Diurnal and annual variations in 10-kHz radio noise

It is shown that the investigation of lightning activity levels, using very low frequency radio emissions near 10 kHz, needs to take into account the significant influences of subionospheric propagation. Ten-kilohertz spectra l power measurements made at Halley, Antarctica (76 degrees S, 26 degrees W ), show at least two significant periodicities. The largest variation (10 d B) is due to the diurnal cycle and is strongly dominated by South American thunderstorm activity levels. The effect of African thunderstorm activity i s observable in the diurnal variation plots but is strongly reduced by disa dvantageous propagation conditions. The analysis suggests that given two id entical tropical source regions, one situated east and one situated west, o f a polar receiver site, the region to the west will tend to dominate the d iurnal variation. The smaller annual variation at Halley (3 dB) can be expl ained by the improved subionospheric propagation conditions during the Sout hern Hemisphere winter counterbalancing the seasonal effects of decreased s ource activity in South America.