Study On Ion Heating In The Fast Solar Wind

The mechanism that how to heat fast solar wind has been an important unresolved question for several decades.Observations showed that the fast solar wind will expand when travelling in the interplanetary space,during this period,the temperature of fast solar wind also decreases.However,observation showed the temperature drops much more slowly than the adiabatic expansion,which indicates there is local heating during fast solar wind travelling.Based on the temperature of ions of fast solar wind observed by SOHO(Solar and heliospheric observatory),we found that there exists obvious temperature anisotropy in solar wind,i.e.T(?)>T?·In addition,the observations also showed that the temperature of the heavy ions was much higher than that of the protons,which means that the heavy ions were heated preferentially.These observations provide an important basis for evaluating different solar wind heating mechanisms.In the first chapter,we mainly introduce several main local heating mechanisms of fast solar wind,including:turbulent heating,vertical shock wave heating,large amplitude nonlinear resonance Alfven wave heating,large amplitude alfven wave parametric decay,large amplitude Alfven wave-wave coupling,electromagnetic ion/ion instability.This paper focuses on these mechanisms:ion/ion instability,wave-wave coupling of large amplitude Alfven waves and turbulence heating.In chapter 2,we study the wave-wave coupling heating of Alfven wave with large amplitude.Observation showed that the energy of Alfven wave in the fast solar wind is concentrated in the low frequency band,it is difficult to directly transfer energy to particles via wave-particle interaction.However,if there is a certain angle between the propagation direction of Alfven wave and the background magnetic field,the wave has not only an electromagnetic component,but also an electrostatic component.The results of one-dimensional hybrid simulation show that the electrostatic component of the oblique propagating Alfven wave can be coupled with the electromagnetic component to produce higher frequency harmonics,which can effectively heat the ions in the perpendicular direction to the background magnetic field.Considering the Alfven wave in fast solar wind is not a monochromatic wave but waves with a certain bandwidth,we also study the coupling of the Alfven spectrum composed of multiple oblique propagating waves.In addition to the coupling within the electromagnetic components and electrostatic component of same mode,the electromagnetic components and electrostatic component of different modes can also couple to make a much complicated spectrum.It is worth noting that the wave coupling not only produces higher frequency harmonic waves,but also the wave mode with lower frequency.In chapter 3,electromagnetic ion/ion instability in the fast solar wind are studied by two-dimensional hybrid simulation.Firstly,we studied the effect of beam ratio on electromagnetic ion/ion instability.As the beam ratio increases,the amplitude of the excited Alfven wave increases and becomes closer to linear polarization,what's more,the frequency of the excited Alfven wave increases.As the frequency increases,more ambient protons can satisfy the cyclotron resonance condition and can be heated in the direction of perpendicular to the background magnetic field.We obtain similar results from the linear Vlasov equation,that is,the growth rate and the peak frequency of the instability increase with the increase of the beam ratio.However,the frequency of Alfven wave obtained by linear theory is always lower than proton cyclotron frequency,and the peak frequency of excited Alfven wave will exceed that of proton cyclotron when the beam ratio is large enough.By studying the evolution of waves' frequencies over time,it is found that the wave frequency gradually increases with time,and the high-frequency wave appears in the later period of evolution.We propose that the wave mode with frequency higher than the proton cyclotron frequency is generated by the nonlinear evolution between wave modes,but the specific mechanism still needs further study.In addition,the effects of beam velocity and plasma beta on the electromagnetic ion/ion instability were investigated.As the beam velocity increases,the greater the amplitude of the excited Alfven wave,the earlier the instability starts,and the more obvious the perpendicular heating of the background protons.When the plasma beta value was increased,the amplitude of the excited Alfven wave decreased,the time of instability was delayed,the frequency of the excited Alfven wave decreased,and the vertical heating effect of the background protons was weakened.In addition,we also studied the existence of helium beam and proton beam at the same time.It is found that the excitation frequencies of single helium ion beam and single proton beam are both between helium ion cyclotron frequency and proton cyclotron frequency,but the frequencies of waves excited by helium beam are lower.When two kinds of beam exist simultaneously,the frequency range of excitation wave is wider and the peak frequency is higher.The heating mainly occurs in the direction of perpendicular to the ambient magnetic field,and the perpendicular heating of helium is more obvious.When there are two beam components,the perpendicular heating of beam protons is almost unchanged compared with the heating of single beam component,but the vertical heating of beam helium ion is more obvious.Chapter 4 is a summary of our recent work on turbulent heating.The Orazag-Tang vortex is used as the initial condition for turbulence excitation.It is found that the energy cascade of plasma turbulence is accelerated near the scale of ion inertia length.The development process of magnetic turbulence is a process of continuous accumulation of current in the system.The turbulence evolution process is related to the initial conditions.When the energy injected into the system is low,the current sheet generated at the central position of the simulation region will continuously elongate and thin,at last,magnetic reconnection will occur.Two reconnection points are generated during this process,and secondary magnetic islands are generated between the two rejoin points at last.Reconnection of small scale current sheet may play an important role in energy cascade of magnetic field turbulence,which accelerates the cascade of energy near the ion inertial scale.