Studies On Non-thermal Radiations Of Solar Flares

Solar flares represent one of the most explosive energy release processes in solar atmosphere.During flares,the energies stored in the coronal magnetic fields are released through magnetic reconnection,producing energized particles（over 10 GeV）and hot plasmas（over 10 MK）in solar active regions,resulting in enhanced emissions at all wavelengths（from radio to γ-rays）.The non-thermal radiations at hard X-ray（HXR）and radio wavelengths,produced by energetic electrons,play a crucial role in diagnosing the physical conditions of the source region,and in revealing the natures of energy release and particle acceleration.Although the basic pictures have been widely accepted,a complete understanding of the flare processes has not been achieved.For various types of radio bursts accompanying flares,the mechanisms of coherent radio emissions are still controversial.In this thesis,we present studies on the non-thermal HXR and radio radiations relevant to energetic electrons of solar flares.Chapter 1 introduces the primary concepts of solar flares,HXR and radio bursts,and the coherent emission mechanisms.In Chapter 2-3,we present two observational studies of flare events with data at wavelengths of X-rays,EUVs,and microwaves,to investigate the processes of energy release and electron acceleration.In Chapter 4-5,we investigated the coherent electron cyclotron maser emission（ECME）mechanism driven by energized electrons in the flare region,with fully-kinetic electromagnetic particle-in-cell（PIC）simulations.In Chapter 6 we conclude the major results of the thesis and present the prospect for future studies.We investigated a two-stage energy release process of a confined flare,with multi-wavelength observational data analysis.Earlier studies on confined flares mainly analyzed disk events to reveal the magnetic topology and causes of confinement.In this study,taking advantage of a tandem of instruments working at different wavelengths of X-rays,EUVs,and microwaves,we present dynamic details of a confined flare occurring on the northwestern limb of the solar disk on July 24th,2016.The entire dynamic evolutionary process starting from its onset is consistent with a loop-loop interaction scenario.The X-ray profiles manifest an intriguing double-peak feature.According to the spectral fitting results,it is found that the first peak is non-thermally dominated while the second peak is mostly multi-thermal with a hot（～10 MK）and a super-hot（～30 MK）component.This double-peak feature is unique in that the two peaks are clearly separated by 4 minutes,and the second peak reaches up to 25-50 keV;in addition,at energy bands above 3 keV the X-ray fluxes decline significantly between the two peaks.This,together with other available imaging and spectral data,manifest a two-stage energy release process.A comprehensive analysis is carried out to investigate the nature of this two-stage process.We conclude that the second stage with the hot and super-hot sources mainly involves direct heating through loop-loop reconnection at a relatively high altitude in the corona.We also report a broken-up spectra of the loop-top HXR source of a solar flare.Solar HXR sources（both footpoint and coronal sources）provide critical information on the physics of particle acceleration and plasma heating.It has been found that the HXR spectra of some flares manifest significant features of spectral hardening at high energies（over hundreds of keVs）,i.e.,broken-up power-law spectra.However,earlier studies are mainly based on spatially-integrated spectral analysis.without distinguishing the contributions from individual sources.We present an observational study of broken-up spectra of a coronal source of the SOL2017-09-10T16:06 X8.2-class flare,using HXR data recorded by RHESSI.The flare occurred behind the western limb and its footpoint sources were mostly occulted by the disk.The coronal source is consistent with the nature of coronal thick-target bremsstrahlung.We could clearly identify such broken-up spectra pertaining solely to the coronal source during the flare peak time and after.Since a significant pileup effect on the RHESSI spectra is expected for this intense solar flare,we have selected the pileup correction factor,p=2.In this case,we found the resulting RHESSI temperature（～30 MK）to be similar to the GOES soft X-ray temperature and break energies of 45-60 keV.Above the break energy,the spectrum hardens with time from spectral index of 3.4 to 2.7,and the difference in spectral indices below and above the break energy increases from 1.5 to 5 with time.However,we note that when p=2 is assumed,a single powerlaw fitting is also possible with the RHESSI temperature higher than the GOES temperature by～10 MK.Possible scenarios for the broken-up spectra of the looptop HXR source are briefly discussed,including termination shock acceleration model,trap-plus-precipitation model,and stochastic acceleration model.Flare are usually accompanied by various types of solar radio bursts,e.g.,typeⅢ,Ⅳ radio bursts and millisecond radio spikes,observed with very high brightness temperatures.Such radiations are caused by coherent emission mechanisms driven by energetic electrons with different types of anisotropy,like plasma emission（PE）and ECME.In the flare region the magnetic fields are strong,and the plasma oscillation frequency is generally lower than the electron cyclotron frequency（ωpe<Ωce）,thus ECMEs can be generated.However,when applied to solar radio bursts,there exists a major problem,i.e.,the escaping difficulty,while the fundamental emission could be effectively absorbed when passing through the second-harmonic absorption layer.Emissions at second or higher emissions are more likely to escape.ECME is the most favored mechanism for solar radio spikes,and has been investigated extensively since 1980s.Most studies employ loss-cone type electrons,generating waves mainly in fundamental X/O mode（X1/O1）,which can hardly escape through the second-harmonic absorption layer.In this letter,we perform PIC simulation on the horseshoe-driven ECMEs,with ωpe/Ωce=0.1,corresponding to the plasma condition in flare region with strong magnetic field.Such horseshoe-like distribution has been measured in the source area of auroral kilometric radiations（AKR）.We found that waves in both Z-mode and 2nd harmonic X-mode（X2）can be amplified efficiently in our simulation,with a relatively weak growth of 01 and X3.We also varied the density ratio of energetic electrons to total electrons（ne/n0）in the simulation.With a higher density ratio,the X2 emission becomes more intense,and the rate of energy conversion from energetic electrons into X2 modes can reach～0.06%and 0.17%,with ne/n0=5%and 10%,respectively.The results provide novel means for resolving the escaping difficulty of ECME when applied to solar radio spikes.The simultaneous growth of X2 and X3 can be used to explain some harmonic structures observed in solar spikes.To explore more possibilities of harmonic emissions,we further present a 2D3V fully kinetic electromagnetic particle-in-cell（PIC）simulation to investigate the wave excitations and subsequent nonlinear processes induced by the energetic electrons in the loss-cone distribution.The characteristic frequency ratio ωpe/Ωce is set to 0.25,adequate for solar active region conditions.As a main result,we obtained strong emissions at the second harmonic X mode（X2）.While the fundamental X mode（X1）and the Z mode are amplified directly via the electron cyclotron maser instability,the X2 emissions can be produced by the nonlinear coalescence between two Z modes and between Z and X1 modes.This represents a novel generation mechanism for the harmonic emissions in plasmas with a low value of ωpe/Ωce,which may resolve the escaping difficulty of explaining solar radio emissions with the ECME mechanism.In this thesis,we mainly studied non-thermal radiations in HXR and radio wavelengths,generated by energetic electrons of solar flares.With observational data analysis,we mainly investigated the spectral features of HXR emissions,to study the processes of energy release and electron acceleration in solar flares,and properties of electrons.We further conducted PIC simulations to study the coherent radio radiations amplified by energetic electrons,to explore more possibilities of harmonic maser emissions,to resolve the long-standing problem of escaping difficulty of ECME,and to explain the radio bursts accompanying solar flares,e.g.,solar spikes.