EPP

Polar Mesosphere Summer Echoes (PMSEs) are very strong radar echoes observed at altitudes near the polar summer mesopause. One of the essential properties of these radar echoes is that they can give useful diagnostic information about the physics of the scattering process. In this paper, the related characteristics of PMSEs measured with the European Incoherent SCATter Very High Frequency (EISCAT VHF) 224 MHz radar on 13–15 July 2010 are studied at different elevation angles from 78° to 90°. It is found that the PMSEs peak power and strongest PMSEs average power occur at the same elevation angles. Also interesting is that the strongest PMSEs occur at off-vertical angles when a PMSEs has a layered (multilayer) structure. And reflection may have more significant effects on PMSEs when there are double or multilayer PMSEs. Possible explanations regarding these observations are discussed.

Polar mesosphere summer echoes (PMSEs) are very strong radar echoes in the polar mesopause in local summer. Here we present the frequency dependence of the volume reflectivity and the effect of energetic particle precipitation on modulated PMSEs by using PMSEs observations carried out by European Incoherent SCATter (EISCAT) heating equipment simultaneously with very high frequency (VHF) radar and ultra high frequency (UHF) radar on 12 July 2007. According to the experimental observations, the PMSEs occurrence rate at VHF was much higher than that at UHF, and the altitude of the PMSEs maximum observed at VHF was higher than that at UHF. Overlapping regions were observed by VHF radar between high energetic particle precipitation and the PMSEs. In addition, high-frequency heating had a very limited impact on PMSEs when the UHF electron density was enhanced because of energetic particle precipitation. In addition, an updated qualitative method was used to study the relationship between volume reflectivity and frequency. The volume reflectivity was found to be inversely proportional to the fourth power of radar frequency. The theoretical and experimental results provide a definitive data foundation for further analysis and investigation of the physical mechanism of PMSEs.

This paper briefly reviews ionospheric irregularities that occur in the E and F regions at mid-latitudes. Sporadic E (E_S) is a common ionospheric irregularity phenomenon that is first noticed in the E layer. E_S mainly appears during daytime in summer hemispheres, and is formed primarily from neutral wind shear in the mesosphere and lower thermosphere (MLT) region. Field-aligned irregularity (FAI) in the E region is also observed by Very High Frequency (VHF) radar in mid-latitude regions. FAI frequently occurs after sunset in summer hemispheres, and spectrum features of E region FAI echoes suggest that type-2 irregularity is dominant in the nighttime ionosphere. A close relationship between E_S and E region FAI implies that E_S may be a possible source of E region FAI in the nighttime ionosphere. Strong neutral wind shear, steep E_S plasma density gradient, and a polarized electric field are the significant factors affecting the formation of E region FAI. At mid-latitudes, joint observational experiments including ionosonde, VHF radar, Global Positioning System (GPS) stations, and all-sky optical images have revealed strong connections across different scales of ionospheric irregularities in the nighttime F region, such as spread F (SF), medium-scale traveling ionospheric disturbances (MSTID), and F region FAI. Observations suggest that different scales of ionospheric irregularities are generally attributed to the Perkins instability and subsequently excited gradient drift instability. Nighttime MSTID can further evolve into small-scale structures through a nonlinear cascade process when a steep plasma density gradient exists at the bottom of the F region. In addition, the effect of ionospheric electrodynamic coupling processes, including ionospheric E-F coupling and inter-hemispheric coupling on the generation of ionospheric irregularities, becomes more prominent due to the significant dip angle and equipotentiality of magnetic field lines in the mid-latitude ionosphere. Polarized electric fields can map to different ionospheric regions and excite plasma instabilities which form ionospheric irregularities. Nevertheless, the mapping efficiency of a polarized electric field depends on the ionospheric background and spatial scale of the field.

An extraordinary (X-mode) electromagnetic wave, injected into the ionosphere by the ground-based heating facility at Tromsø, Norway, was utilized to modify the ionosphere on November 6, 2017. The high-power high-frequency transmitter facility located at Tromsø belongs to the European Incoherent Scatter Scientific Association. In the experiment, stimulated electromagnetic emission (SEE) spectra were observed. A narrow continuum occurred under cold-start conditions and showed an overshoot effect lasting several seconds. Cascading peaks occurred on both sides of the heating frequency only in the preconditioned ionosphere and also showed an overshoot effect. These SEE features are probably related to the ponderomotive process in the X-mode heating experiment and are helpful for understanding the physical mechanism that generated them during the X-mode heating experiment. The features observed in the X-mode heating experiments are novel and require further investigation.