The time delay between pulses is equal to the IAR resonant period.
Thus, a possible role of distant thunderstorms in the IAR stimulation is still disputable.
Thus, the pulse repetition period is about the IAR fundamental period.
A possible scheme of the IAR excitation by a lightning is as follows (Fig. 9).
The other possible energy source for the IAR excitation is related to lightning discharges.
While the spectral properties of the IAR are well reproduced by numerous analytical and numerical models (e.g., Prikner et al., 2000), a physical mechanism of the IAR excitation has not been found yet.
Both direct and reflected pulses reach an observation site with a negligible delay for the IAR and SR frequency ranges.
The "feedback instability" due to the ionospheric modification by the energetic electron precipitation can also stimulate the IAR (Lysak, 1991; Pokhotelov et al., 2001).
The damping rates and Q-factors of the IAR modes strongly depend on the Alfven velocity differential in the upper ionosphere and on the lower ionosphere conductivity.
The coupling between shear and fast modes in the IAR can be illustrated by observations of the spectral resonance structure of these modes.
Therefore, the fundamental eigenmode of the IAR is 0.38 to 0.61 Hz for the good conductivity and poor conductivity cases, respectively.
