Relationship between electrojet current strength and ELF signal intensity in modulated heating experiments

The High Frequency Active Auroral Research Program (HAARP) facility is used to generate waves in the extremely low frequency (ELF) range via modulated HF heating of the ionosphere. This HF heating modulates the electron temperature in the D region ionosphere and leads to modulated conductivity and a time-varying current which then radiates at the modulation frequency. We investigate the relationship between the intensity of the HAARP-generated ELF signal and the strength of the east-west component of the auroral electrojet as measured by a ground-based magnetometer. We find that under all magnetic conditions, HAARP can generate ELF radiation detectable 37 km away with 73% of tones having an amplitude exceeding 0.15 pT. While strong ELF amplitudes (>1.5 pT) were most common during an enhanced electrojet, a weak electrojet can also support equally high ELF amplitudes. The relative change in ELF amplitude per unit change in electrojet current strength is inversely proportional to the absolute current strength. We interpret the dynamic relationship between ELF amplitude and electrojet current strength in terms of the time-variable ionospheric parameters and HF heating efficiency.