Reported herein are first observations of narrowband stimulated electromagnetic emissions (NSEEs) within ±30 Hz of the second harmonic of the pump wave frequency during ionospheric heating experiments at the High Frequency Active Auroral Research Program facility. The observations are juxtaposed with contemporaneous NSEEs observed within ±30 Hz of the pump wave frequency. The transmit power of the pump wave is linearly increased, and the pump wave frequency is stepped through frequencies near the third electron gyroharmonic frequency during the experiment. The first observation is that there is a transmit power threshold for the occurrence of NSEEs downshifted from the second harmonic of the pump wave frequency. Secondly, when downshifted NSEEs are present at both the pump frequency and its second harmonic, the transmit power threshold for the onset of both are comparable. Third, the frequency offset of the downshifted NSEEs spectral line from the second harmonic frequency is approximately twice the frequency offset of the downshifted NSEEs spectral line from the pump wave frequency when the downshifted NSEEs are present near both the pump and second harmonic frequency. Finally, the NSEEs at the second harmonic of the pump wave frequency are suppressed at pump frequencies very close to the third electron gyroharmonic frequency.

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Due to the potential for new diagnostic capabilities, there has been renewed interest in the generation of ionospheric stimulated electromagnetic emissions (SEEs) near the second harmonic of the pump frequency ( 0 ), a process known as second harmonic generation (SHG). Observations of SHG during experiments at the High Frequency Active Auroral Research Program facility in which 0 was stepped near the third harmonic of the electron gyrofrequency, 3 ce , and the transmit power linearly increased over the heating cycle at each 0 , were reported recently. A key observation was the linkage between SEEs within ±30 Hz of 0 , due to stimulated Brillouin scatter, and within ±30 Hz of 2 0 . This current work reports further High Frequency Active Auroral Research Program observations that compare the time evolution of SEEs including SHG under the following two transmit power conditions (I) linear power ramp (II) maximum available power (2.8 MW). During these experiments, 0 was stepped near 3 ce and also 2 ce . The results show that SEEs within ±100 Hz of 0 and 2 0 are both suppressed within a few seconds when the ionosphere is irradiated with the maximum available power. These SEEs appear to be suppressed before the onset of field-aligned irregularities at the upper hybrid layer which is not in line with previous reports. Thus, other mechanisms, which are discussed, could possibly be responsible for the observed suppression of stimulated Brillouin scatter and SHG. Some preliminary diagnostics are derived from the SHG spectra temporal evolution by leveraging concepts from the field of laser plasma interactions.

Contained herein are results from an experimental study of geomagnetic field (B0) impacts on second harmonic generation (SHG) during the interaction of high power, high frequency radio waves with the ionosphere. These magnetic field effects on SHG are assessed by (1) considering the proximity of the pump frequency to the electron gyroharmonic frequency and (2) considering the pump antenna beam angle relative to the geomagnetic field direction. The observations show evidence of ion Bernstein mode wave-mixing processes in stimulated electromagnetic emissions (SEEs) simultaneously near the pump wave frequency (f0) and its second harmonic (2f0). In an experiment performed at the High Frequency Active Auroral Research Program (HAARP) facility, the transmit antenna beam was either oriented along or tilted away from B0, and f0 was stepped near the second harmonic of the electron gyrofrequency (2fce). SEEs ordered by the oxygen ion gyrofrequency (fci) are observed in spectra near 2f0 for: (1) a B0-oriented antenna beam when f0≈2fce and (2) f0<2fce, when the beam is tilted away, but not beyond a maximum angle from B0. The observations motivate further studies to develop SHG into important diagnostics for the modified ionosphere.