Radar spectral observations of HF‐induced ionospheric Langmuir turbulence with improved range and time resolution

Abstract

The technique of coded long radar pulses was used to obtain radar backscatter spectra from HF‐induced Langmuir turbulence every millisecond for height intervals 150 m apart. The most interesting spectra were obtained by transmitting HF radio wave pulses of 5‐ms length every second, with a frequency of 5.1 MHz and with equivalent radiated powers (ERPs) of 40 or 60 MW ERPs. Using pulses of that length or shorter appears to have avoided the generation of density irregularities. Moreover, using these short HF pulses assured the effectiveness of the novel technique used to achieve the 1 ms temporal resolution. Under those conditions 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 narrow spectral “free mode” type peak above the pump frequency. Those two spectral features have been predicted by the numerical simulation spectra of DuBois et al. [1990]. In the subsequent spectra, free mode spectral peaks appear at lower heights but without accompanying broad spectra. Those free mode spectral peaks show clearly the pump standing wave structure (Airy function structure) in their distribution with height. For 50‐ms‐long HF pulses the free mode peaks are seen for only about the first 10‐20 ms of the HF pulse. At about the time when the free mode peak “disappears,” strong decay line type spectra appear at more than two heights near and above the matching height at the same frequency; both effects are tentatively attributed to the development of density irregularities in the plasma density.