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

Abstract

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 fH = 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.

Perturbing the high-latitude upper ionosphere (F region) with powerful HF radio waves: A 25-year collaboration with EISCAT

“…The HF-enhanced plasma line powers are shown at two fi xed heights where they had maximum values. The same behavior was also observed for HF-enhanced ion lines [41].…”

“…Large electrondensity increases of 50% to 70% were observed in a wide altitude range up to 600 km during all X-mode pump pulses (Figures 5a and 5d). Such e N enhancements were a typical feature of X-mode pumping at diff erent frequencies ( [31,35,36,41]. These e N enhancements occurred only in the magnetic fi eldaligned direction in a narrow-angle beam with a width of 3° to 4°around the magnetic fi eld line [34,35].…”

“…The appearance of the X-mode Langmuir and ionacoustic plasma waves was delayed by 15 s to 30 s from the heating onset. Their intensity then smoothly enhanced and maximized within about 1 min, when artifi cial fi eldaligned irregularities were generated [41]. Such a behavior of HF-induced plasma and ion turbulence corresponded to the development of the non-resonance type instability in which artifi cial fi eld-aligned irregularities played a key role.…”

“…We analyzed the distinctive features and behaviors of HF-enhanced Langmuir and ion-acoustic turbulences induced by the O-mode and X-mode HF pump waves radiated into the high-latitude ionospheric F region towards the magnetic zenith [36,41,46,47]. Results came from a large number of alternating O/X-mode experiments carried out at the pump frequencies of 5.423 953 MHz, 30 s before the HF pumping onset and during the fi rst 2 min of the heating cycles [36].…”

“…It was shown that excitation thresholds for the parametric decay instability (PDI), represented in the radar spectra as HF-enhanced plasma and ion lines, signifi cantly diff ered for the O-mode and X-mode HF pumping towards the magnetic zenith [41]. Results from a power stepping experiment at 7.953 H f  MHz demonstrated that the X-mode parametric decay instability excitation threshold was 0.47 V/m.…”