Theoretical And Observational Study Of Ion Beam Instability In The Inner Heliosphere

The Sun is continuously blowing outward hot and high-speed plasma,which further affect the whole heliosphere.Since the 1970s,various space satellite missions have performed in-situ detection for the plasma environment in the heliosphere,and they have usually found that the ion velocity distributions generally deviate from the thermal equilibrium distribution.As a consequence,the researchers have been trying to distinguish the formation mechanism of the non-thermal equilibrium ion velocity distribution in the heliosphere.For this fundamental problem,one mechanism that has received much attention is the ion kinetic instability mechanism,which can effectively constrain the configuration of the proton velocity distribution.Over the last decade or so,owning to that a plenty of quasi-monochromatic ion-scale waves were found by the satellite observations,the importance of the ion kinetic instability is greatly emphasized as this instability is the only excitation mechanism of these ion-scale waves.Recently,based on the Parker Solar Probe(PSP),the researchers have exhibited two distinctive properties for the ion kinetic instability and its produced waves in the near-Sun solar wind:one is the dominant free energy responsible for the ion kinetic instability being the relative drift speed between two components(i.e.,core and beam)in the proton velocity distribution;and the another is the occurrence rate of quasi-monochromatic ion-scale waves is unexpectedly high.These information indicate that the instability driven by proton beam occurs frequently in the near-Sun solar wind.Because the study of the ion beam instability and related ion-scale waves still stays an initial stage,there are many unknown problems that are being explored.This thesis focuses on the ion beam instability in the inner heliosphere,and three research works have been done by using the plasma theory and PSP observations.First,basing on the plasma kinetic theory,this thesis studies the radial distribution of ion beam instability and related wave-particle interaction process in the inner heliosphere.Second,for a typical wave event in the near-Sun solar wind,this thesis studies the excitation mechanism of the observed waves and identifies the actual role of the ion beam component in triggering the instability through combining the plasma wave and instability theory.Third,based on electromagnetic field and plasma instrument data,this thesis performs the statistical analysis of quasi-monochromatic ion-scale wave events in the inner heliosphere,indirectly conjecturing excitation of the ion beam instability therein.The main results are summarized as follows.(1)It provides theoretical predictions of the parameter control regimes of four main types of ion beam instabilities in the radial distance and velocity space in the inner heliosphere for the first time,and it also clarifies dominant wave-particle interactions responsible for the excitation of each ion beam instability.Using the usually used models of the magnetic field,number density and temperature in the inner heliosphere and the general assumption conditions,a comprehensive theoretical analysis is performed for the proton beam instability in a four-component(i.e.,proton core,proton beam,alpha particle,and electron)plasma.This work finds the existence of four main types of instabilities.According to the mode nature of the excited wave and wave propagating direction,these four instabilities are named as the oblique fast-magnetosonic/whistler instability,oblique Alfvén/ion-cyclotron instability,oblique Alfven/ion beam instability,and parallel fast-magnetosonic/whistler instability.This work clarifies that the Alfvén-Ⅰ instability,which was found more than twenty years but its nature was unknown,is actually the oblique Alfven/ion beam instability.The radial distributions of these four instabilities are obtained for the first time,in which the control regimes of these instabilities in the parameter space of the radial distance and beam drift velocity are explored and the relative small excitation threshold for the three oblique instabilities in the typical solar coronal environment are found.Through the energy transfer rate method,this work identifies the dominant waveparticle interactions contributing to the excitation of each instability for the first time:the oblique fast-magnetosonic/whistle instability is mainly triggered by the Landau and transit-time resonance mechanisms relating to the proton beam component;the oblique Alfvén/ion-cyclotron instability is mainly triggered by the Landau,transit-time,and anomalous cyclotron resonance mechanisms associating with the proton beam component;the oblique Alfvén/ion beam instability is mainly triggered by the anomalous cyclotron resonance mechanism of the proton beam component;and the parallel fastmagnetosonic/whistle instability is only excited by the anomalous cyclotron resonance mechanism of the proton beam component.In addition,this work performs the investigation of the dependence of the instability on plasma parameters in a typical near-Sun solar wind environment(at 10 solar radius),and it finds that the occupation of the proton beam component significantly affects the growth rate and control regime of each proton beam instability,while the temperature anisotropy of the proton core,proton beam,or electron population weakly affects the instability growth rate.(2)Using PSP,it provides direct observational evidences of the occurrence of the ion beam instability in the near-Sun solar wind,which lead to the explicit explanation of the nature of the observed left-and right-handed polarized ion-scale waves.Based on PSP observations,this work picks up an event that contains a significant proton beam component,where beaming protons mainly come from leaking protons from the reconnection jet region in the heliospherical current sheet.Through the analysis of quasi-monochromatic waves in this event,this work finds the existence of two types of waves,that is,the left-and right-handed polarized waves.The former appears almost simultaneously with the leaking protons,while the latter are independent of the leaking protons.In order to determine the possibility of local excitation of the ion beam instability,a fitting method for the proton velocity distribution function observed by PSP is developed,and it gives the information of the plasma parameters(e.g.,number density,drift velocity,parallel and perpendicular temperatures)of the proton core and beam component under the assumption of their distribution functions followed the drift bi-Maxwellian distribution function.Then,this work thoughtfully analyzes the instability induced by the proton beam component,and it finds that the left-and righthanded polarized waves are excited by the oblique Alfvén/ion-cyclotron instability and the parallel fast-magnetoacoustic/whistler instability,respectively.Consequently,the left-and right-handed polarized waves correspond to the Alfvén/ion-cyclotron and fastmagnetosonic/whistler wave,respectively.Through the analysis for the dependence between the instability and plasma parameters,this works exhibits the preferential excitation parameters for these two types of instabilities in actual observations:for the oblique Alfvén/ion-cyclotron instability,the excitation occurs when the leaking protons are the majority of the proton beam population,corresponding to the proton beam component having a relatively large number density ratio and low relative drift velocity;for the parallel fast-magnetosonic/whistler instability,the excitation arises as the proton beam component has a relatively small number density ratio and a large relative drift velocity.(3)The radial distribution of quasi-monochromatic ion-scale waves is obtained by using PSP observations,and the wave occurrence rate in the near-Sun solar wind is found to be much larger than that in the solar wind far from the Sun.Based on the magnetic field and plasma observations during Encounters 1～11,the procedure for identifying and collecting quasi-monochromatic ion-scale wave events is developed through wavelet and SVD methods.The data of the wave events of interested are collected under the following criteria:the wave lasting time≥20 seconds,the wave normal angle≤30 degrees,the absolute value of ellipticity≥0.60,and the degree of polarization(DOP)≥0.65.The most important finding is that the occurrence rate of quasi-monochromatic ion-scale waves exhibits an increasing trend as the radial distance decreases.In the 0.3～0.7 AU solar wind,the wave occurrence rate is about 6%～14%;in the near-Sun solar wind below 0.3 AU,the wave occurrence rate can be 21%～29%.For this change of the wave occurrence rate,this work analyzes the satellite sampling effect,and it finds that this effect is insufficient to explain such change.Therefore,this work proposes a physical explanation,that is,the ion beam instability is more easily triggered in the near-Sun solar wind,naturally resulting in high wave occurrence rate therein.In addition,this work explores the information of the wave frequency and wave scale by using the observational parameters and plasma wave theory and by referring to the known knowledge of the ion beam instability and ion-scale waves given by previous works.The findings in this thesis exhibit the physical mechanism and excitation process of ion-scale waves excited by the ion beam instability in the inner heliosphere,and also through the observations of ion-scale waves,conjecture indirectly this instability being frequently triggered under the plasma conditions in the near-Sun solar wind.These findings can enhance our understanding on the ion kinetic instability and related ionscale waves in the inner heliosphere.