Studies On Physics Of Solar Radio Bursts

Solar radio bursts are important in the study of solar physics and radio astronomy.The radio emission characteristics(intensity,frequency,spectrum,etc.)are closely related to the properties of plasmas,the magnetic field,and energetic particles of the source,and can be used to diagnose physical parameters of the solar atmosphere,so as to better understand the process of the conversion and release of magnetic energy,the acceleration of energetic particles and the mechanism of solar radio bursts.This thesis presents our observational and theoretical studies relevant to type ? and type ? bursts.Chapter 1 mainly introduces relevant background,including the activities in the solar atmosphere and related solar radio bursts,two relevant coherent emission mechanisms that are electron cyclotron maser emission and plasma emission.In Chapter 2 we investigated EUV and magnetic activities associated with type-? solar radio bursts.The underlying dynamic process and exact emission mechanism of Type-Is still remain unresolved.Magnetic activities in the so-lar atmosphere that are relevant to type-Is have not been revealed clearly.Here,with a combined analysis of multi-wavelength of extreme ultraviolet(EUV),radio and photospheric magnetic field data of unprecedented quality recorded during a type-? storm on 30 July 2011,we identify a good correlation between the ra-dio bursts and the co-spatial EUV and magnetic activities.The EUV activities manifest themselves as three major brightening stripes above a region adjacent to a compact sunspot,while the magnetic field there presents multiple moving magnetic features(MMFs)with persistent coalescence or cancelation and a mor-phologically similar three-part distribution.We find that the type-? intensities are correlated with those of the EUV emissions at various wavelengths with a correlation coefficient of 0.7-0.8.In addition,in the region between the bright-ening EUV stripes and the radio sources there appear consistent dynamic motions with a series of bi-directional flows,suggesting ongoing small-scale reconnection there.Mainly based on the induced connection between the magnetic motion at the photosphere and the EUV and radio activities in the corona,we suggest that the observed type-? noise storms and the EUV brightening activities are the consequence of small-scale magnetic reconnection driven by MMFs.This is in support of the original proposal“MMFs are at the origin of the observed metric noise storms" made by Bentley et al.(2000).This work is of significance for understanding the long-term evolution of active regions and the gradual release of magnetic field energy.From the above study on type-Is,it can be suggested that type-Is are related to energetic electrons released by small-scale magnetic reconnections.These elec-trons are trapped by the large-scale closed magnetic loops overlying the active region.For type-? radio bursts,it has also been prosed that energetic electrons trapped by eruptive magnetic structure account for the radiation.The electron cyclotron maser instability with the parameter regime of ?pe/?ce>1 and subse-quent wave-coupling process has been suggested to be the emission mechanism of both the type-? and type-? continuum.In Chapter 3,studied the effect of the temperature of background plasma and the energy of energetic electrons on Z-mode waves excited by energetic electrons with a loss-cone type velocity dis-tribution.And in Chapter 4 we further explore the effect of varying ?pe/?ce and ve on the ECMI instability and the emission characteristics.Both the temperature of background plasma(T0)and the energy of energetic electrons(ve)are considered to be important to the variation of the maximum growth rate(?max).In Chapter 3 we present a detailed parameter study on the effect of To and ve on Z-mode waves excited by ECMI,within a regime ofthe frequency ratio(10??pe/?ce?30).In addition to ?max,we also analyze the effect on the corresponding wave frequency(?maxr)and propagation angle(?max).We find that(1)?max in-general decreases with increasing ve,while its variation with T0 is more complex depending on the exact value of ve;(2)with increasing T0 and ve,?maxr presents step-wise profiles with jumps separated by gradual or very-weak variations,and due to the warm-plasma effect on the wave dispersion relation ?maxr can vary within the hybrid band(the harmonic band containing the upper hybrid frequency)and the band higher;(3)the propagation is either perpendicular or quasi-perpendicular,and 0max presents variations in x line with those of ?maxr,as constrained by the resonance condition.We also examine the profiles of ?max with ?pe/?ce for different combinations of To and ve to clarify some earlier calculations which show inconsistent results.The calculations are significant for explanations of type-IV zebras and type I-IV continuum,and further studies of coronal conditions,such as the magnetic field strength in the source region.In addition,it should be highlighted that,both magnetic field and plasma density change rapidly during solar flares,and this may also have an important effect on the wave growth rate as well as the presence of zebra stripes and their spectral morphology.In Chapter 4,we studied the effect of varying ?pe/?ce and ve on the wave modes excited by ECMI and relevant plasma process using PIC simulation.Pre-vious studies mostly discussed growth rates of ECMI instability of electrostatic upper hybrid(UH)waves based on the qusi-linear theory,neglecting nonlinear wave mode coupling and conversion process.Ni et al.(2020)used PIC simulation to study the plasma emission process induced by ECMI(ECMI-Plasma Emis-sion).They discussed the properties of UH mode,Z mode,and W mode,and subsequent nonlinear wave coupling or conversion process for a specific ratio of plasma frequency to gyrofrequency(?pe/?ce=10).Here we further explored this process focusing on the effect of ve and ?pe/?ce·The results show that the linear growth rates as well as the wave intensities of UH and Z modes present a local maximum at ?pe/?ce?10.25 and a local minimum at ?pe/?ce?10.75.This roughly agrees with the linear analysis present,ed in Chapter 3.It was found that the intensity profiles of UH and H modes(O-F and Z modes)are similar to each other.This provides support to the occurrence of the ECMI-plasma emission pro-posed by Ni et al.(2020).In addition,we found that the directional pattern of the H emission strongly depends on values of ve and ?pe/?ce;and the harmonic emission is much stronger in comparison to that of the fundamental emission with a higher contrast between its maximum and minimum values of emission intensity,therefore we suggest that it is the harmonic mode that accounts for the emission stripes of zebras.These results are significant to studies on emission mechanisms of type-Is and type-IVs and studies using zebras to perform coronal diagnostics.Chapter 5 summarizes major results of this thesis and presents suggestions for further studies.