Formation of radio auroras

I’ve described elsewhere on this site what happens when the E Region gets a higher density of electrons because of the Sun’s energy. Ions and electrons are produced, and they live in a soup or plasma, trapped there because of atmospheric tides and winds and interaction with the Earth’s geomagnetic field. I have also described how Sporadic E propagation comes about because of ablated particles from meteors that ionise and increase the E Region electron density.

The aurora forms in a somewhat similar but very unique way.

Plasma from the Sun is propelled towards Earth in the solar wind. It interacts with the Earth’s magnetic field – its magnetosphere. The plasma tracks down the field lines, entering the Earth’s atmosphere at the poles.

As soon as the plasma as ions and electrons from the Sun hit the particles of the thermosphere above about 120km, they interact. More ions and electrons are produced. Above the E Region where there’s a less dense atmosphere, many pairs recombine and emit a flash of light. This is the visible aurora.

Particles track down the Earth’s magnetic field lines. As the atmosphere becomes denser in the E Region, more ions and electrons are produced. Such events are described in the diagram below.

How plasma forms

The graph on its side to the left of the image shows the all-important levels of electron density with height above the Earth. The highest electron density is found in the E Region. The radio aurora is, therefore, a dense patch of E Region ionisation. But it has a very specific nature.

While in other E Region phenomena, plasma is held for a time as a patch because of tides, winds and the Earth’s geomagnetic field, the aurora forms a unique set of spinning columns.

The ions and electrons track down the lines of the Earth’s magnetic field. As a result, they form into areas of dense and less dense plasma in the E Region. This is shown in the image below.

How spirals form

This image illustrates how the ionisation occurs at the top of the E Region, and as the plasma tracks down, it forms into high and low electron density clouds. As these interact with the Earth’s geomagnetic field, the columns spin.

It’s those columns of high-density spinning plasma that radio amateurs point their antennas at and that enable back scatter.