Dynamics and Electrodynamics of the Mesosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 353

Resolute bay VHF radar

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Day: 18-Jun-98 (169)

Figure 7.9 Contour plot of the signal-to-noise ratio at Resolute Bay, Canada (75 ◦ N,
95 ◦ W). The time axis is in UT. [After Huaman and Kelley (2002). Reproduced with
permission of Elsevier.]

in Chapter 6 and most likely is related to tidal and/or gravity wave perturbations.
(The radar frequency is 51.5MHz corresponding to backscatter from structures
of 2.91m wavelength, which is smaller than the Kolmogorov microscale in the
mesosphere.) Radiowaves scatter most easily from electrons and, when they are
present, usually dominate the radiowave physics. Indeed, since the summer polar
zone is in full sunlight, the ionosphere actually extends downward to the altitudes
of these echoes. But this is true at other latitudes in daytime, so simply having
electrons present in a turbulent atmosphere (the mesopause is below the tur-
bopause) is not sufficient to yield these huge echoes. Rayleigh scatter is not a
viable explanation.
The rocket data in Fig. 7.7 that show the electron density structure in the
medium provided the first clear indication that the echoes were related to the
coupling of electrons and small particles (Kelley et al., 1987; Kelley and Ulwick,
1988). Figure 7.10 shows simultaneous VHF scatter echo profile and the electron
density during a strong radar event. A severe, sharp, bite-out in the latter is
coincident with the echoing region. It seems curious that removing electrons can
increase the signal, but if the resulting coupled electron-ice gas has very sharp
edges or small eddies, the crucial requirement that structure at half the radar
wavelength exists may be accomplished. Analysis of the rocket data indeed shows
that for this VHF (53.5MHz) study the electron gas was indeed structured at this
small scale. Wavelets have been used to probe the sharp edges in the data set to see
if they are steep enough to partially reflect VHF radar signals. Indeed, as shown
in Fig. 7.11, the Canny edge detector wavelets are strong deep into the meter
range responsible for edge scatter (Alcala and Kelley, 2001; Alcala et al., 2001).
With edges removed, these authors found some regions that appeared turbulent