We present European Incoherent Scatter (EISCAT) Svalbard Radar observations of enhanced incoherent scatter ion-acoustic spectra with transmitted frequencies slightly shifted relative to the radar central frequency. This study confirms the initial findings that (1) the naturally enhanced ion acoustic lines sometimes show asymmetric power profiles and (2) the amplitude enhancement of upgoing and downgoing ion-acoustic waves occurs at somewhat different altitudes and different power levels. These observational facts are modeled using the parametric decay of beam driven Langmuir wave in an inhomogeneous medium. It is shown that the density gradient with altitude as well as the collisions contribute to an altitude variation of ion-acoustic spectra and may account for the observed signatures. It is also responsible for the transition from upshifted to downshifted enhanced ion lines. Our model shows that the power radiated by enhanced spectra varies with the wavelength and that the power difference between two wavelengths varies with altitude. ¿ 2001 American Geophysical Union