RESONANT DECAY OF UPPER HYBRID WAVES AND ELECTROMAGNETIC EMISSIONS NEAR CYCLOTRON HARMONICS IN THE IONOSPHERE

An O mode wave launched into the ionosphere around harmonics of cyclot ron frequency excites a large-amplitude upper hybrid wave via resonanc e instability near the critical layer. The upper hybrid wave may param etrically decay into a lower hybrid wave and an electromagnetic wave. This process may account for the appearance of downshifted maximum in the spectrum of stimulated electromagnetic emissions. The threshold po wer density P-th for the process depends inversely on the spatial exte nt xi(o) of the localized upper hybrid pump. Near the seventh cyclotro n harmonic, (omega(o) approximate to 7 omega(c)), P-th approximate to 1 mu W/m(2) at xi(o) = 50 m, k approximate to 0.085 cm(-1). However, O mode frequency omega(o) must be significantly away from cyclotron har monics to avoid heavy cyclotron damping of the upper hybrid wave. At t he fourth and seventh harmonics, (omega(o) - n omega(c)) must exceed 1 0 and 1 KHz, respectively, to avoid damping.