Strictly speaking Aurora are not an astronomical effect but a terrestrial effect. In fact many planets and moons in our solar system experience this phenomenon.

There many types of atomic particles ejected by the sun that get caught in the earth’s gavitational field. Because they are so small they are easily influenced by the earth’s weaker magnetic field which, like a magnet, is strongest at the poles. Normally these particles stream harmlessly to the earth. Again because they are so small they can’t be seen.

However during a solar storm great quantities of much faster moving particles are ejected. As these are much higher in energy they have the ability to knock electrons off the stable gases that form our atmosphere momentarily making ions of them. It takes energy to do this and the resulting ions aren’t happy to remain in this state and so immediately start recombining with lost electrons. Bond forming is an exothermic process and so light is given off. Four-fifths of our atmosphere is nitrogen and the light given off is UV which we humans can’t see. However the gas making up most of the remaining one-fifth of our atmosphere is Oxygen and it’s isotopes. The light that they emit is visible to us as greens and reds.

Others - unknown

Portland Bill, Dorset

Cumbria

Durdle Door, Dorset

Fortunately in the UK those clever people at Lancaster University have developed an app to show us when we are most likely to see Aurora. When the bars go red the chances are far increased.

Click on the image to go to the site.