EPP

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.

In this study, we present three experiments carried out at the EISCAT (European Incoherent Scatter Scientific Association) heating facility on October 29 and 30, 2015. The results from the first experiment showed overshoot during the O-mode heating period. The second experiment, which used cold-start X-mode heating, showed the generation of parametric decay instability, whereas overshoot was not observed. The third experiment used power-stepped X-mode heating with noticeable O-mode wave leakage. Parametric decay instability and oscillating two-stream instability were generated at the O-mode reflection height without the overshoot effect, which implies suppression of the thermal parametric instability with X-mode heating. We propose that the electron temperature increased because X-mode heating below the upper hybrid height decreased the growth rate of the thermal parametric instability.

Parametric decay instability (PDI) is an important process in ionospheric heating. This paper focuses on the frequency and wavevector matching condition in the initial PDI process, the subsequent cascade stage, and the generation of strong Langmuir turbulence. A more general numerical model is established based on Maxwell equations and plasma dynamic equations by coupling high-frequency electromagnetic waves to low-frequency waves via ponderomotive force. The primary PDI, cascade process, and strong Langmuir turbulence are excited in the simulation. The matching condition in the initial PDI stage and cascade process is verified. The result indicates that the cascade ion acoustic wave may induce or accelerate the formation of cavitons and lead to the wavenumber spectrum being more enhanced at 2k_L (where k_L is the primary Langmuir wavenumber). The wavenumber spectra develop from discrete to continuous spectra, which is attributed to the caviton collapse and strong Langmuir turbulence.