Laboratory Investigation Of The Boundary Layer Processes Of Artificially-created Ionospheric Depletion

The Earth’s ionosphere, a ionized region of upper atomsphere, plays an important role in atmospheric electricity and formation of the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propaga-tion to distant places on the Earth, especially in the space era. Therefore, it is crucial to progress the study of the artificial modification of the ionosphere. Ionospheric de-pletion, produced by artificial release of attachment chemicals, was widely investigated and taken as a potential technique for the artificial modification of space weather in the past decades. Active release experiment and numerical simulation are the primary men-thods to study the ionospheric depletion until now. However, limited to the resolution of the dignostic tools, some micro-processes, especially the onset and evolution of the electron-ion hybrid(EIH) instability in the the boundary layer, have been few observed and analysed until recently. The EIH mode is believed to play important roles in the evolution of the ionospheric depletion, such as the anomalous trasport and the origin of irregularities.In our work, a new approach for investigating ionosphere chemical depletion in the laboratory is introduced. Air glow discharge plasma closely resembling the ionosphere in both composition and chemical reactions is used as the artificially created ionosphere. The ionospheric depletion experiment is accomplished by releasing chemicals such as SF6, CCl2F2, and CO2into the model discharge. The evolution of the electron density is investigated by varying the plasma pressure and input power. It is found that the neg-ative ion (SF6-,CCl2F2-) intermediary species provide larger reduction of the electron density than the positive ionC(CO2+) intermediary species. The negative ion intermedi-ary species are also more efficient in producing ionospheric holes because of their fast reaction rates. Airglow enhancement attributed to SF6and CO2releases agrees well with the published data. Compared to the traditional methods, the new scheme is sim-pler to use, both in the release of chemicals and in the electron density measurements. It is therefore more efficient for investigating the release of chemicals in the ionosphere.Based upon the previous study, we have experimentally investigated the boundary layer processes of artificially-created ionospheric depletions. Those ionospheric deple-tions were modeled via releasing attachment chemicals, such as SF6, CF4, and CO2, into the ambient plasmas. Boundary layer of width of electric scale length emerged and separated those plasmas into two regions, the ambient plasmas and the negative ions plasmas. In the localized boundary layer, those fluctuations of the electron den-sity and the floating potential were investigated varying with the plasma pressure and the partial pressure of released chemicals. The electron density decreased sharply that yielded steep density gradients▽ne, and the floating potential increased which gen-erated sheared electron flows. It is found that the magnitude of fluctuating floating potential is proportional to that of the▽ne. Those fluctuations were analyzed in detail using digital spectra analysis techniques. Vortex-like coherent structures were observed in the fluctuations of electrostatic potentials. These coherent frequencies are sensitive to the mass of the negative ions, and all lie in the lower hybrid(LH) range. By compar-ing the experimental results with theoretical predictions, the modes have been identified as the coherent structures resulting from the electron-ion hybrid instability. Our results are important to study the early phase nonlinear evolution of the ionospheric depletion, and also may be applied to the plasma sheet boundary layer in where often encounters the narrow electron density gradients and sheared electron flows.In addition, we also studied the spontaneously generated electromagnetic fluctua-tion in the boundary layer of laboratory-created ionospheric depletion. These depletions were modelled via releasing attachment chemicals into the ambient plasmas. Electron density gradients and sheared flows appeared in the boundary layer of nagative ions and positive ion plasmas. The electromagnetic fluctuation lies in the LH range, and it was indentied as the right-hand polarized whistler wave branch. Besides, we also con-firmed that thees whistler modes were tranformed from the electrostatic EIH modes in the boundary layer. The research can be applicated to the geospace enviroment such as the early phase nonlinear evolution of the ionospheric depletion, and also may be ap-plied to the plasma sheet boundary layer in where often encounters the narrow electron density gradients and sheared electron flows.In summary, we have studied the nonlinear evolution of the bounday layer of lab-oratory. Electrostatic and electromagnetic fluctuations were successfully observed in the region, and the EIH mode and whistler mode also were studied in detail. Besides, we found that the EIH mode can transform to the whistler mode via nonlinear scatter-ing processes. Therefore,the project provide a good supplement for the active release experiment and numerical simulation, and will certainly gain experiences for the future active releasing experiments of China.