Magnetospheric MHD Resonances and ULF Pulsations - Ultra and Extremely Low Frequency Electromagnetic Fields - page 253

spectrum must fall off with a decrease in frequency followed by the decrease in
inclination angle of the lines 2 0 and 3 0 as shown in Fig. 6.14 .
The lines 3 and 3 0 , which correspond to the nighttime parameters of the
ionosphere, lie below the experimental data. To explain this discrepancy with
observations, one may assume the presence of supplementary sources, which
contribute the ULF noise at nighttime.

6.4.7 Neutral Gas Turbulence

Turbulence of neutral gas flow in the altitude range of the E layer can serve as an
alternative excitation source of the ULF electromagnetic noise. As we have noted
above, if a neutral gas flow is stirred in some region with size , turbulization of
flow may occur in a so-called inertial subrange, 1 k 1 Re 3=4 ,ink

space. It is usually the case that the Reynolds number, Re, tends to maximize in
the vicinity of turbopause and it can be large enough in the E-layer, that is about
10 2 -10 4 as it follows from the assessment we made in Sect. 5.3.6 . The Kolmogorov
spectrum covers the frequency range given by Eq. ( 5.72 ). Assuming for the moment
that the smoothed mean mass velocity of the gas flow is V D 10 2 m/s, and the
typical scale of the turbulization of flow is D 10 2 km we get the estimate
1:6 10 4 f .0:005-0:16/ Hz. According to the Kolmogorov theory for

an isotropic homogeneous medium, in this frequency region the mechanical energy
of the turbulent flow has a power law spectrum / k 5=3 .
The correlation matrix of the ionospheric wind-driven current can be expressed

through the spectral density of the mass velocity fluctuations D ıV l ıV p E which in
turn is proportional to the spectral density of the mechanical energy. Since the
typical frequencies of turbulent pulsations are evaluated as ! kV , we can
thus assume that the functions F lp .!/ / ! 5=3 . Considering 2D distribution
of the height-integrated currents in the ionosphere we come to the dependence
‰ . B /

xx .!/ / ! 11=3 . The spectral index 11=3 of the correlation function ‰ . B / xx .!/

slightly differs from the best fit value 3, which corresponds to the data shown in
Fig. 6.14 . The best hope for that is the case of 1D distribution of the wind-driven
ionospheric currents when we obtain ‰ . B /

xx .!/ / ! 8=3 .

6.4.8 Random Variations of Background Atmospheric

Current and Conductivity

The mean value of the background atmospheric current density due to atmospheric
conductivity is about .3:5-4/ 10 12 A=m 2 . Recently Davydenko et al. ( 2004 )
have studied the electric environment of a mesoscale convective system (MCS).
The typical size of the MCS-trailing stratiform region was estimated to be about