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Modulated heating of the lower ionosphere with the HAARP HF heater is used to excite 1–2 kHz signals observed on a ship‐borne receiver in the geomagnetic conjugate hemisphere after propagating as ducted whistler‐mode signals. These 1‐hop signals are believed to be amplified, and are accompanied by triggered emissions. Simultaneous observations near (∼30 km) HAARP show 2‐hop signals which travel to the northern hemisphere upon reflection from the ionosphere in the south. Multiple reflected signals, up to 10‐hop, are detected, with the signal dispersing and evolving in shape, indicative of re‐amplification and re‐triggering of emissions during successive traversals of the equatorial interaction regions.

We have constructed instruments to measure the atmospheric conduction current and the atmospheric electric field: two fundamental parameters of the global-electric circuit. The instruments were deployed at the Amundsen-Scott South Pole Station in January 1991 and are designed to operate continuously for up to one year without operator intervention. The atmospheric current is measured by a sensor that uses a split-hemispheric conducting shell of 17.8-cm radius, separated by a thin Teflon insulating disk. The detection electronics are inside the sphere. In principle, the atmospheric current flows into one hemisphere, through the electronics where it is measured, and out the other hemisphere. The electric field is measured by a field mill of the rotating dipole type. The electric field sensing elements are two 30-cm-long antennas, driven to rotate in the vertical plane at 1800 rotations per minute. Two arrays of identical instruments have been deployed, separated by 600 m, in order to distinguish between atmospheric electrical signals of local and global origin. The separation distance of the arrays was determined by the climatology of the Antarctic plateau. Sample data from the first days of operation at the South Pole indicate variations in the global circuit over time scales from minutes, to hours, to days. 1. 2612 BYRNE ET •,L.: ATMOSP•SRIC ELSCTRIC M.•,*,SURSMSNTS AT SouT• POLE tricity [Park, 1976; Cobb, 1977]. The Antarctic plateau has a desertlike climate with predominantly clear skies, very low atmospheric-aerosol content, and with prevailing winds which are generally light, flow in a nearly constant direction, and are relatively free of turbulent and convective motions [Dalrymple, 1966]. The Antarctic plateau is also thunderstorm free. Hence atmospheric-electrical measurements made at South Pole are relatively unperturbed by local meteorological conditions. Secondly, South Pole is located within or near the geomagnetic polar cap. Therefore atmospheric electricity measurements made at the pole are useful in investigating large-scale electrical processes unique to high latitudes [Byrne et al., 1991]. Thirdly, the downward air-Earth current being delivered to the surface of the Antarctic polar plateau is larger (• a factor of 2) than the

We present the performance characteristics of a high-sensitivity radio receiver for the frequency band 0.5-470 kHz, known as the Low Frequency Atmospheric Weather Electromagnetic System for Observation, Modeling, and Education, or LF AWESOME. The receiver is an upgraded version of the VLF AWESOME, completed in 2004, which provided high sensitivity broadband radio measurements of natural lightning emissions, transmitting beacons, and radio emissions from the near-Earth space environment. It has been deployed at many locations worldwide and used as the basis for dozens of scientific studies. We present here a significant upgrade to the AWESOME, in which the frequency range has been extended to include the LF and part of the medium frequency (MF) bands, the sensitivity improved by 10-25 dB to be as low as 0.03 fT/ √ Hz, depending on the frequency, and timing error reduced to 15-20 ns range. The expanded capabilities allow detection of radio atmospherics from lightning strokes at global distances and multiple traverses around the world. It also allows monitoring of transmitting beacons in the LF/MF band at thousands of km distance. We detail the specification of the LF AWESOME and demonstrate a number of scientific applications. We also describe and characterize a new algorithm for minimum shift keying demodulation for VLF/LF transmitters for ionospheric remote sensing applications.

A digital signal processing program has been developed to measure the phases of coherent VLF signals from analog tape recordings made in the field. The program uses a constant frequency pilot tone recorded with the VLF data to correct tape speed errors and reconstruct the signal phases. We analyze several examples of whistler mode signals from the VLF transmitter at Siple Station, Antarctica, as received at Roberval, Quebec. Pulses with temporal growth show a relative phase advance with time, and thus a positive frequency offset from the transmitted signal, often from the beginning of the pulse. Amplitude beating is often seen toward the end of a pulse, sometimes with phase cycle‐skipping as the emission becomes unlocked from the input signal. Current theories of wave‐particle interaction are reviewed and found to explain some of the observed signal features, though no theory predicts the initial frequency offset of a growing pulse.