X‐mode suppression of artificial E region field‐aligned plasma density irregularities

Abstract

Ionospheric modification experiments have been performed at the High frequency Active Auroral Research Program involving the creation and suppression of artificial field‐aligned density irregularities (AFAIs) using O‐mode and X‐mode continuous wave emissions, respectively. The emission frequencies were offset so that the O‐mode upper hybrid interaction height nearly matched the X‐mode reflection height in the ionospheric E region. AFAIs created by O‐mode heating were observed using a 30 MHz coherent scatter radar imager. Simultaneous X‐mode heating was found to suppress the AFAI intensity and increase the threshold power for excitation by approximately a factor of 5 in our experiments. The effects are attributed in part to the broadening of the upper hybrid interaction region and in part to increased O‐mode absorption, which reduces the amplitude of the standing wave pattern in that region. Preliminary estimates based on local calculations suggest that the electron temperature in the E region was increased by a factor of about 1.6 in these experiments.