Not only does the Sun radiate the light we see – and that we do not see – but it also continually ejects into space 1 million tonnes of hydrogen per second. This wind is minute by astronomical standards; it carries a very small fraction of the solar energy output, and compared to the violent explosions pervading the universe it blows rather gently. Yet it has amazing effects on the solar surroundings. It blows a huge bubble of supersonic plasma – the heliosphere – which engulfs the planets and a host of smaller bodies, shaping their environments. It also conveys perturbations that can be seen in our daily life.

The object of this chapter is twofold. To give a concise historical account of the key ideas and observations that made our modern view of the solar wind emerge; and introduce the main properties of the Sun and of its wind, and their interpretation in terms of basic physics. The latter goal requires some tools of plasma physics, and will be developed in the rest of the book.

A brief history of ideas

The idea that planets are not moving in a vacuum is very old.