Ionospheric Alfvén Resonator (IAR) - Ultra and Extremely Low Frequency Electromagnetic Fields - page 178

0.06

0.055

7

0.05

6

0.045

4

5

0.04

3

0.035

2

0.03

1

0.025

0.02

0.5 1 1.5 2 2.5 3 3.5 4

f , Hz

Fig. 5.16 Square root of the spectra of correlation function, . xx =2/ 1=2 , on the ground for
nighttime conditions. The thunderstorm sites were randomly distributed along the circumferences
as shown in Fig. 5.15 . In the model the spectra shown with lines 1-7 correspond to different number
of the thunderstorms, which make a contribution to the spectra. These numbers are defined by the
radius of area occupied by the thunderstorm sites. In making the plots of xx shown with lines 1-7
we have used r D 0:55, 1:1, 1:65, 2:2, 3:3, 4:4,and5:5 thousands km. Taken from Surkov et al.

( 2006 )

It is obvious from Fig. 5.16 that all the spectra exhibit distinct SRS and the
resonant frequencies are close to the IAR eigenfrequencies. Starting with distance
of 1-2 thousand kilometers the relative peaks weakly depend on the thunderstorm
number. This means that only a few nearby thunderstorms dominate in the SRS
signature which can be detected on the ground. The remote thunderstorms are of
little importance since the relative amplitudes of peaks are practically independent
of distance as r>.1:5 2/ 10 3 km. The plots shown in Fig. 5.17 partly contradict
to this notation since the first two spectra do not exhibit any distinct SRS. By
chance, in this case the nearby thunderstorm distribution was so symmetric that
all the random angles ' were close to =2 and 3=2. In such a case the resonant
component g r makes a little contribution to the correlation function xx .
Model calculations of the spectrum peaks versus distance at fixed frequencies are
displayed in Fig. 5.18 . The fixed frequencies f D 0:5, 1:0, and 1:5 Hz correspond to
the first nighttime IAR eigenmodes. As before the distance, r, denotes the radius of
the area, S, which makes a contribution to the spectra. In making the plot of xx we
have used the model sketched in Fig. 5.15 and r 1 D 400 km (lines 1-3) and 550 km
(lines 4-6).