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

Abstract

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.

Simultaneous Action of X- and O-Mode HF Pump Waves on the High-Latitude Upper (F-Region) Ionosphere at EISCAT

“…In addition to the nf ci spectral structures, the downshifted spectral component at 0.5f ci can be seen on the spectrogram. Similar multiple ion gyro-harmonic structures in the NSEE spectra were observed in our other X-mode experiments at different heater frequencies [25][26][27]. The observed structures in the NSEE spectra did not occur under "pure" O-mode HF pumping from 15:31 to 15:41 UT.…”

“…The features and excitation mechanisms of various discrete structures in the NSEE spectrum were extensively investigated (see, for example, [13,[41][42][43][44]). Our X-mode HF pumping experiments at EISCAT made it possible to observe for the first time the NSEE at a large distance from the EISCAT/heating facility [25][26][27]. Alternating O/X-mode pumping has demonstrated clearly that only X-mode heating is able to produce NSEE observed far away from EISCAT.…”

“…Multiple decays can be realized in both mechanisms, which leads to the development of a cascading process in the ion gyro-harmonic excitation. FAIs, generated in the artificially disturbed ionospheric patch, have a dominant role in NSEE observations far away from the EISCAT/heating facility [27]. Kalishin et al [27] proposed two relevant mechanisms responsible for the excitation of multiple discrete spectral structures induced by the X-mode HF pumping and observed at a large distance from the EISCAT/heating facility.…”

“…FAIs, generated in the artificially disturbed ionospheric patch, have a dominant role in NSEE observations far away from the EISCAT/heating facility [27]. Kalishin et al [27] proposed two relevant mechanisms responsible for the excitation of multiple discrete spectral structures induced by the X-mode HF pumping and observed at a large distance from the EISCAT/heating facility. The first is magnetized stimulated Brillouin scatter (MSBS) when an X-mode electromagnetic wave decays into a backscattered electromagnetic wave and an electrostatic ion gyro-harmonic mode.…”

“…As a result, X-mode HF pump waves should not generate such waves and, as a consequence, the excitation of the artificial plasma turbulence and phenomena accompanying them. However, a large number of X-mode experiments, performed by the Russian team at EISCAT, clearly demonstrated for the first time that the X-polarized HF pump waves, injected into the high latitude ionospheric F-region towards the magnetic zenith, are able to produce the excitation of a large variety of HF-induced phenomena, which can even be much stronger than the O-mode effects [20][21][22][23][24][25][26][27][28][29][30][31].…”