|Lightning discharges between thunder clouds and the ground radiate powerful radio noise bursts over a wide frequency range from a few Hz to higher than several hundreds of Megahertz. Radio noises in the ELF (3 Hz -3 kHz) and VLF (3-30 kHz) range can propagate over long distance through the Earth-ionosphere waveguide [Barr et al., 2000]. Especially, radio noises in the frequency range less than 50 Hz can propagate globally with extremely low (less than 1 dB/Mm) attenuation rates [Jones, 1999]. Interferences of these globally propagating waves result in the Earth-ionosphere cavity resonances known as Schumann resonances (SR) with their fundamental mode at 8 Hz [Burke and Jones, 1996; Sentman, 1996; Huang et al., 1999; Jones, 1999]. The continuous SR background noises of amplitudes of ~1 mV/m or 3 pT are a superposition of individual pulses arriving from random lightning strokes whose occurrence rate is about 100 strokes/sec all over the world [Belyaev et al., 1999]. From these characteristics of SR waves, it is possible to monitor lightning activity in the world even if measurements of the magnetic or electric field of SR waves are carried out at only a single station.|
Occasionally, transient oscillations with a dominant frequency of 8 Hz are observed above the background SR noises. The amplitudes of these transient SR are a factor of 10 or more greater than the background noises. It is suggested that these transients are the Earth-ionosphere cavity resonances excited by extremely large lightning strokes and could be observed globally [Sentman, 1989]. Burke and Jones  suggested that the transient SR waves are likely to be produced by positive cloud-toground (+CG) discharges characterized as large current moments and slightly longer continuing currents. Additionally, recent observations of SR by Boccippio et al.  and Füllekrug and Reising  revealed that many of these transient SR waves are associated with transient luminous events in the mesosphere called gspritesh. They suggested that particularly large +CG discharges would excite both the Earth-ionosphere cavity resonances and the sprites simultaneously. Huang et al.  developed a technique to estimate the charge moments of lightning discharges associated with sprites from ELF data. They showed that the derived charge moments Qdl of 200-2000 C-km are sufficient to trigger conventional breakdown at sprite altitudes of ~50-70 km.
|Using these transient SR waves observed at multi observation point, it is possible to geo-locate CG location exciting these transients. Right figure shows the global IR cloud image and estimated CG location [Sato et al., 2002; Sato et al., 2003]. In this figure regions of high cloud top altitude are indicated white color, and red dots correspond to the estimated CG location. CG locations are estimated by using ELF data obtained at Syowa station in Antarctica and Onagawa observatory in Japan. At high cloud top regions strong upward convections are developed. That is enough to create strong thundercloud and intense lightning discharges. Continuous measurement of SR waves is the best way to monitor global lightning activities.|
It is also possible to estimate global occurrence regions and rates of sprites. First, intense transient SR waves are selected from ELF waveform data. Then, charge moments of CG discharges inducing these waves are estimated by using equations of normal mode expansion for SR waves. From the results by Hu et al. , sprite initiation probabilities of these CG discharges can be calculated. Finally, the occurrence rates and locations are derived.
Right figure shows a averaged diurnal sprite
occurrence rate in the summer season of northern hemisphere [Sato et
al., 2002]. It is found that the global occurrence rate of
sprites is estimated to be about 720 events/day on average. It is also found that the active regions of
sprite occurrences are located in North and South America, Africa and
Barr, R., D. Llanwyn Jones, and C.J. Rodger, ELF and VLF radio waves, J. Atmos. Solar-Terr. Phys., 62, 1689, 2000.
Belyaev, G.G., A.Yu Schekotov, A.V. Shvets, and A.P. Nickokaenko, Schumann resonances observed using Poynting vector spectra,
J. Atmos. Solar-Terr. Phys., 61, 751, 1999.
Boccippio, D.J., E.R. Williams, S.J. Heckman, W.A. Lyons, I.T. Baker, and R. Boldi, Sprites, ELF transients and positive ground strokes,
Science, 269, 1088, 1995.
Burke, C.P., and D.L. Jones, On the polarity and continuing currents in unusually large lightning flashes deduced from ELF events,
J. Atmos. Terr. Phys., 58, 531, 1996.
Füllekrug, M., and S.C. Reising, Excitation of Earth-ionosphere cavity resonances by sprite-associated lightning flashes,
Geophys. Res. Lett., 22, 4145, 1998.
Huang, E., E. Williams, R. Boldi, S. Heckman, W. Lyons, M. Taylor, T. Nelson, and C. Wong, Criteria for sprites and elves based on
Schumann resonance observations, J. Geophys. Res., 104, 16943, 1999.
Jones, D.L., ELF sferics and lightning effects on the middle and upper atmosphere, Modern Radio Science 1999, pp. 171, 1999.
M. Sato, H. Fukunishi, Y. Takahashi, T. Adachi, A. Okubo, and W.A. Lyons: Charge moment differences in winter of Japan and
summer of the United States, EOS Trans. AGU, 83(47), Fall Meet. Suppl., F136, 2002.
M. Sato, H. Fukunishi, M. Kikuchi, H. Yamagishi, and W.A. Lyons, Validation of sprite-inducing cloud-to-ground lightning based on
ELF observations at Syowa station in Antarctica, Journal of Atmospheric Solar-Terrestrial Physics, 65, 607-614, 2003.
Sentman, D.D., Schumann resonance spectra in a two-scale-height Earth-ionosphere cavity, J. Geophys. Res., 101, 9479, 1996.
Sentman, D.D., Detection of elliptical polarization and mode splitting in discrete Schumann resonance excitation,
J. Atmos. Terr. Res., 51, 507, 1989.