A SURVEY OF NATURAL ELECTROMAGNETIC NOISE IN THE FREQUENCY-RANGE F = 1-10 KHZ AT HALLEY STATION, ANTARCTICA - 1 - RADIO ATMOSPHERICS FROM LIGHTNING

This paper presents results from the first systematic survey of VLF wa ve activity at Halley, Antarctica (76 degrees S, 27 degrees W, L = 4.3 ). Beginning in 1971, the peak, average and minimum (P, A, M) signal l evels observed in four frequency bands centred on 0.75 kHz, 1.25 kHz, 3.2 kHz and 9.6 kHz have been recorded every 5 min. At these frequenci es the observed radio noise is largely natural, the waves being genera ted either in the magnetosphere (e.g. chorus, hiss, etc.) or near the ground, the latter principally from lightning discharges (radio atmosp herics, or ''spherics'') which reach receiver after propagating some d istance in the Earth-ionosphere waveguide (lightning does not occur in the immediate vicinity of Halley). Here we analyse the observations f or 1984, the first complete year for which we have data in digital for m, in terms of thunderstorm regions, as a benchmark for more extended studies of possible long-term change in global lightning activity. The data are presented in compressed colour graphic format which facilita tes the identification of periodic (diurnal and annual) and aperiodic variations. At 3.2 kHz, attenuation in the Earth-ionosphere waveguide is severe, and only relatively few spherics, from close lighting sourc e regions, are observed. Thus, whilst the 3.2 M channel is insensitive to lightning, and responds mostly to magnetospheric emissions, the 3. 2 P channel is dominated by spherics. The 3.2 P data show a marked diu rnal and seasonal variation symmetrical about Halley local noon and ab out the solstices, consistent with nearby sources and attenuation rate s for subionospheric propagation which are much greater during the day than at night. At 9.6 kHz, waveguide attenuation is much lower (and t here is less difference between day and night), and the minimum channe l is dominated by a continuum of spheric noise originating from global ly distributed distant source regions, notably those in the tropics. C onsequently, there is no control by the local dawn-dusk terminator, th e diurnal and seasonal variation is not symmetrical about Halley local noon and the solstices but consists of a quasi-sinusoidal diurnal var iation, in which the phases of the minimum and maximum vary during the year: similar to 07 LT (LT similar or equal to UT-2 h at Halley) and similar to 17 LT in December (summer) and similar to 10 LT and similar to 21 LT in June (winter). Agreement between the observations and the CCIR (1983) empirical model is poor. A somewhat better fit is given b y a simple model in which thunderstorm regions consist of point source s having radiated powers which vary with local rime and season, the to tal effect at Halley being modelled as the sum of contributions from t hese sources. (C) 1998 Elsevier Science Ltd. All rights reserved.