Numerical Study On Parametric Instabilities Of Alfven Waves With A Spectrum And Associated Ion Heating

As a planet,the earth,human's only home,has been orbiting around the sun for about 4.6 billion years.The sun in the center of the solar system is a giant hot plasma sphere and in the core of the sun the violent nuclear fusion reactions occur releasing a large quantity of energy into the interplanetary space.'The corona is the outmost layer of the sun's atmosphere and its temperature can be heated to the order of one million degree.Due to such high temperature of the corona,the huge difference of pressures between the corona and the interplanetary space can drive the corona to blow out plenty of ionized solar plasma with magnetic field,which is called solar wind.In the space observations,it is found that a large scale low frequency magnetic fluctuation ubiquitously exists in the solar wind.Furthermore,such above low frequency magnetic fluctuation is called Alfven wave and is considered to be a probable candidate for explaining the corona heating and solar wind acceleration.This doctoral dissertation is mainly about the parametric instabilities of Alfven waves with an incoherent spectrum and associated ion heating during the evolution of the parametric instabilities.(1)Although Alfven wave is an exact solution of the ideal incompressible magnetohydrodynamic(MHD)equations,three types of parametric instabilities can occur for Alfven wave,which are called decay instability,beat instability and modulation instability,respectively.The parametric instabilities are involved with nonlinear wave-wave interactions,which can transfer energy between different wave modes providing a way of dissipation for Alfven wave.In the first work of this doctoral dissertation,by using a one-dimensional(1-D)hybrid simulation model in a low beta electron-proton plasma system,we have investigated the parametric instabilities of Alfven waves with an incoherent spectrum and found that the evolution of the parametric instabilities has two stages.In the first stage,the envelope modulation of the pump Alfven waves with a spectrum occurs generating a series of low frequency density modes,and then the new generated density modes couple with the pump Alfven waves further generating new high frequency magnetic modes.Therefore,much more density modes and magnetic modes with a broadband spectrum are excited.In the second stage,each monochromatic Alfven wave from the initial pump Alfven waves decays separately.Proton beam along the background magnetic field B0 can be generated in the saturation stage of the parametric instabilities of the pump Alfven waves.(2)Preferential heating of heavy ions in solar corona and solar wind has been a long-standing hot topic.In the second work of this doctoral dissertation,also by using a 1-D hybrid simulation model in a low beta electron-proton-alpha plasma system,we have investigated the heating of alpha particles during the parametric instabilities of parallel propagating Alfven waves with an incoherent spectrum.Similar to the results of the first work above,the evolution of the parametric instabilities including alpha particles here also experiences two stages corresponding to the two stages of alpha particle heating.In both stages of the evolution,alpha particles can be effectively heated in the direction perpendicular to B0,which is caused by cyclotron resonance between the alpha particles and the high frequency magnetic modes.Meanwhile,Landau resonance between the alpha particles and the density modes leads to the parallel heating of alpha particles.Protons can be heated only in parallel direction,but can not be heated in perpendicular direction,during the entire evolution of the parametric instabilities.(3)For further studying alpha particle heating during the parametric instabilities of the initial oblique propagating Alfven waves with an incoherent spectrum,we have generalized the second work above to a 2-D hybrid simulation model in the third work of this doctoral dissertation and investigated the relationships between the ion heating and the power spectra evolution of the density modes and the magnetic modes excited from the parametric instabilities.It is found that the wave-wave couplings and wave-particle interactions play key roles in ion heating.The initial pump Alfven waves with a spectrum at a small oblique propagation angle(e.g.?k0,B0 ?15°,is an oblique angle between the initial pump Alfven wave vector ko and the background magnetic field Bo)can more effectively heat alpha particles in perpendicular direction than the cases of a monochromatic pump Alfven wave and pump Alfven waves with a spectrum at a larger oblique propagation angle.Meanwhile,in parallel direction both protons and alpha particles can be also more effectively heated by pump Alfven waves with a spectrum at a small oblique propagation angle.Moreover,the ion(proton or alpha particle)beam is formed parallel to Bo regardless of the initial oblique propagation angle of the pump Alfven waves,which is mainly caused by Landau resonance between the ions and the excited ion acoustic waves(IAW)in latter phase of the evolution of the parametric instabilities.However,the perpendicular heating of alpha particles is mainly due to cyclotron resonance between the alpha particles and the high frequency Alfven waves(AW)generated via nonlinear wave-wave interactions in both early and latter phase of the evolution of the parametric instabilities.The study of the third work has not yet been totally completed and the current results still need to be improved further.