Harmonics of the ion acoustic frequency in the heater induced ion spectrum

Odd harmonics of the ion acoustic frequency in the 224 MHz radar ion Line spectrum have been found during HF heating experiments at Tromso. This new feature appears in a narrow height range near the matching height where the ion line is characterized by enhanced ion acoustic shoulders and cascade or line spectra are observed in the plasma line channel. An explanation and a first estimate are given in terms of the parametric decay theory. A numerical calculation using an extended Zakharov model also demonstrates the existence of such ion Line harmonics.