ELF/VLF wave generation by modulated HF heating of the auroral electrojet

The generation of electromagnetic waves in the Extremely Low Frequency (ELF, 3-3000 Hz) and Very Low Frequency (VLF, 3-30 kHz) bands by modulated High Frequency (HF, 3-30 MHz) heating of the auroral electrojet current system is investigated experimentally, and observations are compared against the predictions of a theoretical model.; Experimental evidence is presented to demonstrate the regular occurrence of ELF/VLF amplitude saturation as a function of peak HF power level. The observed ELF/VLF amplitude saturation is examined as a function of modulation frequency and as a function of time of day. For modulation depths less than 100%, the dependence of ELF/VLF amplitude on average HF power is investigated, yielding an optimal average HF power level which maximizes the observed ELF/VLF amplitude. Observations of ELF/VLF amplitude saturation under a large range of geomagnetic conditions indicate that the identified saturation process occurs on a regular basis. Furthermore, observations indicate that the spatial distribution of the modulated ionospheric conductivity can be remotely sensed using ground based measurements of the horizontal component of the generated ELF/VLF magnetic flux density.; Each of these experimental observations is interpreted in the context of an HF heating model which accounts for the Earth's magnetic field and for the altitudinal variation of several ionospheric constituents. This model indicates that the variation in ELF/VLF amplitude at low HF power levels is dominated by the HF self-absorption process, whereas the variation in ELF/VLF amplitude at high HF power levels results from the competition between the electron energy losses associated with the rotational excitation of molecular nitrogen and the vibrational excitation of molecular oxygen.