Geomagnetic field impacts on second harmonic generation during high power radio wave-ionosphere interaction

Abstract

Contained herein are results from an experimental study of geomagnetic field (${\mathbf{B}}_{\mathbf{0}}$ ) 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 ($2f_0$ ). 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 ${\mathbf{B}}_{\mathbf{0}}$ , and f₀ was stepped near the second harmonic of the electron gyrofrequency ($2f_{ce}$ ). SEEs ordered by the oxygen ion gyrofrequency (f_ci) are observed in spectra near $2f_0$ for: (1) a ${\mathbf{B}}_{\mathbf{0}}$ -oriented antenna beam when $f_0\approx 2f_{ce}$ and (2) $f_0<2f_{ce}$ , when the beam is tilted away, but not beyond a maximum angle from ${\mathbf{B}}_{\mathbf{0}}$ . The observations motivate further studies to develop SHG into important diagnostics for the modified ionosphere.