the original design goal of HAARP was for low frequency communications with submarines. low frequencies was chosen because of their ability to cover distances beyond the line of sight.High Frequency Active Auroral Research Program. it transmits at 2.8-10 MHz radio frequencies. so it isn't low frequency. plus the 3.6MW is naff all power in the scheme of things.
i believe it's around 5 hertz.what frequency is this resonance?