The Sun is a huge ball of gas. On the surface there are continual nuclear explosions. From time-to-time plasma escapes from the Sun and roars towards Earth riding the solar wind.
Plasma is a hot gas, hot enough for its atoms to routinely split into, and exist as, electrons and ions. Because plasma is made of changed particles, it is influenced by electromagnetic fields like that surrounding the Earth.
Ordinarily, the Earth’s magnetic field is strong enough to deflect much of the Sun’s plasma away with little able to penetrate the atmosphere. The Earth’s magnetic field is termed the magnetosphere. The image below shows conceptually the Sun on the left and the tiny Earth and its lines of magnetic flux coloured blue that form the magnetosphere with particles from the Sun deflected round the magnetosphere creating a ‘bow wave’ as if Earth is a ship on an ocean.
From time to time the energy in the plasma and the speed of the solar wind is such that significant plasma enters the magnetosphere and penetrates down into the Earth’s atmosphere.
There are various theories about how exactly the plasma couples with the Earth’s magnetic field and subsequently how exactly plasma ends up rushing towards Earth. One theory suggests a direct coupling and transit of particles. Another theory is that plasma is continually stored in the ‘tail’ of the magnetosphere and is jolted into play by a solar wind shock. Either way, plasma makes an entrance. The following image shows two ideas:
So, by a complex mechanism, plasma from a high-energy solar wind following a solar flare or other solar event couples with the magnetosphere. The ions and electrons of the plasma scream down into the Earth’s atmosphere at the North and South Poles.
As the plasma meets the increased density of the Earth’s atmosphere, it splats round the atmosphere like an egg cracked on a football. How far it descends from its entry points at the poles depends on the energy in the plasma, and hence ultimately on the magnitude of the solar event that launched it.
This ‘splatting’ over the poles and down creates the north and south auroral ovals. The arrival of high energy plasma disturbs the Earth’s magnetic field and is detectable with a magnetometer. This disturbance can be used to determine if a usable radio aurora is imminent.
The following image shows an auroral oval in green looking down on the South Pole.
The auroral oval therefore contains both plasma from the sun and particles from the ionosphere in varying densities and temperatures. Its energy may dissipate at high latitudes and few people will see a visual aurora or be able to exploit a radio aurora. With high energy events though, it’s a plasma on the move, rushing down and spinning round the lines of the Earth’s magnetic field.
So, a radio aurora starts as a huge solar flare or other ejection of plasma from the Sun. If there’s enough energy, the plasma hits the Earth’s atmosphere three days later and tracks south and north from the poles. Again, if there’s enough energy, the result may be a visible aurora, and more rarely, a radio aurora.
See my other pages on this site for more on how, when and where radio auroras occur.
And finally, just for a little relaxation, check out this video:
First and third images and video from https://www.ucl.ac.uk/mssl/research/solar-system/space-plasma-physics/what-space-plasma accessed on 18th October 2021.