The statistics of natural ELF/VLF waves derived from a long continuous set of ground-based observations at high latitude

This paper analyses a unique set of continuous high-quality well-calibrated observations of natural ELF/VLF radio waves, in the range 0.3–10 kHz, made at Halley Research Station, Antarctica (76∘S,27∘W,L=4.5)(76∘S,27∘W,L=4.5) over one and a half solar cycles (1992–2007). Reference is also made to similar but shorter data sets obtained from other Antarctic stations. The observed waves vary over a very wide dynamic range, from the receiver noise level of ∼20dB wrt 10−33T2Hz−1 (at 1 kHz) up to 40–50 dB above it, over a wide range of timescales. However, the long continuous data set allows us to average out the random and aperiodic variations to extract the underlying dependence of the wave characteristics on local time, time of year, solar cycle, etc. Below about 5 kHz the received waves are predominantly whistler-mode waves, notably chorus, which are generated in the magnetosphere and propagate on geomagnetic field-aligned (“ducted”) paths to low altitudes. At the top of the frequency range (∼10kHz) the observed waves are mostly atmospherics from tropical lightning. The spectrum, and dependence on local time and season, are discussed in terms of a source function and a propagation function from the source region through the ionosphere (in the case of the magnetospheric waves) and under the ionosphere. The dependence of the waves on latitude, geomagnetic activity, solar cycle and day of the week are also described.