Generation of whistler waves by continuous HF heating of the upper ionosphere

Abstract

Broadband VLF waves in the frequency range 7–10 kkHz and 15–19 kHz, generated by F region CW HF ionospheric heating in the absence of electrojet currents, were detected by the DEMETER satellite overflying the High Frequency Active Auroral Research Program (HAARP) transmitter during HAARP/BRIOCHE campaigns. The VLF waves are in a frequency range corresponding to the F region lower lybrid (LH) frequency and its harmonic. This paper aims to show that the VLF observations are whistler waves generated by mode conversion of LH waves that were parametrically excited by HF-pump-plasma interaction at the upper hybrid layer. The paper discusses the basic physics and presents a model that conjectures (1) the VLF waves observed at the LH frequency are due to the interaction of the LH waves with meter-scale field-aligned striations—generating whistler waves near the LH frequency; and (2) the VLF waves at twice the LH frequency are due to the interaction of two counterpropagating LH waves—generating whistler waves near the LH frequency harmonic. The model is supported by numerical simulations that show good agreement with the observations. The (Detection of Electromagnetic Emissions Transmitted from Earthquake Regions results and model discussions are complemented by the Kodiak radar, ionograms, and stimulated electromagnetic emission observations

Low‐Frequency Waves in HF Heating of the Ionosphere

“…For the above-mentioned heating settings, LH waves are known to be generated by parametric instabilities that are triggered by the HF-pump-plasma interactions. This is confirmed by SEE observations (not shown here) available during experiment 2 [Vartanyan et al, 2016].…”

“…Since the only requirement for the mechanism is the presence of LH waves (known to be generated in less than 20 ms), whistler waves were indeed observed by DEMETER during both experiments. If the two simulations were combined into one, then the frequency spectrum would reveal peaks at both the LH frequency and its harmonic [Vartanyan et al, 2016], as in the o bservations (Figure 3.9a).…”

“…While in the first mechanism LH waves linearly coupled to whistler waves via the external irregularities (striations), this mechanism relies on LH waves nonlinearly coupling to whistler waves via the density fluctuations of another (counterpropagating) LH wave, and as such does not directly rely on striations. Using the above-m entioned LH-whistler mode conversion model with two counterpropagating LH waves, but generalizing the model to include the nonlinear coupling, this mechanism could be simulated to yield whistler waves at the LH h armonic [Vartanyan et al, 2016]; the simulation was achieved by placing two counterpropagating LH waves on top of each other and observing the spectrum of the subsequently modeconcerted whistlers. Since the only requirement for the mechanism is the presence of LH waves (known to be generated in less than 20 ms), whistler waves were indeed observed by DEMETER during both experiments.…”

“…Another mechanism for generating low-frequency waves in the ELF range was motivated by observations from the DEMETER satellite during experiments at HAARP with no modulation of the HF power. These waves have been identified as whistlers with frequencies close to that of lower hybrid waves and generated by parametric p rocesses [Vartanyan et al, 2016]. The mid-latitude ionosphere has no steady large-scale current or electrojet, and modulated HF heating is expected to generate low-f requency waves due to the processes similar to those of the high-latitude ionosphere.…”

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

“…6. A new mechanism generating EM VLF waves in frequency ranges 7-10 kHz and 15-19 kHz with constant HF heating was recently proposed by Vartanyan et al [2016]. It does not rely on any VLF modulation of HF emissions and was observed in absence of any electrojet.…”