Ion Gyro‐Harmonic Structures in Stimulated Emission Excited by X‐Mode High Power HF Radio Waves at EISCAT

The distinctive features of the ion gyro-harmonic discrete structures in the narrowband stimulated electromagnetic emission (NSEE) spectra (within ±1 kHz of the heater frequency), excited by an extraordinary (X-mode) polarized high frequency (HF) pump wave in the F-region of the high latitude ionosphere, are reported. Results were obtained from three sets of experiments at the EISCAT (European incoherent Scatter) HF Heating facility at pump frequencies of 5.423, 6.77 and 7.953 MHz, which are below the fourth, fifth, and sixth electron gyro-harmonics. High power HF radio waves were radiated in the magnetic field-aligned direction with an effective radiated power of 238–740 MW. Discrete spectral features, ordered by the ion gyro frequency (for O+ ions), were recorded at a large distance (1,200 km) away from the Heating facility. Up to 10 downshifted discrete ion gyro-harmonic structures paired with the upshifted spectral components (Stokes and anti-Stokes modes) in the NSEE spectra have been excited in the course of these experiments. It was found that preconditioning effects have a significant impact on the temporal evolution of the discrete spectral structures. The ion gyro-harmonic structures coexisted with field-aligned artificial irregularities (FAIs). Comparing the NSEE and EISCAT incoherent scatter radar observational results, it was concluded that the electron accelerations along and across the geomagnetic field coexist during X-mode heating. The magnetized stimulated Brillouin scatter and stimulated ion Bernstein scatter are discussed as relevant processes for the observed multiple down- and upshifted ion gyro-harmonic structures in the NSEE spectra.Plain Language SummaryExperiments in the Earth's ionosphere (part of the upper atmosphere) using high power high frequency (HF, or shortwave) radio waves may be used to study a wide variety of fundamental phenomena. We discuss recent advances in observations of stimulated radio emissions generated by a powerful HF radio transmitter at the European incoherent Scatter observatory in northern Norway. We found that the high power HF wave is able to generate radio emissions that can be observed more than one thousand kilometers away, in Saint Petersburg, Russia. We found and investigated distinctive frequencies of the radio emissions, and we believe, based on past results, that such emissions are not observable from locations close to the radio transmitter. The emissions we observed can be exploited for diagnostics of the Earth's environment, and they are similar to emissions seen in laser interactions with ionized gases such as occur in laser fusion experiments, and also in piezoelectric semiconductor devices, in fiber optics, and in many biological systems. This opens a window for utilizing our results to assist in these and other related areas.</div

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

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