Generation of ULF, ELF & VLF waves

What is the simplest way to describe the function of an antenna?

Electrons are made to move back and forth within a metal stick.

When an electromagnetic wave arrives onto the antenna, it does that, thereby generating an electric current.

When an electric current is applied to the antenna, it does that, thereby generating an electromagnetic wave.

What if it was possible to have an antenna in the ionosphere? 

What if it was possible to move electrons in the ionosphere in such a way so that a current would generate an electromagnetic wave?

It is possible to do that in order to generate ELF and VLF waves. A fundamental condition is that the electrons in the ionosphere have to be energized or as said commonly to be "heated" by an electromagnetic wave. To this purpose, high frequencies (HF) are used which are between 3 and 30 MHz, and which bounce back from the ionosphere. The corresponding HF transmitters are called ionospheric heaters. The HF signal may be modulated (shaped) in different manners.

There are two main ways of generating ELF and VLF waves.

1) Use the natural electric currents that flow in the ionosphere over the North and the South pole, which are called "electrojets". It would be required to shape them, or in other words to modulate them. This is polar electrojet modulation.

In practice, a high frequency (HF) transmitter sends an HF signal with a modulation in the ELF and VLF range (i.e. less than 20 kHz). The electrojet is then caused to vary in the same ELF/VLF rate, thus producing the corresponding waves.

2) Drive a current in the ionosphere. This is called ionospheric current drive. It constitutes a complex process involving initially the creation of a primary antenna which radiates a special category of waves called magnetosonic waves. That antenna induces the creation of a secondary antenna that radiates ELF.

These two ways are shown in the attached slides by D. Papadopoulos which have been explained extensively in videos by Climate Viewer - J. Lee.