Last Updated on November 26, 2023 by John Berry
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. The result is possible formation of radio auroras.
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.
At the E Region
Particles track down the Earth’s magnetic field lines. As the atmosphere becomes denser in the E Region, more ions and electrons are produced. I’ve described such events with the diagram below.
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.
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.
As I illustrate in this image, 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 leading to possible formation of radio auroras.
It’s those columns of high-density spinning plasma that radio amateurs point their antennas at and that enable back scatter.