100 days of ELF/VLF generation via HF heating with HAARP

Abstract

Extremely low frequency/very low frequency (ELF/VLF) radio waves are difficult to generate with conventional antennas. Ionospheric high frequency (HF) heating facilities generate ELF/VLF waves via modulated heating of the lower ionosphere. HF heating of the ionosphere changes the lower ionospheric conductivity, which in the presence of natural currents such as the auroral electrojet creates an antenna in the sky when heating is modulated at ELF/VLF frequencies. We present a summary of nearly 100 days of ELF/VLF wave generation experiments at the 3.6 MW High Frequency Active Auroral Research Program (HAARP) facility near Gakona, Alaska, at a variety of ELF/VLF frequencies, seasons, and times of day. We present comprehensive statistics of generated ELF/VLF magnetic fields observed at a nearby site, in the 500–3500 Hz band. Transmissions with a specific HF beam configuration (3.25 MHz, vertical beam, amplitude modulation) are isolated so the data comparison is self‐consistent, across nearly 5 million individual measurements of either a tone or a piece of a frequency‐time ramp. There is a minimum in the average generation close to local midnight. It is found that generation during local nighttime is on average weaker but more highly variable, with a small number of very strong generation periods. Signal amplitudes from day to day may vary by as much as 20–30 dB. Generation strengthens by ∼5 dB during the first ∼30 min of transmission, which may be a signature of slow electron density changes from sustained HF heating. Theoretical calculations are made to relate the amplitude observed to the power injected into the waveguide and reaching 250 km. The median power generated by HAARP and injected into the waveguide is ∼0.05–0.1 W in this baseline configuration (vertical beam, 3.25 MHz, amplitude modulation) but may have generated hundreds of watts for brief durations. Several efficiency improvements have improved the ELF/VLF wave generation efficiency further.

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

“…At the Arecibo Observatory in Puerto Rico and at the Jicamarca Observatory in Peru HF facilities were used to generate weak ELF signals with the equatorial electrojets [ Ferraro et al , ; Lunnen et al , ]. The European Incoherent Scatter facility located in Tromsø, Norway, the High Frequency Active Auroral Research Program (HAARP), and the Sura ionospheric heating facility in Russia have been extensively utilized for the modulation of auroral electrojets [ Stubbe et al , ; Barr and Stubbe , ; Moore et al , ; Fujimaru and Moore , ; Cohen et al , ; Gołkowski et al , ; Cohen and Golkowski , ; Kotik et al , ]. The two main modes of heating the ionosphere are called the ordinary mode ( O mode) and the extraordinary mode ( X mode).…”

Self-consistent modeling of induced magnetic field in Titan’s atmosphere accounting for the generation of Schumann resonance

“…The first point of divergence concerns the natural generation mechanism of ELF magnetic field components presumably being the energy source of the Titan's SRs (Béghin, 2014), while the same process does not seem to occur naturally on Earth. However, the first successful attempt of artificial triggering of ELF emissions in the polar electrojet by High Frequency (HF) heating experiments (Mc Carrick et al, 1990) and furthermore performed quasi-routinely in VLF range (e.g., Cohen and Golkowski, 2013) bring a significant support to the model proposed here, though no experimental evidence is known to date about a natural ELF self-modulation of terrestrial electrojets. This is probably because the necessary increase of conductivity which is thought to exist naturally in Titan's ionopause for triggering the ion-acoustic instability and subsequently yielding the ELF modulation of the current, seems not occurring on Earth.…”

“…The physical process of ion-acoustic instability is basically the same as observed with the experiments of HF heating of the ionosphere in the Earth's Polar Regions in which the increase of Pedersen conductivity is triggered artificially (e.g. Stubbe and Koppa, 1977;Mc Carrick et al, 1990;Cohen and Golkowski, 2013), contrary to the natural process involved for Titan. The specific criterion for triggering the ion-acoustic instability is when the electron-ion drift velocity V drift exceeds the adiabatic ion-acoustic speed c s , which reads…”

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

“…Observations at HAARP are $5-100 times stronger than at the Sura heating facility [19]. Cubic ELF and VLF wave generation is $30 dB weaker than auroral electrojet modulation in the 1-5 kHz range [29] and $10 dB weaker in the 16-20 kHz range [30]. It is also $3-6 dB weaker than the signals reported to be generated by the ionospheric current drive (ICD) mechanism [15], but cubic generation is strongest at higher frequencies (> 10 kHz), whereas the ICD is reported to be strongest at lower frequencies (< 100 Hz).…”