Atmospheric Electricity - Ultra and Extremely Low Frequency Electromagnetic Fields - page 63

1

2

1′

2′

Fig. 3.5 Vertical and horizontal dipoles above the perfect conducting ground. The vertical dipole
is approximately duplicated due to the electric images in the ground while the horizontal dipole is
converted to quadrupole

at stratospheric and mesospheric altitudes (Franz et al. 1990 ) is discussed in the
Sect. 1.3.2 . The IC discharges are strongly prevalent over the CG and transient
luminous events (TLEs). Note that the peak current of the IC discharges is one
order of magnitude smaller than that for CG discharges. However the IC discharges
may have a length up to 50-150 km.
Effectiveness of the vertical and horizontal discharges, considered as electromag-
netic wave transmitter, is extremely different (Kudintseva et al. 2009 ; Hayakawa
et al. 2012 ). To illustrate this, we approximate the actual discharge with the effective
electric dipole/antenna shown in Fig. 3.5 with lines 1 (CG) and 2 (IC). If the ground
is considered as a perfect conductor, the electric field of induction electric charges
arising on the ground surface is equivalent to the electric field of dipole image,
which is in the ground symmetrically with respect to the ground surface as shown
in Fig. 3.5 with lines 1 0 and 2 0 . In the vertical case it practically produces the
duplication of the net dipole moment, whereas in the horizontal case the net dipole
moment vanishes, so that the horizontal antenna is equivalent to a quadrupole.
This implies that CG lightning can be a much stronger radiator as compared to
IC especially in ULF/ELF (extremely low frequency, <3kHz) range. One more
argument in favor of this statement is the presence of the so-called continuing
current (CC) following the CG flash because the CC may greatly contribute to the
ULF/ELF portion of the lightning spectrum. The IC lightning cannot, in general, be
observed at distances as great as CG lightning (Heavner et al. 2003 ). The energetic
intracloud (EIC) discharges are the most powerful source of lightning radiation
in the HF (high frequency) and VHF (very high frequency) radio bands (e.g., see
Smith et al. 1999 , 2004 ; Jacobson 2003 ). Certainly the portion of the EIC with
vertical channels may also contribute to the background ULF/ELF electromagnetic
noise produced by the global lightning activity. However in what follows the main
emphasis is on the CG lightning as the most creditable candidate for the excitation
of global electromagnetic low-frequency resonances.