Electrojet Modulation ELF Communications

The causes for the inefficient conversion of HF to ELF waves by ionospheric heating observe in recent experiments are examined. It is found that for the low frequency HF range 2.5-4 MHz used in the present experiments the modified conductance saturates during a fraction of the HF irradiation cycle. As a result most of the HF energy is transferred to processes that do not affect the conductance significantly. This was experimentally confirmed using the HIPAS facility. Techniques relying on fast HF antenna sweeping by which the efficiency can increase by more than two orders of magnitude are presented.