Abstract
Abstract. The ESRAD 52-MHz and the EISCAT 224-MHz radars in northern Scandinavia observed thin layers of strongly enhanced radar echoes from the mesosphere (Polar Mesosphere Winter Echoes - PMWE) during a solar proton event in November 2004. Using the interferometric capabilities of ESRAD it was found that the scatterers responsible for PMWE show very high horizontal travel speeds, up to 500 ms-1 or more, and high aspect sensitivity, with echo arrival angles spread over as little as 0.3°. ESRAD also detected, on some occasions, discrete scattering regions moving across the field of view with periodicities of a few seconds. The very narrow, vertically directed beam of the more powerful EISCAT radar allowed measurements of the spectral widths of the radar echoes both inside the PMWE and from the background plasma above and below the PMWE. Spectral widths inside the PMWE were found to be indistinguishable from those from the background plasma. We propose that scatter from highly-damped ion-acoustic waves generated by partial reflection of infrasonic waves provides a reasonable explanation of the characteristics of the very strong PMWE reported here.
Highlights
It has been known for several decades that radar echoes can be obtained from the mesosphere by sufficiently powerful radars operating at VHF frequencies
Turbulent fluctuations in neutral density are expected at scale sizes of a few meters or more and these can be expected to lead to similar fluctuations in ion and electron density, at least during daylight or in the upper mesosphere, and so provide at least a qualitatively possible explanation for the observed radar echoes (Hill, 1978)
It was not expected that turbulent structures would be present in the electron density in the upper mesosphere at shorter scale sizes than a few meters, due to rapid damping by the high viscosity prevailing at those heights
Summary
It became clear that the echoes observed by EISCAT could not reasonably be explained by turbulence alone since turbulent eddies at the scale size needed to cause EISCAT echoes (67 cm) should be strongly damped – they should not be able to produce such strong echoes as observed by the radar The physics behind these “Polar Mesosphere Summer Echoes” (PMSE) has since been the subject of a very large number of scientific studies both by radars and by sounding rockets, and it is nowadays considered well-proven that layers of charged aerosol particles are responsible for their existence since these hinder the viscous damping of small-scale structure in the electron gas (Kelley et al, 1987; Cho and Rottger, 1997; Rapp and Luebken, 2004). The relatively low-power 52-MHz ESRAD radar (Table 1), sometimes simultaneously with the EISCAT VHF radar, has observed surprisingly strong radar echoes from the winter mesosphere during solar proton events and on other occasions when electron densities at mesospheric heights are enhanced by energetic particle precipitation (Kirkwood et al, 2002a, 2002b, 2005; Belova et al, 2005). In this paper we are not primarily concerned with the causes of variations in the echo power but rather in the spatial and spectral fine-structure of the radar echoes so we choose to study the 10 November because the strength of the echoes allows us to do this as accurately as possible
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