The observation of ultra-high frequency radar during an ionospheric heating experiment carried out at TromsФ site of European Incoherent Scatter Scientific Association, Norway, is analyzed. When pump is operating slightly above the fifth electron gyrofrequency, some strong enhancements in radar echo and electron density occur in a wide altitude range and are in sync with the shifting and spread of plasma line around the reflection altitude, which may be due to the focusing or collimating of radar wave by irregularities. While some strong enhancements in electron density and radar echo around the reflection altitude do not correspond to the true increase in electron density, but due to the enhanced ion acoustic wave by parametric decay instability and oscillation two stream instability. In addition, the different heating rates and cooling rates at the pump frequencies below, around and above fifth gyrofrequency respectively result in the dependence of the enhancements in electron temperature on the pump frequency.
Observations are presented of the phenomenon of the enhancement in electron density and temperature that is caused by a powerful pump wave at a frequency near the fifth gyrofrequency. The observations show that the apparent enhancement in electron density extending over a wide altitude range and the enhancement in electron temperature around the reflection altitude occur as a function of pump frequency. Additionally, the plasma line spectra show unusual behavior as a function of pump frequency. In conclusion, the upper hybrid wave resonance excited by the pump wave plays a dominating role and leads to the enhancement in electron temperature at the upper hybrid altitude. The phenomenon of apparent enhancement in electron density does not correspond to the true enhancement in electron density, this may be due to some mechanism that preferentially involves the plasma transport process and leads to the strong backscatter of radar wave along the magnetic line, which remains to be determined.
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.
