A 25-year record of 10 kHz sferics noise in Antarctica: Implications for tropical lightning levels

We report measured levels of very low frequency (VLF) radio noise at simila r to 10 kHz, due to lightning sferics, observed at Halley Station, Antarcti ca (76 degrees S, 27 degrees W) between 1971 and 1996. The observed VLF noi se levels at Halley are a product of the thunderstorm source function and t he transfer function for propagation to the receiver in the waveguide forme d by the Earth's surface and the ionosphere. Least squares fitting enables us to confirm the characteristic diurnal, annual and semi-annual periodicit ies found by the present authors in a separate paper. That method and also cross correlation of annual averaged 10 kHz VLF power with sunspot number s hows a similar to 4 dB peak-to-peak fluctuation at the similar to 11-year s olar cycle period, believed to be due to the influence of EUV flux on the t he ionospheric D region. Finally we constrain any linear trend to 1.4 +/- 2 .6 dB in 25 years. If a positive trend is present and is interpreted as a c hange in tropical South American lightning hash rate, it is less than 10 %.