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Time (UT)
Figure 10.19 Comparison of time series data for HF (heavy crosses) and incoherent
scatter (light crosses) velocity estimates calculated for the returns received from the nearby
volumes. [After Ruohoniemi et al. (1987). Reproduced with permission of the American
Geophysical Union.]
Unless some additional instability arises at small scales, a possibility we address
soon, the only way waves with
λ
10m can arise is through a cascade of structure
from long to short
. There is a curious difference between high- and low-latitude
irregularities in this regard. As discussed in Chapter 4, convective ionospheric
storms evolve in such away that a very steep spectrum ( k 5 ) evolves for
λ
n 2
n 2
δ
(
)/
k
when
100m. This spectral range looks very much like a viscous subrange in
neutral turbulence and has been termed the diffusive subrange. No analogy to
the viscous or diffusive subrange has been reported at high latitudes.
Measurements on satellites and rockets do show that the spectrum of plasma
density fluctuations varies as k n in the wavelength regime from tens of meters
to tens of kilometers (Dyson et al., 1974; Sagalyn et al., 1974; Cerisier et al.,
1985), where typically 1
λ
5. As discussed in Chapter 4, this spec-
tral form has also been reported for equatorial measurements and barium cloud
striations. In both bottomside spread F and low-altitude barium striations such
spectra were shown to be due to wave steepening and not to turbulence (Costa
and Kelley, 1978; Kelley et al., 1979). There has been no evidence to date
that steepening occurs at high latitudes; rather, turbulent processes appear to
dominate. An example from a rocket flight is reproduced in Fig. 10.20. The
.
5
n
2
.
 
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