Wave Injection with the HAARP HF Heater: Conditions and Coherence Limits for Magnetospheric Amplification

Controlled magnetospheric wave injection experiments are carried out with ELF/VLF waves generated by heated modulation of the ionospheric auroral electrojet currents. In an on-going experiment with the High Frequency Active Auroral Research Program (HAARP) facility located at L = 4.9, 500 Hz - 5 kHz waves are used to trigger wave-particle interactions that result in the non-linear amplification of the wave, known as the coherent whistler mode instability of VLF triggered emission phenomenon. The amplified signals show temporal growth rates of 15-30 dB/sec as well as triggering of free running emissions. The amplification process is sensitive to select frequency bands and to selectively amplify specific frequency-time formats in these bands. It is found that propagation to the magnetospheric interaction region via magnetospheric ducts may be one of the factors determining the occurrence of observations. Special formats designed to test the coherence limits of the input wave for the interaction are transmitted. Results show that while increasing the bandwidth of the input wave does not quench amplification, a minimum temporal duration for input pulses is necessary.