Distinctive Features of Langmuir and Ion-Acoustic Turbulences Induced by O- and X-Mode HF Pumping at EISCAT

摘要

We report experimental results regarding the distinctive features and behaviors of high-frequency (HF)-induced Langmuir and ion-acoustic turbulences induced by ordinary (O-mode) and extraordinary (X-mode) polarized powerful HF radio waves injected into the high-latitude ionosphere F region toward the magnetic zenith at EISCAT (European Incoherent SCATter). Alternating O-/X-mode pumping was produced at high heater frequencies in the range f_H = 6-8 MHz during magnetically quiet background geophysical conditions. An effective radiated power of 450-600 MW was utilized. The radical distinction between the temporal evolution of the O- and X-mode Langmuir and ion-acoustic plasma waves after the HF heater is switched on in conjunction with the development of artificial field-aligned irregularities (FAIs) was analyzed. It was found that the excitation thresholds of the Langmuir and ion-acoustic turbulences significantly differed for the O- and X-mode HF pumping. We have revealed that the X-mode excitation thresholds for HF-induced plasma and ion line backscatter are 0.47 and 0.61 V/m, respectively. The persistent O-mode plasma and ion line backscatters that coexisted with FAIs showed excitation thresholds of 0.62 and 0.73 V/m, respectively, which exceeded the thresholds for the X-mode plasma and ion lines, while their intensity was 2 orders of magnitude less. For the same background conditions, the "classic" resonance parametric decay instability and modulation instability, excited as the momentary response to the O-mode HF pump wave switching on (the so-called overshoot), have thresholds of 0.17 and 0.25 V/m, respectively, which are much below the excitation thresholds of persistent O-mode plasma and ion lines.