The measurements of the potential distributions in the boundary layer near meshes with different mesh spacing were conducted in weakly collisional plasmas using a fine-structured emissive probe and the results of the sheath thickness and electric field at the sheath-presheath edge were compared with theoretical models of collisional presheath and collisionless sheath.It was shown that,because the meshes are partially transparent to ions,the sheath is thinner and the electric field is stronger for the mesh of higher transmissivity,owing to the increased ion density in the sheath contributed from the ions transmitted from the other side of the mesh.However,the potential profiles in the presheath remain almost the same for different meshes except for the shift of the sheath-presheath edge.The thickness of the sheath decreases while the electric field at the edge increases with the increase of the neutral gas pressure.Furthermore, depending on the pressure,the measured electric fields at the edge are close to that from the models of a transition region.
Time evolution of ionospheric D-region plasmas including the perturbations of electronsand charged aerosol particles is investigated under the conditions of polar mesosphere summerechoes (PMSE).It is shown that the time scale of decay of the electron density is in the orderof an hour under typical PMSE conditions,in the majority of cases,the electron density is anticorrelatedto the ion density,except that the radius of aerosol particles is greater than 50 nm.Also,the evolutions under varied parameters,such as the amplitude and width of perturbation,theaerosol particle radius,and the altitude of the PMSE occurrence are investigated.The obtainedresults are useful for interpreting the experimental observations.
