Electrojet-independent ionospheric extremely low frequency/very low frequency wave generation by powerful high frequency waves

Results of extremely low frequency/very low frequency (ELF/VLF) wave generation by intensity-modulated high frequency (HF) heaters of 3.2 MHz in Gakona, Alaska, near local solar noon during a geomagnetic quiet time, are presented to support an electrojet-independent ELF/VLF wave generation mechanism. The modulation was set by splitting the HF transmitter array into two subarrays; one was run at cw full power and the other run alternatively at 50% and 100% power modulation by rectangular waves of 2.02, 5, 8, and 13 kHz. The most effective generation was from the X-mode heater with 100% modulation. While the 8 kHz radiation has the largest wave amplitude, the spectral intensity of the radiation increases with the modulation frequency, i.e., 13 kHz line is the strongest. Ionograms recorded significant virtual height spread of the O-mode sounding echoes. The patterns of the spreads and the changes of the second and third hop virtual height traces caused by the O/X-mode heaters are distinctively different, evidencing that it is due to differently polarized density irregularities generated by the filamentation instability of the O/X-mode HF heaters.