ELF Generation Using HAARP

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The polar electrojet, as this phenomenon is called, carries currents that often exceed a million amperes. The current is distributed within a sheet 100 km or more wide so that the current density in any given region of the layer is low. The latitude at which the polar electrojet is overhead is dependent on the time of day (since the current flows in a roughly circular arc centered on the magnetic pole which, in turn is offset from the geographic pole). The equatorward extent of the polar electrojet is also a strong function of solar activity, reaching lower latitudes during active and storm level conditions and receding to the north as the geomagnetic field becomes quiet. Depending on the time of day, the current may be flowing east-to-west or west-to-east.

Active ionospheric techniques are able to temporarily affect a small portion of the layer in which the electrojet flows causing a minute change in the bulk electrical conductivity of that region. Because electrical current tends to favor higher rather than lower conductivity media, current flowing through the affected volume will decrease slightly relative to the current flowing through surrounding unaffected regions. When the stimulation is removed, the electrical properties in the volume under study rapidly return to ambient levels and the electrojet currents return to their natural distribution. The rate at which the conductivity can be increased and decreased is a critical physical parameter because it determines the highest frequency at which variations can be induced onto the current flowing within the study volume.

A propagating electromagnetic field is generated whenever a varying current flows along a conductor. In the study of this application, an active ionospheric research facility such as HAARP transmits a fundamental signal using a frequency chosen to deposit energy into the layer carrying the electrojet currents. Under typical ionospheric conditions, this frequency would be near the lower end of the facility's 2.8 - 10 MHz operating range. The signal is caused to vary in amplitude by a low frequency modulation such that the strength of the fundamental HF signal varies in a regular (or periodic) manner. During the peaks of the transmitted signal (when the transmitted power is larger), the layer volume absorbs energy and its local conductivity decreases. This causes a small net decrease in the local current flowing through that volume.

During troughs (or minima) in the fundamental HF signal, as the transmitted power approaches zero, the volume conductivity returns to normal and the net current in the region returns to its original value. Because the current in the layer is varying over a volumetric region having dimensions of a few tens of kilometers, a weak but scientifically useful electromagnetic signal is generated at the modulation frequency.

The upper limit of frequencies that can be generated using this technique depends on the rate at which the layer's electrical properties can be changed and at which they return to normal. Studies at other active research facilities have shown that frequencies up to approximately 20 - 30 kHz can be generated in this manner. The level of the signal measured on the ground is quite small and can only be detected using special purpose, correlating receivers. Typically, the level is on the order of 1 - 3 pT in the ELF range. Based on many ground observations and mathematical models of the physical process, it is believed that, out of the million amperes naturally flowing in the electrojet, only about one ampere can be influenced in this manner to generate a useful signal.

Much of the early work in this research area was conducted by Prof. Anthony Ferraro [1] of Penn State University and by scientists from the Max Planck Institute at Tromsoe, Norway as reported by Stubbe [2]. Recent research at HIPAS in Fairbanks, Alaska by McCarrick and others was reported in the journal Radio Science [3].

[1] Ferraro, A. J., Lee, H. S., Allshouse, R., Carroll, K., Tomko, A. A., 
    Kelly, F. J., and Joiner, R.G., VLF/ELF Radiation from the ionospheric dynamo 
    current system modulated by powerful HF signals, J. Atmos. Terres. Phys., 
    44, 1113-1122, 1982.
    
[2] Stubbe, P., Kopka, H., Rietveld, M. T. and Dowden, R. L., ELF and VLF wave 
    generation by modulated HF heating of the current carrying lower ionosphere, 
    J. Atmos. Terr. Phys., 44, 1123-1135, 1982b.
    
[3] Several papers in Radio Science, Volume 25, Number 6, 
    November-December 1990.

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