Modulated high frequency (HF, 3–10 MHz) heating of the ionosphere in the presence of the auroral electrojet currents is an effective method for generating extremely low frequency (ELF, 3–3000 Hz) and very low frequency (VLF, 3–30 kHz) radio waves. The amplitudes of ELF/VLF waves generated in this manner depend sensitively on the auroral electrojet current strength, which varies with time. In an effort to improve the reliability of ELF/VLF wave generation by ionospheric heating, recent experiments at the High‐frequency Active Auroral Research Program (HAARP) facility in Gakona, Alaska, have focused on methods that are independent of the strength of the auroral electrojet currents. One such potential method is so‐called “beat‐wave” ELF/VLF generation. Recent experimental observations have been presented to suggest that in the absence of a significantD‐region ionosphere (∼60–100 km altitude), an ELF/VLF source region can be created within theF‐region ionosphere (∼150–250 km altitude). In this paper, we use a time‐of‐arrival analysis technique to provide direct experimental evidence that the beat‐wave source region is located in theD‐region ionosphere, and possibly the lowerE‐region ionosphere (∼100–120 km altitude), even when ionospheric diagnostics indicate a very weakD‐layer. These results have a tremendous impact on the interpretation of recent experimental observations.

[1] Modulated high frequency (HF) heating of the lower ionosphere in the presence of auroral electrojet currents has become an important method for generating electromagnetic waves in the extremely-low frequency (ELF) and very-low frequency (VLF) bands. Recent research efforts focus on improving the efficiency of ELF/VLF wave generation. One method to do so involves the spatial mapping of modulated currents that result from HF heating for comparison with HF heating models. As a first step toward providing a spatial map of the modulated ionospheric currents, we introduce time-of-arrival (TOA) observations performed during a series of experimental research campaigns conducted at the High-frequency Active Auroral Research Program (HAARP) in Gakona, Alaska. The TOA method provides a measurement of the ELF/VLF amplitude and phase detected at a ground-based receiver as a function of time, and this information may be used to estimate the distribution of ELF/VLF source currents within the HF heated region. In an effort to test and improve the TOA method, the University of Florida conducted ELF/VLF wave generation experiments using the HAARP HF transmitter under varying ionospheric conditions and using various transmission formats. In this paper, we summarize our experimental results and compare observations with the predictions of a theoretical model.

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.

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This paper presents experimental observations of ELF/VLF wave generation performed during multi-beam HF heating experiments at the High-frequency Active Auroral Research Program (HAARP) observatory. The primary objective of these experiments is to advance the scientific understanding of the nonlinear absorption processes occurring within the collisional D-region ionosphere. During the February, May, and August 2012 HAARP campaigns, a series of multi-beam HF transmissions were designed to produce inter-harmonic modulation products in the ELF/VLF range. Experiments were performed using a variety of simultaneous 2-to 6-frequency HF transmissions spaced at ELF/VLF frequencies. For instance, in order to generate the 3-frequency experiment using frequencies of 3,248,485 Hz, 3,251,515 Hz, and 3,250,200 Hz, one half of the array broadcast a synthesized-two-frequency (STF) modulation format centered on 3.25 MHz (X-mode) with a modulation frequency of 1515 Hz while the other half of the array broadcast CW at 3.250200 MHz (Xmode). In the presence of the auroral electrojet, this transmission format generated ELF/VLF tones at 1315 Hz, and 1715 Hz, and 3030 Hz, as expected. Additionally, in order to generate ELF waves by the cubic nonlinearity, one half of the array broadcast at 2,750,250 Hz while the other half broadcast at 5,499,500 Hz, generating ELF waves at 1000 Hz. These experiment were repeated using different HF frequencies, different modulation frequencies, and different HF power levels. During the May and August 2012 experiments, a frequency-time variation was imposed on the transmissions in order to enable time-of-arrival signal processing.Experimental observations are compared with the initial results of a multi-beam HF heating model that accounts for inter-harmonic mixing among the various field components. We demonstrate that not all observations (particularly those for the cubic nonlinearity generation method) can be explained without accounting for the electron temperature oscillations that occur at the second harmonic of the HF waves.