On the origin of narrow non‐ion‐acoustic coherent radar spectra in the high‐latitude E region

Abstract

Many types of coherent radar spectra have a width in Doppler velocity units that is less than the ion acoustic speed of the medium. In spectra labeled as type 1 the mean Doppler shift of these narrow spectra matches the ion acoustic speed of the medium. There also exist narrow high-latitude spectra for which the mean Doppler shift is either markedly less or markedly more than the ion acoustic speed. We propose that electron density gradients with scale lengths as small as 100 m are at the origin of a large fraction of these narrow spectra near 50 MHz. The sharp density gradients in that case are created in regions of discrete auroral precipitation associated either with multiple narrow arcs or with sharp edges of broader features. Using the same principle at radar frequencies in the 10-to 20-MHz range, we find that gradient scales from 20 to 30 km in size create a combination of fast and slow phase velocities closely resembling the spectral characteristics expected from NO + ion cyclotron waves. However, gradients are not always responsible for slow narrow spectra; a detailed analysis of available observations has led us to conclude that the high-latitude E region cannot always be considered as fully turbulent even when appreciable coherent echo returns are registered by the radars. In particular, slow narrow spectra at 50 MHz are at times produced under gradient-free weakly turbulent conditions. In addition, at lower radar frequencies (10 to 20 MHz) the narrow spectral width of slowly moving waves and the morphology of these waves both suggest that the irregularities are generated indirectly via mode coupling of linearly unstable modes and that these "secondary" waves are themselves not coupling eflqciently. This implies that processes other than mode coupling are contributing to the overall wave energy budget. In that case we suggest that the convective properties of the slowly growing modes are an important factor in removing wave energy• even for waves as small as a few meters in wavelength. We also propose that there may be two distinct generation mechanisms for secondary waves at 10 MHz, each with its own mean Doppler shift behavior. doupis et al. [1991] were not convinced by that explana-6447 6448 ST.-MAUR/CE ET AL.: NARROW E REGION SPECTRA weakly or strongly turbulent [Hamza and St.-Maurice, 1993a] (hereinafter referred to as paper 1). For nonion-acoustic Doppler shifts, on the other hand, it may not be so obvious that narrow wave spectra are made of primaries particularly if their mean Doppler shift is ST.-MAURICE ET AL.: NARROW E REGION SPECTRA 6449smaller than the ion acoustic speed. As we will show, a more precise probing of the waves properties or of the circumstances that lead to their occurence can help sorting out whether the observed waves are primaries or secondaries.Weakly turbulent ideas for the generation of secondary waves are rather well known, and we only raise a few points about the role of anomalous diffusion in complicating the interpretation of the mean Doppler shifts somewhat for thes...

Investigation of the relationship between optical auroral forms and HF radar E region backscatter

“…It should be noted that horizontal gradients should play a more in¯uential roà le in the generation of irregularities in the HF regime than the VHF regime (e.g. St.-Maurice et al, 1994). In the present study, we found the best co-location of radar and optical aurora if the emission altitude was assumed to be 250 and 110 km in study intervals I and II, respectively.…”

CUTLASS HF radar observations of high-velocity E-region echoes

“…According to this interpretation, the present observations and the observations by Villain et al (1987Villain et al ( , 1990 can be consistently explained. To apply the interpretation by St-Maurice et al (1994) to the observations, however, one has to be sure that strong plasma gradients associated, for example, with auroral arcs, were present. We do not have data to support or reject this interpretation.…”

“…-Maurice et al (1994) proposed an alternative interpretation of the high velocity echoes. These authors argued that large velocity waves are simply the F-B waves excited in the presence of strong plasma gradients and electric fields.…”

The role played by thermal feedback in heated Farley-Buneman waves at high latitudes

“…Even the di erent``types'' that have mean Doppler shifts that are markedly di erent from the nominal ion-acoustic speed of the medium have been linked to threshold conditions through the in¯uence of gradients (e.g. St.-Maurice et al, 1994) or through electron heating e ects (e.g. Farley and Providakes, 1989;Shalimov and Haldoupis, 1995).…”