Majority of PMSE spectral widths at UHF and VHF are compatible with a single scattering mechanism

Abstract

Motivated by a recent controversy in the literature about the agreement or disagreement of spectral widths (after conversion to Doppler-velocities) of PMSE observed simultaneously with the EISCAT VHF and UHF radars in Tromsø, Norway, this issue was reconsidered taking into account a much larger data set than in any of the previous studies. While these previous studies only considered case studies of a few minutes in length, we considered a total of 380min of simultaneous and common volume observations with these two radars at a time resolution of 30s, i.e., a total of 760 samples. This study is important to discern whether PMSE at these two frequencies originate from the same coherent scattering mechanism or if different physical mechanisms are at work. We considered both shape and width of the considered spectra and found that – within the available spectral resolution – the VHF spectra are on average well described by a Gauss shape (with spectral parameter n=2—which is derived from a generalized formulation of the autocorrelation function), whereas the UHF-spectra show a small deviation from this shape with an average n of 1.6. Spectral widths do largely agree but show a small systematic difference, i.e., the UHF spectra are on average 0.1m/s narrower than the VHF spectra at an average spectral width of 3.5m/s. This small systematic effect is largely explained considering the overall effect of beam-, shear-, and wave-broadening. This means that the slightly different beam widths of the VHF and UHF-radars and the hence slightly different observing volumes account for the small systematic difference in mean spectral width. Finally, we demonstrated that the small deviation of UHF-spectra from a perfect Gauss shape can be either due to relatively small signal-to-noise ratios of the UHF-data and/or a superposition of incoherent and coherent scatter in cases where both contributions add equally to the total observed power. Since VHF PMSE are much more stronger than the incoherent scatter background this effect cannot be recognized in VHF observations. This superposition effect may occasionally also lead to larger deviations between VHF and UHF spectra in that it can potentially narrow the UHF-spectra significantly. However, excluding these rather rare and exotic cases, we conclude that the majority of our observations are compatible with a single coherent scattering at both VHF and UHF.