Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> We present a model that describes the decay of beam generated Langmuir waves into ion-acoustic waves in the topside ionosphere. This calculation is done within the frame of the weak turbulence approximation. We study the spectral signature of such a process as seen by a VHF incoherent scatter radar. An incoherent scatter (IS) spectrum is characterized by two maxima at k<i><sub>radar</sub></i> and -k<i><sub>radar</sub></i>, the right and left ion lines respectively. It is shown that, for reasonable beam parameters, the parametric decay of beam-generated Langmuir waves can enhance either the right, the left or both ion lines simultaneously. The shape of the spectrum can change drastically on time scale of about 0.1 to 1 s. The role of the beam parameter as well as the ionospheric parameters is also investigated. For a given beam number density, the beam energy or the background density are important to trigger either the left or the right ion line. A large energy spread of the beam or low electron collision frequencies can explain the simultaneous observations of the left and the right ion line. The importance of the electron collision frequency can explain the altitude distribution of the coherent echoes observed by incoherent scatter radars.<br><br><b>Key words.</b> Ionosphere (plasma waves and instabilities) · Space plasma physics (waves and instabilities; wave-wave interactions)
Highlights
Powerful spectra have been unambiguously observed with incoherent scatter radars both at UHF and VHF frequencies (Foster 1988; Collis et al, 1991; Rietveld et al, 1991)
The ®rst calculation has been made for n~0 1011 mÿ3, n~b 2 104 mÿ3, E~b 45 eV, DE~b =E~b 1=4 and ~me
We have modeled the decay of beam-generated Langmuir waves into ion-acoustic waves using the wave kinetic equations within the framework of the weak turbulence approximation
Summary
Powerful spectra have been unambiguously observed with incoherent scatter radars both at UHF and VHF frequencies (Foster 1988; Collis et al, 1991; Rietveld et al, 1991). Such coherent spectra all exhibit one or two strong ion-acoustic peaks enhanced by one or two orders of magnitude. These echoes are observed along the magnetic ®eld line, from the topside ionosphere all the way down to 140 km. (1996) showed that such echoes statistically exhibit dierent features according to the altitude. The upper shoulder (ion-acoustic waves going down) is enhanced more frequently than the lower shoulder.
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