There are two ways to heat the ionosphere and perform ELF generation: polar electrojet heating and ionospheric current drive heating
Ionospheric heaters are powerful HF transmitters (2.8-10 MHz) that induce controlled temporary modification to the electron temperature at desired altitude. Creation of virtual ULF/ELF/VLF antennas.
Selected sources cited at "ELF Generation 101" (video/slides) by Climate Viewer managed by J. Lee, including mainly the presentation and discussion of three slides from D. Papadopoulos' 2013 presentation. Suggested reading: Papadopoulos et al 2011.
Ionosphere heaters at high latitudes (near polar regions), such as EISCAT, HAARP and HIPAS, can use the polar electrojet, a naturally-occurring electric current in the D/E region (70-90 Km) of the ionosphere as both an amplifier and a virtual antenna. The polar electrojet is depicted in Climate 3D Viewer in Figure 1.
There also exists an equatorial electrojet. The mid-latitude Arecibo heater (Puerto Rico) and the equatorial Jicamarca heater (Peru) have used the equatorial electrojet (Cohen et al 2010, Lunnen et al 1984).
Figure 1: Climate 3D viewer screen capture showing the electrojet in the polar region.
The electrojet consists of natural currents which flow in the auroral and equatorial ionosphere at the E region which is found at approximately 100 Km (Figure 2). The high frequency (HF) signal of the ionospheric heater modulated at ELF/VLF frequencies is directed to the electrojet where it performs heating and thereby causes changes in the local conductivity of the ionosphere (Figure 2). As a result, the electrojet current is caused to vary at the same ELF/VLF rate. A virtual antenna is created in the E region which radiates at the frequency of the modulation. Propagating ELF/VLF waves are radiated by this virtual antenna. Polar electrojet heating will produce waves from 0.001 Hz to 20 KHz with a 2.8 to 8 KHz peak efficiency.
Different modulation techniques exist such as amplitude modulation (changing the intensity of the beam), beam painting (similar to using the beam to create strokes on the ionospheric canvas) and geometric modulation (when geometric patterns are formed with the beam) (Guo et al 2021 - Figure 4).
Figure 2: Slide 5 from D. Papadopoulos' presentation (PDF).
Figure 4: From http://roma2.rm.ingv.it/en/themes/22/magnetic_pulsations
"They're going to heat it way out here (arrow on Fig. 4). Now the MS (magnetosonic) waves, the very low frequency waves are what's creating the antenna now. They're not actually just powering the ionosphere - the polar electric jet - they're shooting it way out here into space; and these waves are traveling down and creating another antenna. This is very important and I know this is high-level but there are people who are going to understand this that need to hear it and they will do something about it I am sure."
"What they're doing is creating a virtual antenna in the sky that radiates extremely low-frequency signals that travel worldwide and can be heard in the deepest depths of our oceans."
Figure 5: Magnetostatic waves and shear Alfvén waves. Slide 14 from D. Papadopoulos' presentation (PDF).
Ionospheric Current Drive (ICD) heating can produce 0.1 Hz magnetosonic (MS) waves, 2.5 Hz shear Alfvén waves (SAW) and ULF/ELF waves up to 60-70 Hz (Figure 5).
Transmitted electromagnetic radiation may propagate along the magnetic field lines of the Earth termed L-shells (ducted propagation)
Figure 6: Slide 15 from D. Papadopoulos' presentation (PDF).
"Then some of it bounces off - and, you know, aggravates the heads of people all in this region - then bounces back into space. Now, once it makes it all the way back to HAARP, that's called a hop. Now why am I telling you this? Well, they have a thing out here in the middle of the ocean called the HAARP buoy and it's part of the one-hop-experiment." "It's a VLF buoy, so basically (at) the place where HAARP lands in the ocean, they put a receiver out there to listen for it. And that conjugate point is right off the shore of Australia." (Stanford VLF buoy shown in Figure 7.)
Figure 7: Figure 1 from Golkowski M. et al, 2018 illustrating ducted propagation excited by the HAARP HF heater and image of Stanfold VLF buoy (HAARP receiver).
It is possible to generate VLF waves by using interference between two different ionospheric heaters or two subarrays of a heater in which case we refer to "split-beam". This is described by Kuo et al (2012). A mobile source of VLF waves can be produced in this way.
The mechanism known as "excitation of the thermal cubic nonlinearity" (Moore et al 2013) may represent an interference case.
Broadband VLF waves in the frequency range of 7–10 KHz and 15–19 KHz, generated by F region continuous HF ionospheric heating in the absence of electrojet currents, were detected by satellite and are believed to have been generated by LH-to-whistler mode conversion (Vartanyan et al 2017).
The use of VLF waves is important for the Navy and especially for submarine communications. The Navy’s VLF antenna in Cutler, Maine occupies 2000 acres on a peninsula and consists of 26 towers 850 to 1000ft high. It consumes approximately 20 MW from a dedicated power plant (ref). It is composed of two umbrella or snowflake arrays (Wikipedia).
"Very Low Frequency (VLF, 3-30 kHz) and Extremely Low Frequency (ELF, 3-3000 Hz) electromagnetic waves are generated by various natural and anthropogenic processes in the lower atmosphere. On a global basis by far the most significant source is ELF/VLF radiation from lightning propagating in the Earth-ionosphere waveguide."
I. Whistlers - source is lightning.
II. ELF/VLF Emissions - not associated with lightning.
1. Continuous or noise-like. Cf. "hiss".
2. Discrete. Cf. "chorus"
III. Interactions Between Whistlers and ELF/VLF Emissions.
IV. Power Line Harmonic Radiation (PLHR)
"ELF-VLF harmonic radiation from power systems penetrates the ionosphere and propagates into the inner magnetosphere, where it is amplified by unstable ambient plasma and may take the form of a self-sustained ELF-VLF emission." https://www.sgo.fi/Publications/SGO/thesis/ManninenJyrki.pdf
A primer is available starting at e-page 15 of the reference https://ir.uiowa.edu/etd/4549/.
Whistler | VLF | 1 kHz to 30 kHz https://en.wikipedia.org/wiki/Whistler_(radio)
Hiss | ELF-VLF | 300 Hz – 10 kHz https://en.wikipedia.org/wiki/Hiss_(electromagnetic)