Observations of amplitude saturation in ELF/VLF wave generation by modulated HF heating of the auroral electrojet

Abstract

We present detailed observations of the onset of amplitude saturation in ELF/VLF waves generated via modulated HF heating of naturally‐forming, large‐scale current systems, such as the auroral electrojet. Broadband ELF/VLF measurements at a ground‐based receiver located near the High‐Frequency Active Auroral Research Program (HAARP) HF transmitter in Gakona, Alaska, exhibit variations in signal amplitude which are qualitatively consistent with a hard‐limiting approximation of the saturation process. A method to approximate the saturation curve as a function of HF power from experimental data is presented, and the results indicate that a ∼5–10% reduction in generated ELF signal amplitude is typical at the maximum radiated HF power level (771 kW) for modulation frequencies between 1225 Hz and 3365 Hz. For HF transmissions using sinusoidal amplitude modulation, the saturation dominantly affects the second harmonic of the generated ELF/VLF signal, with amplitudes on average 16% lower than expected at the maximum HF power level.

The relationship between geophysical conditions and ELF amplitude in modulated heating experiments at HAARP: Modeling and experimental results

“…The HF heating component of the model is described in detail by Cohen et al [2010a] and builds off the work of Tomko et al [1980], Rodriguez [1994], Payne [2007], and Moore [2007]. To summarize, the three‐dimensional model uses a grid, which in this case is defined from 60–95 km in altitude and spans 160 km in each horizontal direction.…”

Generation of ELF and ULF electromagnetic waves by modulated heating of the ionospheric F2 region

“…A most unexpected result revealed by the simulations is the distribution of the ELF magnetic amplitude on the ground signatures as a function of distance from the heater, shown in Figure 9. Previous experience with ELF and ULF waves generated by modulating the Polar Electrojet (PEJ) [ Rietveld et al , 1984, 1987, 1989; Papadopoulos et al , 1990, 2005; Moore , 2007; Payne et al , 2007] indicated that the wave amplitude had a maximum in the vicinity of the heater while monotonically decreasing with distance in a fashion consistent with guided wave propagation. However, as noted in Section 3.3, ICD driven waves have a minimum at the ground location defined by its interception with the magnetic field line that passes through the heated volume.…”

“…The dynamics of the plasma is then governed by the inertia‐less electron momentum equation and the ion momentum equation where m e and m i are the electron and ion masses, v en and v in are the electron‐neutral and ion‐neutral collision frequencies, and P e is the externally modulated electron pressure, which provides the source of ELF/ULF electromagnetic waves. The model does not take into account nonlinearities associated with HF wave propagation through the D‐region, frequently used to generate ELF and ULF waves by modulating the Polar Electrojet (PEJ) [ Rietveld et al , 1984, 1987, 1989; Papadopoulos et al , 1990, 2005; Moore , 2007; Payne et al , 2007]. We have assumed that collisions with neutrals are dominant.…”

Low‐Frequency Waves in HF Heating of the Ionosphere

“…Previous results with ELF and ULF waves generated by modulating the polar electrojet (PEJ) [Rietveld et al, 1984[Rietveld et al, , 1987[Rietveld et al, , 1989Papadopoulos et al, 1990Papadopoulos et al, , 2005Moore, 2007;Payne et al, 2007] indicated that the wave amplitude had a maximum in the vicinity of the heater while monotonically decreasing with distance in a fashion consistent with guided wave propagation. In general, ICD driven waves have a minimum at the ground location defined by its intersection with the magnetic field line that passes through the heated volume.…”