Spectral observations of polar mesospheric summer echoes at 33 cm (450 MHz) with the Poker Flat Incoherent Scatter Radar

In this paper, we report on multi-beam spectral observations of polar mesospheric summer echoes (PMSE) at 450 MHz (Bragg scattering wavelength of similar to 33 cm) carried out with the Poker Flat Incoherent Scatter Radar (PFISR) located near Fairbanks, Alaska. The observations presented in this paper occurred with auroral particle precipitation, which enhanced the otherwise low nighttime D-region ionization. The observations indicate two classes of spectra associated with PMSE at this frequency: a relatively rare, "broad" class of spectra that seems to be particularly turbulent with spectral widths (root mean square velocity fluctuations) of 6-7 m/s, and the more common "narrow" spectra, with spectral widths close to I m/s. The results are discussed in terms of the turbulence scattering theory of PMSE. Using the theories of Rapp and Lubken [2003. On the nature of PMSE: electron diffusion in the vicinity of charged particles revisited. Journal of Geophysical Research 108, 8437, doi: 10.1029/2002JD002857], we find that neutral turbulence together with enhancement of the Schmidt number by the presence of charged ice can indeed explain the observations, even at these small scales. The echoes are likely associated with large charged ice particles (a few 10 s of nanometers in radius). The narrowest echoes, while seemingly resulting from relatively modest neutral turbulence, thus acquire long diffusion times which allow them to drift with the background winds for tens of seconds, possibly explaining the predominance of these narrow-width echoes. (C) 2008 Elsevier Ltd. All rights reserved.