RADAR SPECTRAL OBSERVATIONS OF HF-INDUCED IONOSPHERIC LANGMUIR TURBULENCE WITH IMPROVED RANGE AND TIME RESOLUTION

The technique of coded long radar pulses was used to obtain radar back scatter spectra from HF-induced Langmuir turbulence every millisecond for height intervals 150 m apart. The most interesting spectra were ob tained by transmitting HF radio wave pulses of 5-ms length every secon d, with a frequency of 5.1 MHz and with equivalent radiated powers (ER Ps) of 40 or 60 MW ERPs. Using pulses of that length or shorter appear s to have avoided the generation of density irregularities. Moreover, using these short HF pulses assured the effectiveness of the novel tec hnique used to achieve the 1 ms temporal resolution. Under those condi tions initially only a weak decay line type spectrum is observed from near the 430-MHz radar matching height. One or two milliseconds later a broad asymmetric spectrum appears at a height greater by about 1 km, with most of the energy below the pump frequency, together with a nar row spectral ''free mode'' type peak above the pump frequency. Those t wo spectral features have been predicted by the numerical simulation s pectra of DuBois et al. [1990]. In the subsequent spectra, free mode s pectral peaks appear at lower heights but without accompanying broad s pectra. Those free mode spectral peaks show clearly the pump standing wave structure (Airy function structure) in their distribution with he ight. For 50-ms-long HF pulses the free mode peaks are seen for only a bout the first 10-20 ms of the HF pulse. At about the time when the fr ee mode peak ''disappears,'' strong decay line type spectra appear at more than two heights near and above the matching height at the same f requency; both effects are tentatively attributed to the development o f density irregularities in the plasma density.