EVOLUTION OF LANGMUIR TURBULENCE AND STIMULATED ELECTROMAGNETIC EMISSIONS EXCITED WITH A 3-MHZ PUMP WAVE AT ARECIBO

We present the results of a comparative experimental study at Arecibo of the initial temporal development of stimulated electromagnetic emis sions (SEE) and HF-enhanced plasma lines (HFPLs) excited in the ionosp heric F region by a high-power O mode radio wave at 3.1745 MHz. These results show that the portion of the SEE spectrum which extends from a pproximately -30 kHz to +10 kHz in the sidebands around the injected H F wave, and which is usually referred to as the narrow continuum SEE c omponent, exhibits an overshoot behavior very similar to that of the u pshifted HFPL of the 430-MHz radar. Over timescales of a few hundred m illiseconds, the narrow continuum emission weakens at all frequencies and gradually changes shape to become more structured. At 3.1745 MHz t he amplitude of the HFPL is significantly smaller than that observed a t higher HF frequencies; after an overshoot period of 300-500 ms, the HFPL often disappears completely.