Upper hybrid wave excitation due to O-mode interaction with density gradient in the ionosphere

Summary form only given, as follows. It has been well recognized that upper hybrid (UH) waves play a key role in various wave processes occurring in the upper hybrid resonance (UHR) region of the ionosphere leading to the observed stimulated electromagnetic emissions (SEE) during artificial heating by ordinary mode (O-mode) electromagnetic waves. Hence it is important to investigate how the UH waves get excited from the incident O-mode. It has been generally suggested that the UH waves are excited by O-mode interaction with nonuniform ionospheric plasma. For instance, direct conversion of the O-mode into UH waves due to pre-existing short scale irregularities was reported earlier. Here we consider the role of large-scale, smooth density gradient in exciting the UH waves from the O-mode. The model used is that of a driven harmonic oscillator in which the source term arises from the O-mode interaction with local density gradient. For a slab model with density gradient in the x-direction, and the geomagnetic field in the x-direction, we obtain an inhomogeneous fourth order ordinary differential equation governing the UH wave excitation: /spl delta//sup 2//spl psi/""+P/spl psi/"+Q/spl psi/\´+R/spl psi/=f, where /spl delta//sup 2/=(p/sub e//sup 2//h/sup 2/)/spl Lt/1 gives the thermal correction to the cold UH mode. This equation has been analyzed in the vicinity of the UHR. The pertinent solutions are presented and discussed for the typical parameters of heating experiments.