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 ∼10 kHz, due to lightning sferics, observed at Halley Station, Antarctica (76°S, 27°W) between 1971 and 1996. The observed VLF noise levels at Halley are a product of the thunderstorm source function and the transfer function for propagation to the receiver in the waveguide formed by the Earth's surface and the ionosphere. Least squares fitting enables us to confirm the characteristic diurnal, annual and semi-annual periodicities 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 shows a ∼4 dB peak-to-peak fluctuation at the ∼11-year solar cycle period, believed to be due to the influence of EUV flux on the the 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 change in tropical South American lightning flash rate, it is less than 10%.

[1]  C. Price.  Global surface temperatures and the atmospheric electrical circuit , 1993 .

[2]  N. Watkins,et al.  Diurnal and annual variations in 10‐kHz radio noise , 1999 .

[3]  A. Fraser-Smith,et al.  Global lightning and climate variability inferred from ELF magnetic field variations , 1997 .

[4]  Andy J. Smith,et al.  VELOX: a new VLF/ELF receiver in Antarctica for the Global Geospace Science mission , 1995 .

[5]  J. Houghton,et al.  Climate change 1995: the science of climate change. , 1996 .

[6]  E. Williams.  The Schumann Resonance: A Global Tropical Thermometer , 1992, Science.

[7]  G. Sátori,et al.  Spectral characteristics of Schumann resonances observed in central Europe , 1996 .

[8]  R. Barr.  The ELF and VLF amplitude spectrum of atmospherics with particular reference to the attenuation band near 3 kHz , 1970 .

[9]  E. Williams.  Global Circuit Response to Seasonal Variations in Global Surface Air Temperature , 1994 .

[10]  Steven R. Schroeder,et al.  Widespread tropical atmospheric drying from 1979 to 1995 , 1998 .

[11]  M. Jarvis,et al.  Satellite Experiments Simultaneous with Antarctic Measurements (SESAME) , 1995 .

[12]  D. Rind,et al.  Possible implications of global climate change on global lightning distributions and frequencies , 1994 .

[13]  P. J. Jenkins,et al.  A SURVEY OF NATURAL ELECTROMAGNETIC NOISE IN THE FREQUENCY RANGE F= 1-10 KHZ AT HALLEY STATION, ANTARCTICA : 1. RADIO ATMOSPHERICS FROM LIGHTNING , 1998 .

[14]  R. Markson.  Tropical convection, ionospheric potentials and global circuit variation , 1986, Nature.