Properties of the Stimulated Electromagnetic Emissions During the Inclined High‐Frequency Pumping of the Ionosphere Near the Fourth Electron Gyroharmonic at the High‐Frequency Active Auroral Research Program Facility

Abstract

Many phenomena in the high‐frequency pumped ionosphere exhibit dependence on the pump beam incident angle α. This motivates a systematic study of the α dependence of the stimulated electromagnetic emission (SEE), particularly near electron gyroharmonics. We report the first observations of stationary SEE spectra for α ranging from −28° (north) to 28° (south) at three receiving sites, for the pump frequency (f0) sweeping near the fourth gyroharmonic (4fc). The following is established: (i) For pumping near the magnetic zenith (α = 7°, 14°, 21°), when existent dynamic broad upshifted maximum in the SEE spectrum indicates that artificial ionization layers are excited, suppression of the downshifted maximum at f0 ≈ 4fс is weakest, and 4fс increases. (ii) Weaker similar effects occur for α = −14° (a condition called “mirror magnetic zenith”). (iii) For northern pump beam inclinations, the SEE intensity decreases (in comparison with southern inclinations and vertical), most strongly at the southernmost receiving site.