signifi-cant flux variation on 1 second or even shorter time scales.This fraction increases with the decrease of frequency and reaches?100%at 1 GHz,implying that short timescale processes are universal in solar flares.We also studied the correlation between the im-pulsive radio flux densities and soft X-ray fluxes obtained with the GOES satellites and found that more than 65%of these solar flares with an evident impulsive component have their impulsive radio emission reach their peak values ahead of the soft X-ray fluxes and this fraction increases with the radio frequency.Summary and outlook of this thesis are provided in Chapter 5.