EXCITATION OF ELF WAVES IN THE SCHUMANN RESONANCE RANGE BY MODULATED HF HEATING OF THE POLAR ELECTROJET

We report the successful generation and detection of ELF waves in the frequency range of 6-76 Hz by polar electrojet modification using the High-Power Auroral Stimulation (HIPAS) HF Heater Facility near Fairbanks, Alaska. Magnetic field amplitudes of approximately 1 pT and vertical electric field amplitudes of approximately-0.2 mV have been observed at a receiving site 35 km from the HIPAS site. The wave amplitude does not depend strongly on the ELF frequency, however, the amplitude is closely related to the level of electrojet activity inferred from magnetometer chain data and from ionosonde measurements. The 1-MW HF heater is modulated at low ELF frequencies using an "array dephasing" technique where the eight-element antenna array is alternately phased for peak vertical gain and dephased for a spread pattern at the ELF rate. This method can be employed at any modulation frequency without concern for transmitter power supply resonances. Measurements of the wave amplitude are made by conventional analog lock-in techniques and by continuously digitizing the waveform output of the wideband magnetic search coils and the electric field detector. With the digitized waveform, digital postprocessing can be used to extract the low-level coherent signal from the Schumann resonance noise background using a software lock-in routine.