Comparison of ELF/VLF generation modes in the ionosphere by the HIPAS heater array

The characteristics of three different modes of ELF and VLF excitation using the high-power auroral simulation (HIPAS) HF heater array are compared under different ionospheric conditions. For each of these three methods, amplitude modulation (AM), phase demodulation (DM), and the double-frequency excitation (DF), we observed signals on the order of a few picoteslas at two widely separated receiver sites for both X and O mode heater polarizations. Strong electrojet activity is essential for ELF generation at frequencies close to the Schumann range, whereas at higher frequencies, such as in the VLF range (greater than 1 kHz), signals could be observed under a wider range of ionosopheric conditions. The AM method generally produced the largest signal at both frequency ranges, while the DM mode was approximately half of this signal amplitude. The DF generated signals are comparable in strength with the AM signals at VLF frequencies and are more stable than either method as a function of time. The X mode polarization also produces a stronger signal than the O mode for these three methods of excitation by a factor of two. The polarization of the received signals follow the same variation at VLF frequencies for all three modes, indicating a common height of origin.