The Theoretical Studies Related To Dust Void And Polar Mesosphere Summer Echoes

Dust voids exist in a large numbers of dusty plasmas under the conditions of gravity and micro-gravity, which are usually resulted from the dust clouds nonlinear evolution in the rf discharge conditions. There has been much experimental and theoretical interest in the linear and nonlinear stages of dust voids formation. Experimentally, ionization plays import roles in the plasma maintenance and voids formation. However, it is still not clear about the exact role of ionization effect on voids formation.The polar mesosphere summer echoes (PMSE) are caused by scattering of radar waves from the structures of small scale electron density. It is a long outstanding question in scientific community in how such small scale structure produce, long time be sustained and what the physical mechanism causes the echoes.The main research contents of this dissertation are composed of two parts. In the first part, the theoretical model of dust void formation, considering the effect of ionization, is proposed for the fist time and the dust evolution is studied numerically.Then, the sheath structure of rf discharge in experiments is investigated by numerical simulation and the void formation in the sheath is researched by molecular dynamics (MD) simulation method. In the second part, the small scale density perturbations of plasma and charged aerosol particles and the reflection of oblique incidence of electromagnetic waves by the special distribution of dust cloud are researched.In the first chapter, the experimental, theoretical and computer simulation research of dust voids are reviewed in detail. The experimental study of dust voids under gravity and micro-gravity conditions and the voids formation theories of steady and time evolution are emphasized. In the second chapter, using the fluid model, the dust void formation under the consideration of ionization effect is investigated. Under the appropriate parameters and initial conditions, the voids formation is studied by numerical method. It is shown that dust voids can form only when the ionization rate is larger than the threshold value. With increasing of ionization rate, the time needed for void formation is reduced and the void size increases first and then decreases. When the dust convective term is considered, we can obtain the ring-like dust void.In the third chapter, the plasma sheath of the rf discharge is simulated by the steady fluid model, then, in the sheath, the dust void formation is investigated by MD simulation. The results show that there is a potential bump in the center of sheath and high symmetry circular and irregular voids, which are observed in experiments, can form.In the fourth chapter, the experimental observation and theoretical study of PMSE are summarized. Many characteristics of PMSE in observations and the background investigations of PMSE occurrences are emphasized. Also, the three representative mechanisms of PMSE production are introduced in detail.In the fifth chapter, using the multi-polar diffusion theory, the time evolutions of the small scale plasma and charged aerosol particles perturbations are investigated, considering the initial condition of electron and aerosol particles density perturbations. It is found that, under the typical parameters of PMSE observations, the typical existence time scale of the small scale plasma structures is on an hour order, in the majority cases, the electron and ion densities are anticorrelated, but when the radius of dust particles are larger than the critical value, the electron and ion are correlated. Further, under the varied parameters, such as the amplitude or width of perturbations, the radius of aerosol particles and the altitude of PMSE occurrence, the evolution are also investigated.In the sixth chapter, the oblique electromagnetic waves reflected by the special dust-distributing cloud are investigated, with the consideration of dusts charging effect and the small scale plasma structure produced by dust charging in waves'propagation. The results show that, when the P waves are assumed to be the incident waves, the reflection coefficient is inverse proportional to the incidence frequency approximately. With the increase of incidence angle, the reflection coefficient increases slowly first and then decreases quickly. The variation of the reflection coefficient with the radius of dust particles is effected by linear mode conversion and dusts charging effect.In the last chapter, the conclusions and perspectives of the dissertation are presented.