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 spectral power measurements made at Halley, Antarctica (76 ¿S, 26 ¿W), show at least two significant periodicities. The largest variation (10 dB) is due to the diurnal cycle and is strongly dominated by South American thunderstorm activity levels. The effect of African thunderstorm activity is observable in the diurnal variation plots but is strongly reduced by disadvantageous propagation conditions. The analysis suggests that given two identical tropical source regions, one situated east and one situated west, of a polar receiver site, the region to the west will tend to dominate the diurnal variation. The smaller annual variation at Halley (3 dB) can be explained by the improved subionospheric propagation conditions during the Southern Hemisphere winter counterbalancing the seasonal effects of decreased source activity in South America. ¿ 1999 American Geophysical Union