Nonlinear reflection of a high‐frequency radio wave by the ionospheric grating created by another wave

Abstract

A powerful O‐mode radio wave of frequency ω0, launched from a ground‐based transmitter into the equatorial ionosphere, forms a standing wave pattern below the critical layer, heating the electrons with spatial periodicity. Subsequent ambipolar redistribution of plasma creates a density ripple. When another radio wave, of higher‐frequency ω, is launched into the ripple region, the oscillatory electron velocity due to the ω wave couples to the density ripple to produce a nonlinear current that resonantly drives a reflected wave toward the Earth at frequency ω when k matching condition is satisfied. By this scheme one may reflect radio waves of frequency 20 times the maximum plasma frequency ωpmax of the ionosphere. In the case when the heater wave is mode converted into a plasma wave with Airy function profile, the nonuniform heating of electrons forms a nonuniform density grating of much shorter periodicity length. This grating may cause reflection of much higher frequency waves, however, with lesser amplitude.