Abstract: Using experimental measurements and theoretical analysis, it is shown that the HF/ELF conversion efficiency
is controlled by the timescale for electron temperature saturation. This is a function of the heater ERP and frequency and
the ionospheric electron density profile. For the current HAARP parameters this corresponds to frequencies between 2
and 4 kHz. Efficiency optimization techniques as applied to the projected upgrading of the HAARP heater to its design
power of 3.6 MW are discussed.
A most fascinating and important property of the active ionosphere is its potential to act as a frequency transformer that
converts HF power injected from a high power HF transmitter into the ionosphere, into coherent lower frequency
VLF/ELF/ULF waves. The conversion principle relies on modulating the electrojet currents in the ionospheric D and E
regions by using amplitude modulated HF heating. The low frequency fields subsequently couple to the earth-ionosphere
waveguide, while a fraction of their power upwards towards the magnetosphere. Despite several years of theoretical and
experimental work, many scientific and technical issues remain unresolved. Understanding the physics underlying the
low frequency wave generation is important in increasing the HF to ELF conversion efficiency and utilizing the
technique for ionospheric diagnostics.
A puzzling feature in the results of the experiments conducted using the EISCAT and the HAARP ionospheric heaters,
has been the variation of the conversion efficiency with ELF/VLF frequency and the unusually large relative amplitude
of the harmonics. Prominent features of the data are:
1. Enhanced efficiency relative to the neighboring frequencies at 2 kHz and its harmonics.
2. Maximum efficiency in the frequency range between 2-4 kHz. Efficiency proportional to the frequency f
between 2 kHz and 500 Hz. Weak increase in efficiency between 500 and 100 Hz. Efficiency proportional to
1/f between 4 and 10 kHz.
3. Harmonics with significant relative amplitudes up to ten or larger are present. The amplitudes of the
harmonics are much higher than expected from Fourier analysis of the HF heating waveforms.
These basic features are seen consistently in exp eriments using heater facilities and taken under different heating
parameters and ionospheric conditions. We believe that they are intrinsic features representatives of the ongoing nonlinear
physics. As discussed previously [1-4] and in the absence of propagation effects the conversion efficiency
depends on the value of the ambient electric field in the modified region and the spatio-temporal waveform of the
modified conductivity in response to the HF heating pulse. Since the first factor, that controls the maximum value of the
modified current, is beyond our control, our investigation focused on understanding the physics of the second factor. We
present below the first temporally resolved ELF/VLF waveforms measured during modulated ionospheric heating. The
results are compared with theoretical models and their implications discussed.