Research On Solar-Interplanetary Radio Bursts And Their Emission Mechanism Based On Parker Solar Probe

Solar radio bursts are the direct signal of energy release and the generation of energetic electrons in solar activity,which can reflect physical information such as plasma structure and state in the radiation source region,as well as the movement of the energetic electron beams in it.Therefore,the study of solar radio bursts and their radiation mechanisms can provide abundant information of the physical state of the solar atmosphere and its active phenomenon.The observation characteristics of abnormal bright temperature of solar radio bursts indicate that their radiation should be generated from the coherent process of non-thermal energetic electron beams.Plasma emission based on the nonlinear transformation of Langmuir waves and electron cyclotron maser emission based on direct amplification by electron cyclotron maser instability are two plasma coherent radiation mechanisms.However,the coherent emission process is very sensitive to the physical state of the source region,so researchers have been controversial about the choice of these two coherent radiation mechanisms all the time.It is generally believed that the radiation source region of solar radio bursts with high frequency band(tens of MHz to tens of GHz)is located in the low corona,while the radio bursts with low frequency band(tens of MHz to tens of kHz)mainly originate from the high corona and even interplanetary space(also known as interplanetary radio bursts).It is impossible to detect the low corona in situ,however,Parker Solar Probe(PSP)launched in August 2018 provides an unprecedented opportunity to detect the high corona and interplanetary radiation source regions.The detection frequency band of the high-resolution radio spectrometer on the PSP(19.2 MHz～10.5 kHz)can well cover the frequency range of the radio emissions from the solar wind acceleration region in the high corona to the extensive solar wind region in interplanetary space.Therefore,studying interplanetary radio bursts originating from the Sun by use of PSP not only has a significant unique advantage in the research of the radiation mechanism of radio bursts,but also provides an important new method for studying the origin and acceleration of solar wind.In this thesis,based on the radio observations and in situ plasma measurements by PSP,the theoretical models of low frequency radio bursts originating from the high corona and the radiation mechanism are studied in depth,and some important scientific results are obtained.After a brief introduction of the observational features of solar radio bursts and the plasma coherent emission mechanisms,space and ground low-frequency radio observation equipments,and the research progress of radio bursts by PSP observations(Chapter 1),the thesis discusses all the research works during the doctoral period,including the following three aspects:1)Identification of type Ⅲ radio bursts observed by PSP and statistical analysis of their cutoff frequencies(Chapter 2);2)The generation mechanism of a type Ⅲ radio burst with fine structure(i.e.type Ⅲb)in PSP observations(Chapter 3);3)A new type of solar radio burst called solitary wave radiation discovered by PSP and the physical model of its radiation source(Chapter 4).The main research results are briefly introduced as follows.1)Identification of type Ⅲ radio bursts observed by PSP and statistical analysis of their cutoff frequencies:Based on the approach observations of the PSP radio frequency spectrometer(RFS)in the encounter modes(heliocentric distance less than 0.25 AU)of the first five orbits,we first identify the batch of 176 type Ⅲ radio bursts and analyze their characteristic frequencies statistically.The result shows that their cutoff frequencies flo are mainly distributed between 0.2～1.6 MHz,and the peak frequency appears at about 680 kHz,which is not only generally higher than the local plasma frequency fp(50～250 kHz)measured by PSP,but also significantly higher than the cutoff frequency of interplanetary type Ⅲ bursts(about 100 kHz)observed by Ulysses and WIND at a far heliocentric distance(≥1 AU).It implies that the radial propagation distance of the radiation source of bursts observed by PSP is generally smaller than that observed by Ulysses and WIND.Our preliminary analysis suggests that the reasons for this difference may come from the following three aspects:variations in the intensity of solar activity,different criteria for event selection,and attenuation effects of radiation.In order to further determine the main reason,we use the Canny edge detection method to identify 1194 and 491 type Ⅲ events measured by PSP and WIND in the same period(January 1,2019 to July 31,2020),which can effectively exclude the influence from the difference of solar activity intensity and event selection criteria.However,the statistical results of cutoff frequencies for these events are similar to the previous results,i.e.,the cutoff frequency flo for the events observed by PSP(peak frequency at about 700 kHz)is still significantly higher than that by WIND(peak frequency at about 100 kHz).In particular,when PSP located at the same perspective as WIND,total 212 type Ⅲ bursts are found by approach observations of PSP,while only 18 bursts with stronger intensity are observed by WIND at greater heliocentric distances,which are less than 10 percent of events by the approach observations of PSP.This indicates that there are a large number of weak type Ⅲ bursts in the high corona that can only be detected by approach observations,and also means that radiation attenuation is the main reason for the significant difference of the cutoff frequency of radio bursts observed by spacecraft located at different distances.This is because the propagation of weaker radio bursts sources terminates at a closer heliocentric distance,while the stronger radio bursts sources can travel farther.2)The generation mechanism of a type Ⅲ radio burst with fine structure in PSP observations:The dynamic spectrum of type Ⅲ radio burst with fine structure(i.e.,type Ⅲb radio burst)is mainly composed of a series of quasi-periodic striae that shift from high frequency to low frequency in a manner similar to type Ⅲ radio burst.The appearance of these fine structures provides more abundant physical information for the plasma environment and the structure state of the radiation source,and also provides new clues for a better understanding of radiation mechanism.The approach observation of PSP provides a new opportunity for the observation and study of interplanetary type Ⅲb radio bursts originating from the Sun.A typical interplanetary type Ⅲb burst observed by PSP near the perihelion of the second orbit is analyzed in detail.The duration of the event is about 4 minutes,the radiation frequency range is 19～0.8 MHz,and total 22 clearly identifiable striae appear in the dynamic spectrum.It shows that the maximal power spectral density and duration increase with the decrease of frequency.Combining the empirical model of the solar atmosphere with the plasma measurements by PSP,the analysis shows that the source region of this type Ⅲb radio burst is located in the high coronal region between the heliocentric distance of 1.34～4.18 solar radii,and the radiation mechanism of the event is electron cyclotron maser radiation.The fine structure of the striae can be attributed to the modulation of the growth rate of electron cyclotron maser instability by the ubiquitous Alfvén wave in the coronal atmosphere.In particular,the calculated results using double Alfven wave modulation are in good agreement with the distribution of striae structure and its evolution feature with time,which provides a self-consistent and reasonable physical explanation for the formation of fine structure.3)A new type of solar radio burst called solitary wave radiation discovered by PSP and the physical model of its radiation source:In the analysis of the observations of PSP passing through a low-density magnetic channel in the encounter mode of the second orbit,we find that there are as many as 385 weak radio bursts within about 77 hours.Most of them have the common characteristics of weak intensity,narrow band and short duration.In particular,their dynamic spectra show strong dynamic evolution features,that is,the relative frequency drift rates D decrease rapidly from more than 0.01/s to less than 0.01/s in the frequency range from more than 10 MHz to hundreds of kHz,and the average relative drift rates are about 0.01/s.Such dynamic spectrum features are different from both type Ⅲ solar radio bursts with relative drift rates>0.1/s excited directly by intense electron beams and type Ⅱ solar radio bursts with relative drift rates<0.01/s driven by coronal shock waves.This means that these weak radio bursts come from small-scale radiation sources,and the speeds of the radiation sources are obviously lower than that of the fast electron beams and decelerate rapidly with the propagation process.Using the widely used empirical model of coronal plasma,combined with the plasma measurements by PSP and the radio observations by WIND and LOFAR,we show that the source regions of these weak radio bursts are located in the high coronal region within 1.1～6.1 solar radii from the heliocenter.In particular,this is a typical solar wind acceleration region with low β plasma,in which not only strong Alfven wave turbulence but also a large number of solitary kinetic Alfven waves with the kinetic scale of particles and super-Alfven velocity may exist.We suggest that the weak radio bursts discovered by PSP are a new type of solar radio bursts,which are excited by the electron cyclotron maser emission mechanism of solitary electrons accelerated and captured by solitary kinetic Alfven waves,and we call it solitary wave radiation(SWR).Based on the acceleration model of solitary electrons,the mechanism of electron cyclotron maser emission under the influence of Alfven waves and the theoretical model of propagation and dissipation of solitary kinetic Alfven waves,our results can well explain the main characteristics of the spectrum evolution of these weak radio bursts.As it is the initial stage of PSP observation,the researches on solar radio bursts in this thesis are the first batch of systematic works based on PSP radio observation data,which has attracted extensive interest and special attention by international researchers in the field of solar radio.One of our works was listed as the highlight of solar radio science by the website of the Community of European Solar Radio Astronomers(CESRA)and was reported emphatically.In particular,in a rencent review article by Raouafi et al.in 2023,which mainly summarized the main findings of PSP in the first four years of operation,our series of research works based on PSP observation data are introduced as the main research results obtained by PSP in the field of solar radio.Therefore,our works based on the initial observations of PSP in this thesis creates a promising start for the approach observation of PSP.It is believed that the PSP will provide us more detailed observations with closer to the sun,resulting in more new discoveries.In chapter 5,besides the summary of our main research works,the further detailed observation based on PSP and the joint observation with other satellites in multi-azimuth and multiband are also discussed and prospected.