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10 7
10 6
10 5
10 4
10 3
10 2
k 0
10 1
10 0
k 2
10 1
10 2
10 0
10 1
10 2
Wavenumber (km 1 )
Figure 4.19 Ratio of the spectral density of electric field (total) and density fluctuations
as a function of wave number. The ratio is linear at wavelengths longer than 300meters
but adopts k 2 scaling at shorter wavelengths. At very short wavelengths, the electric field
measurements are dominated by telemetry noise. [After Hysell et al. (1994). Reproduced
with permission of Pergamon Press.]
same spectral form. The electric field indicated by the dashed line is 10mV
m,
which, in turn, indicates a combination of a gravity and a wind-driven process,
since the zonal zero-order electric field is not this large. The physics changes
dramatically near the 100m scale where the ratio begins increasing as k 2 .We
investigate this transition in the next sections. First, however, we briefly discuss
the characteristics of the intermediate wavelength density (or electric field) power
spectra. Computer simulations have been used to predict the power spectrum
of intermediate wavelength density structures for comparison with experiments
(see Fig. 4.5). Keskinen et al. (1980a, b) found isotropic fluctuations at inter-
mediate scales with wave number spectra, which displayed a power law of the
form k n with n
/
5. This result is in good agreement with bottomside vertical
power spectral measurements on rockets. But horizontal satellite spectra often
have spectral indices closer to n
2
.
3.
An explanation for the anisotropic spectra has been proposed by Zargham
and Seyler (1987). Their simulation shows a very strong anisotropy in the devel-
opment of the irregularities with shock-like structures in the direction of
=
5
/
n .
A one-dimensional cut was made through the simulation for near-vertical tra-
jectories, and, as shown in Fig. 4.20, the predicted waveforms and their spectra
are in excellent agreement with the rocket data in Fig. 4.5a (Kelley et al., 1987).
The anisotropy displayed by the Zargham and Seyler simulation is similar to the
 
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