Abstract
We provide the first experimental evidence of the sensitivity of phenomena induced by extraordinary (X-mode) polarized HF high power radio waves to pump frequency stepping across the fifth electron gyro-harmonic (5fce) from below to above. The results were obtained at the EISCAT (European Incoherent Scatter Scientific Association) HF heater facility near Tromsø under effective radiated powers of 456–715MW, when the HF pump wave was transmitted into the magnetic zenith. We have analyzed the behavior and intensities of various spectral lines in the narrowband stimulated electromagnetic emission (SEE) spectra observed far from the heater, HF-enhanced plasma and ion lines (HFPL and HFIL) from EISCAT UHF incoherent scatter radar spectra, and artificial field-aligned irregularities from CUTLASS (Co-operative UK Twin Located Auroral Sounding System) observations, depending on the frequency offset of the pump field relative to the 5fce. At pump frequencies below 5fce the narrowband SEE spectra exhibited very intense so-called stimulated ion Bernstein scatter (SIBS), accompanied by other spectral components, associated with stimulated Brillouin scatter (SBS), which are greatly suppressed and disappeared in the vicinity of 5fce and did not reappear at fH>5fce. As the pump frequency reached 5fce, the abrupt enhancements of the HFPL and HFIL power, the appearance of cascade lines in the plasma line spectra, and the onset of increasing CUTLASS backscatter power occurred. That is opposite to the ordinary mode (O-mode) effects in the vicinity of 5fce. The X-mode pumping at frequencies below and in the vicinity of the fifth electron gyro-harmonic clearly demonstrated an ascending altitude of generation of induced plasma and ion lines from the initial interaction height, whereas for O-mode heating the region of interaction descended. The observations are consistent with the coexistence of the electron acceleration along and across the geomagnetic field at fH<5fce, while only very strong electron acceleration along the magnetic field was observed at fH≥5fce.
Highlights
High-power ordinary mode (O-mode) HF electromagnetic waves radiated from the ground surface are frequently used for the modification of the mid- and high latitude ionosphereF-region
We report here for the first time experimental results demonstrating the sensitivity of phenomena induced by extraordinary polarized HF high power radio waves to pump frequency stepping across the fifth electron gyro-harmonic
The stimulated electromagnetic emission (SEE) band on the spectrograms is represented in such a way that the maximum in the spectrum at the pump frequency corresponded to zero frequency offset at any time
Summary
High-power ordinary mode (O-mode) HF electromagnetic waves radiated from the ground surface are frequently used for the modification of the mid- and high latitude ionosphereF-region. Gyroharmonic effects manifest themselves in a great variety of plasma phenomena, induced by nonlinear interaction between the ordinary polarized pump wave and ionospheric plasma, such as broadband (classical) stimulated electromagnetic emission (SEE) (Leyser, 2001 and references therein ; Carozzi et al, 2002; Kotov et al, 2008), artificial field-aligned irregularities (FAIs). The excitation of parametric decay instability requires that the orientation of the electric field of the pump wave at the reflection height should be parallel to the local magnetic field line This is realized only for O-polarized HF pump waves, while the electric field of the X-mode pump wave is perpendicular to the magnetic field (Robinson, 1989; DuBois et al, 1990; Stubbe, 1996; Gurevich, 2007; Kuo et al 2014)
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