The first thing to remember is that space is not a vacuum through which only radiation can pass, as scientists in the early 20 th century thought. We now know that space is filled with plasma.

The second thing is that the atmosphere is not an electrical insulator, as it’s often thought to be. Recently it’s been recognised that the atmosphere is also a weak plasma; so it’s plasma all the way down from the Sun to the air we’re breathing right here in this room!