Observational And Numerical Studies On Microwave Bursts During Solar Flares

Microwave bursts are mostly gyrosynchrotron emission generated by ener-getic electrons spiraling in the coronal magnetic fields.Therefore,the microwave emission is the indicator of local magnetic morphology and energetic(nonther-mal)electrons and of importance to the flare study.In this thesis,based on microwave emission,we focus on the key processes of magnetic field evolution particle acceleration-microwave emission' and try to understand the physics of solar flares as well as the release of magnetic energy associating with multi-wavelength observations.In chapter 2,we explore the potential of microwave observation in studying magnetic structures in flaring region.In chapter 3,we s-tudy the evolution of flare magnetic morphology and its effect on the acceleration of energetic electrons using multi-wavelength data.In chapter 4,we calculated the microwave emission theoretically for energetic electrons with specified distri-butions for given coronal magnetic field and plasma conditions.And in chapter 5,we summarize this thesis and discuss the following study.We report for the first time the microwave imaging of the magnetic flux rope structure during the pre-impulsive phase of a M7.7 flare on 2012 July 19.During this early stage of the flare,the flux rope is observed by the the hot passbands(94 and 131 A)of the SDO/AIA and is also revealed by the microwave observation at 17 GHz(Nobeyama RadioHeliograph)as an overall arcade-like structure con-sisting of several intensity enhancements.We note that the microwave intensity enhancements(TB in the range of?10000-20000 K)is quite localized,indicat-ing that the distribution of emitting electrons is affected by localized large-scale magnetic structures.We further conduct the wavelet analysis and a pronounced 2 minute period of the microwave TB variation is found at the flux rope area.This period agrees well with the periodic sunward contraction of flare loops and upward ejective plasmoids.This suggests that both the EUV and microwave observations are controlled by the same reconnection process that takes place intermittently at a 2 minute timescale.We infer that at least a part of the e-mission is excited by non-thermal energetic electrons via the gyro-synchrotron mechanism.This work demonstrates that microwave observation is of poten-tial in studying coronal magnetic morphology and evolution which are crucial to understand the physics of solar flares.We then study the coronal double-source structure observed in both HXRs and microwaves during a magnetic-breakout-triggered X1.3-class flare on 2014 April 25.HXR and microwave are generated by a same population of electrons accelerated by flares when precipitating into the chromosphere and spiraling the magnetic fields,respectively.In this event,the lwoer source extends upward from the top of the partially occulted flare loops and the upper source coincides with rapidly squeezing-in side lobes with a speed of?500 km s-1.The X-ray upper source is characterized by flux curves that differ from those of the lower source,a weak energy dependence of projected centroid altitude above 20 keV,a shorter duration,and an HXR photon spectrum slightly harder than those of the lower source.The microwave emission at 17 and 34 GHz also exhibits a similar double-source structure and the microwave spectra present a negative slope which means the microwave emission is produced by non-thermal gyrosynchrotron mechanism.These observations indicate that the upper source is possibly caused by the anti-breakout process and provide new light on the origin of the corona double-source structure observed in both HXRs and microwaves.We further conduct numerical calculation of microwave emission for specified distribution of nonthermal electrons in given coronal magnetic and plasma condi-tion.Recently,some uncommon features of microwave spectra,such as unusually hard(or even positive)spectra,and/or a super-high peak frequency are reported.Due to the limitation of current microwave facility,theoretical calculation should be proceeded with consideration of more factors.We investigate the effect of broken-power-law spectra of energetic electrons on microwave emission on the basis of classical gyrosynchrotron mechanism.The electron broken-power-law spectrum is characterized by three parameters,the break energy(EB)the spec-tral indices below(?1)and above(?2)the break.We find that with the addition of the ?2 component of the electron spectra,the total flux density can increase by several times in the optically-thick regime,and by orders of magnitude in the optically-thin regime;the peak frequency(?p)also increases and can reach up to tens of GHz;and the polarization degree(?c)decreases in general.We also find that(1)the variation of the flux density is much larger in the optically-thin regime,and the microwave spectra around the peak frequency manifest various profiles with the softer or soft-hard pattern;(2)the parameters ?1 and EB affect the microwave spectral index(?)and the polarization degree(?c)mainly in the optically-thick regime,while the effect of ?2 mainly appears in the optically-thin regime.The results are helpful in understanding the lately-reported microwave bursts with unusual spectral features and point out the demands for a more-complete spectral coverage of microwave bursts,especially,in the high-frequency part,say,>10-20 GHz.