Observations of Ionospheric ELF and VLF Wave Generation by Excitation of the Thermal Cubic Nonlinearity

Abstract

Extremely-low-frequency (ELF, 3-3000 Hz) and very-low-frequency (VLF, 3-30 kHz) waves generated by the excitation of the thermal cubic nonlinearity are observed for the first time at the High-Frequency Active Auroral Research Program high-frequency transmitter in Gakona, Alaska. The observed ELF and VLF field amplitudes are the strongest generated by any high frequency (HF, 3-30 MHz) heating facility using this mechanism to date. This manner of ELF and VLF generation is independent of naturally forming currents, such as the auroral electrojet current system. Time-of-arrival analysis applied to experimental observations shows that the thermal cubic ELF and VLF source region is located within the collisional D-region ionosphere. Observations are compared with the predictions of a theoretical HF heating model using perturbation theory. For the experiments performed, two X-mode HF waves were transmitted at frequencies ω1 and ω2, with |ω2-2ω1| being in the ELF and VLF frequency range. In contrast with previous work, we determine that the ELF and VLF source is dominantly produced by the interaction between collision frequency oscillations at frequency ω2-ω1 and the polarization current density associated with the lower frequency HF wave at frequency ω1. This specific interaction has been neglected in past cubic thermal nonlinearity work, and it plays a major role in the generation of ELF and VLF waves.

Multistation observations of the azimuth, polarization, and frequency dependence of ELF/VLF waves generated by electrojet modulation

“…Amplitude modulation and continuous wave modulation techniques were compared by Barr and Stubbe [1997], and all three techniques are experimentally compared by Cohen et al [2010b]. Additional approaches to ELF/VLF generation independent of the ionospheric currents include the so-called ionospheric current drive [Papadopoulos et al, 2011] and cubic nonlinearity [Kotik and Ermakova, 1998;Moore et al, 2013].…”

Past, Present and Future of Active Radio Frequency Experiments in Space

“…expected that f1 wave induces a collision frequency oscillation at 2f1 frequency and that these oscillations of the collision frequency will interact with the oscillations of the polarization current density caused by f2 wave and produce ELF and VLF source current density with the frequency | f2 -2f1|. Moore et al [2013] demonstrate that the cubic generation of ELF and VLF waves is substantially weaker than the electrojet modulation in the 1-5 kHz range. Signals produced by this mechanism are also weaker than the signals reported to be generated by the ICD mechanism at frequencies <100 Hz, but they can be stronger at frequencies >10 kHz.…”

Generation of ELF waves during HF heating of the ionosphere at midlatitudes

“…The HAARP facility has enabled new advances in ionospheric physics, e.g., excitation of ELF waves [Papadopoulos et al, 2011a[Papadopoulos et al, , 2011bMoore et al, 2013], formation of ducts and ion up flows [Kosch et al, 2010, Milikh et al, 2010, and descending artificial ionospheric layers [Pedersen et al, 2010]. The new plasma layers are results of enhanced ionization by electrons, which are accelerated to suprathermal speeds by turbulence resulting from HF heating [Mishin and Pedersen, 2011].…”

“…In another mechanism that relies on nonlinearity, the interaction of two large‐amplitude HF waves produce oscillations in electron temperature, and consequently in the collision frequency. The latter interact with the polarization current associated with the HF wave to produce ELF and VLF waves, which have been observed in HAARP experiments [ Moore et al ., ].…”