Interaction between Ion Beams and Electromagnetic Ion-Cyclotron Waves in the Solar Wind

XIANG Liang

(Purple Mountain Observatory,Chinese Academy of Sciences,Nanjing 210023)

Electromagnetic ion cyclotron wave (EMICW)is a kind of electromagnetic wave whose frequency is lower than or close to the ion cyclotron frequency,and it has two polarization states of left-handed and right-handed.Due to the ion cyclotron resonance,EMICWs can exchange energy with particles directly,which is believed to play an important role in the energization of plasma particles in the solar wind.However,the excitation mechanism of EMICWs and its wave-particle interaction in the solar wind have not fully been understood.In this dissertation,we systematically investigate the influence of ion beams on the generation of EMICWs and its wave-particle interaction in the solar wind plasma environment.The results presented in this dissertation provide a good theoretical basis for further understanding the microscopic plasma process in the solar wind and the physical nature of disturbances as well as the energization phenomena of plasma particles.

Firstly,we introduce the two-fluid and kinetic models of EMICWs,wave properties,observation characteristics of EMICWs in solar wind and ion beams,generation mechanisms of EMICWs and its wave-particle interaction.

Secondly,in the case of ion cyclotron waves (ICWs)excited by proton and electron beams,the relative importance of reactive and kinetic instabilities is compared.The effects of these instabilities on the formation and excitation of ICWs in the solar wind are briefly discussed.In the case of ICWs driven by proton beams,the results show that the kinetic instability has a lower velocity thresholdvbi～vA(wherevbiandvArepresent the protonbeam drift velocity and the local Alfv′en velocity,respectively),but the reactive instability becomes dominant as soon as its threshold is exceeded,i.e.,vbi>2vA.Moreover,the growth rate of the kinetic instability is the largest when the beam velocity is 170vAis satisfied,implying that it is effective to excite ICWs by electron beams in the solar wind.

Thirdly,in view of the ubiquity and importance ofαparticles in the solar wind,we investigate the effect ofαparticles on the excitation mechanism of EMICWs.The results show that the real frequency,growth rate,and instability threshold of EMICWs are highly sensitive to the density and drift velocity ofαparticles.In particular,as the drift velocity ofαparticles increases,the growth rates of both ICWs and magnetosonic waves (MSWs)first decrease then increase,the effect on ICW is more significant.In addition,compared with the observations,the observed drift velocity of proton beams in the solar wind is usually less than or close to the velocity threshold of EMICWs predicted by the theory.This indicates that both ion cyclotron and magnetosonic instabilities in the solar wind can effectively regulate the drift velocity of proton beams.

Finally,in order to examine the deceleration mechanism and evolution process of proton beams in the solar wind,we further investigate the effects of oblique ion cyclotron(OIC)and parallel magnetosonic (PMS)instabilities on the evolution of the proton beam in the solar wind.We also consider the effect of nonlinear wave particle interaction on beam deceleration,so as to establish the physical model of the evolution of proton beams in the solar wind.The results show that there are different instability excitation intervals between the OIC and the PMS.The OIC wave is more likely to grow atβe0.5,while the PMS wave tends to be excited atβe,whereβeis the ratio of electron thermal to magnetic pressure andis the critical value.As the solar wind streams away from the Sun,the OIC can be efficiently excited by proton beams,which constrains the beam velocityvbi/vAbelow the instability threshold.However,when the solar wind exceeds the critical radiusRc(i.e.Rc>0.55 au),the PMS waves can be efficiently excited due to the lower threshold,and the proton beams continue to be decelerated by this instability.Moreover,the nonlinear wave-particle interaction can lead to a further deceleration of proton beams saturating atvbi/vA～1.2.Therefore,these nonlinear wave-particle interaction can explain the observed results that the drift velocity of some proton beams in the solar wind is slightly greater than or close to the local Alfv′en velocity.The present results may play an important role in understanding the deceleration mechanism and evolution of proton beams in the solar wind.