Abstract
Parametric effects of an alternating electric field on an inhomogeneous plasma in a constant magnetic field are studied by taking into account of density gradients. A general dispersion relation governing the four-mode coupling of electrostatic waves is obtained in terms of linear susceptibilities of the plasma. The dispersion relation is applied to the analysis of the parametric excitation of ion-acoustic and Langmuir waves. It is shown that the parametric coupling via density gradient may have important effects in the experimental condition of Stern and Tzoar. It is also shown that if the pumping frequency is about twice the Langmuir (or upper hybrid) frequency, the Langmuir (or upper hybrid) mode can be excited by the inhomogeneity of the plasma. The threshold intensity is sufficiently low to be attainable by microwave irradiations.
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