28 day modulation of global lightning activity and its relation to tropical cloud coverage and solar activity

In order to investigate characteristics of global lightning activity changes, we analyzed Schumann resonance (SR) spectral intensity variation using 1-100 Hz ELF magnetic field waveform data obtained at Syowa station (69.0°S, 39.6°E) for the period between February 2000 and January 2003. We calculated dynamic spectra of ELF data with the fast Fourier transform (FFT) method and extracted spectral powers at the first three resonance modes (8, 14, 20 Hz) as a function of day. Then, we calculated a power spectrum of the SR spectral intensity variation. It is found that there are steep peaks at 28 and 11, and multiple peaks around ~4-6 days. Though it is suggested that the ~10 and ~5 day periodicities are highly associated with planetary wave activity in the tropical region, clear evidence for the presence of the 28 day periodicity over three years has been found for the first time in this study. As a next step, we analyzed the composite infrared cloud images to examine the relationship between the SR spectral intensity variation and the variation of the tropical cloud coverage. From the cross-spectral analysis between these variations it is found that the cross spectrum showed a steep peak at the 24 day period and that the phase histogram showed the clear anti-phase relation. The discovered anti-phase relation may be best explained by the decrease of lightning activity, driven by the decrease of the heat flux and the convective available potential energy (CAPE) needed to induce thunderstorms. Further, we performed dynamic cross-spectral analysis between the SR spectral intensity variation and solar and geomagnetic activity parameters such as F10.7 index, sunspot number, Kp index , Dst index, cosmic ray flux, and relativistic electron and ion fluxes measured by the GOES-8 satellite. Though the cross-spectrum peaks in the period range from 20 to 30 day, the coherence values in this period range are estimated to be less than 0.6. In addition, there is no clear one-to-one phase relationship among these parameters. These facts imply that 28 day modulation of global lightning activity may not be directly controlled by these solar and geomagnetic activity parameters.