The Local Structures Of Dusty Plasmas In Polar Mesosphere

Polar Mesosphere Summer Echoes (PMSE), a kind of strong radar backscatter from the polar mesopause region(80~90km) during the summer time, has attracted a great deal of interest in the last thirty years. It is accepted today that PMSE are radar echoes determined by the local structures of the electron number density, yet the mechanism remains an open question.The abundant water vapor from summer atmospheric circulation is easily condensed to ice particles in the PMSE layer because of extremely low temperature there. These ice particles are believed to affect the local structures of the electron number density greatly. On one hand, ice particles are usually negatively charged by the attachment of electron in mesospheric environment, and thus reduce the number density of free electron. On the other hand, the relatively heavy mass of ice particles might also influence the formation and life time of the electron number density irregularity by reducing the diffusion of electron。In this dissertation, we studied and discussed the possible small scale structures of the plasma densities in typical mesospheric dusty plasmas. We firstly established a one-dimensional fluid model consisting electrons, ions, and heavy ice particles. The ice particles act as a negatively charged background moving with the terminal velocity as a result of the balance between the gravity and neutral air friction. In this moving frame, we studied the diffusion process of ion and electron as well as the local structures of plasma system and electric field therein. We then numerically discussed the local plasma structures with a uniform number density and with a density fluctuation of the ice particles, respectively. The influence of the different parameters of the ice and plasma on the structures is also discussed.When the number density of ice particles is uniform, small structures of electron (ion) number density are obtained. The scalelengths of the structures are of the order of several meters, which are comparable with the VHF radar wavelengths. Therefore, the local plasma structures could be the possible physical mechanism in explaining radar echoes in PMSE layers. In the small structures, the electron density quickly drops to zero but the ion density drops to the value equal to that of the ice particles. It also shows weaker local structures under the condition of higher number density of ice particles, weaker boundary electric field or larger ice particles, respectively. However, the ice particle density around 1000/cm3 and weak boundary electric filed less than 10mV/m are good candidates for reasonable structures. When the number density of ice particles is disturbed, the result indicates a similar overall profile of plasma structures to those in uniform number density of ice particles. However, the disturbed ice particles also result in oscillating and multi-layered plasma structures. When the perturbation amplitude of ice particle density is larger, the oscillation of electron (ion) density is enhanced and multi-layered structures are more easily obtained. When the number density of ice particles is disturbed in a smaller space period, the electron (ion) density oscillates in a corresponding smaller scale. The results in this dissertation are in a good agreement with some observational facts and thus can help in explaining PMSE.