Beat excitation of whistlers in the presence of field aligned irregularities in the ionosphere

Abstract

In equatorial ionospheric plasma, embedded with a density ripple, a low frequency whistler can be resonantly excited by the nonlinear mixing of two high frequency radio waves launched from the ground at an oblique angle. A suitable configuration is where the earth magnetic field is along ẑ (horizontal), ripple wave number q⃗‖ŷ (horizontal), and high frequency wave numbers are k⃗1=k1cosθẑ+k1sinθ(cosϕx̂+sinϕŷ), k⃗2=−k2cosθẑ+k2sinθ(cosϕx̂−sinϕŷ). Phase matching conditions are satisfied for large θ and small ϕ. The radio waves exert a ponderomotive force on electrons at the beat frequency imparting them an oscillatory velocity with a component perpendicular to the magnetic field. This velocity, in conjunction with the density ripple, produces a current that drives the whistler. The Ohmic nonlinearity is found to be relatively weaker in comparison to ponderomotive nonlinearity at higher heights. However in the E-region, this nonlinearity reinforces with the ponderomotive nonlinearity and can improve the whistler power generation.