Ionospheric Alfvén Resonator (IAR) - Ultra and Extremely Low Frequency Electromagnetic Fields - page 165

Fig. 5.7 Same as in Fig. 5.6 , but for other temporal sampling. Taken from Surkov et al. ( 2006 )

5 pT. Total number of the impulses, N, with amplitude, which is greater than
this level, increases with time as shown in Fig. 5.10 . The lines 1 and 2 correspond
to the frequency filters 6-20 and 0:25-4 Hz, respectively. Averaging over interval
of 1 h results in a mean rate number of the impulses of about 1 D 0:144 s 1
and 2 D 0:023 s 1 , which is typical for the nighttime conditions at Karimshino
station. A thunderstorm typically produces the rate number of about 0:03-0:05 s 1 ,
that is close to 2 . From here we may assume that one nearby thunderstorm and
3-6 remote ones make a major contribution to the rate number shown in Fig. 5.10 .
Concerning the signals of remote thunderstorms, it should be noted that the intensity
of the H component is larger than that of the D component. The impulses of the H
component (6-20 Hz) that are displayed in Fig. 5.10 with dashed lines 3 occur more
frequently than in the D component shown with dashed line 4. On the other hand,
in the frequency range of 0:25-4 Hz the rate numbers of both components are very
close to each other.
Most of intense signals, which can be associated with nearby thunderstorm,
have a bipolar structure as is seen from Figs. 5.6 and 5.7 . It appears that the first
impulse in the signals is due to the primary wave radiated by the return stroke.
Since the interval between positive and negative impulses is typically 2 s, one may
assume that the second impulse results from Alfvén wave reflection from the Alfvén
velocity gradient at the upper boundary of the resonance cavity. If the typical size of
the resonance cavity is 500-1;000 km, the arrival time of the reflected Alfvén wave