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.
Figure 2: Slide 5 from D. Papadopoulos' presentation (PDF).
The electrojet consists of natural currents which flow in the auroral and equatorial ionosphere at 80-100 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). The electrojet current is then caused to vary at the same ELF/VLF rate. Propagating ELF wave signals are radiated by this "virtual antenna". Polar electrojet heating will produce waves from 0.001 Hertz to 20.000 Hertz with a 2.8 to 8 KHz peak efficiency.
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 50-70 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).