What is the solar wind?

The Sun is a star, a ball of hot gas held together by its own gravity and lit by fusion in its core. Around it lies a thin, very hot outer atmosphere, the corona. The corona is so hot that the Sun's gravity can't hold all of it. Gas escapes from it all the time, faster than sound, and streams away in every direction.

That stream is the solar wind. It is mostly protons and electrons, the pieces of hydrogen atoms, and it carries the Sun's magnetic field out with it.

Right now it blows past Earth at 340 kilometres per second.

The wind doesn't blow evenly. In the plane where the planets orbit, the usual state is a slow wind of about 400 kilometres per second, NOAA says.

Faster streams pour out of coronal holes. These are dark patches in the corona where the Sun's magnetic field opens straight into space. Their wind runs at between 500 and 800 kilometres per second. The Sun's poles have large coronal holes that last, so fast wind fills the space far above and below the planets' orbits.

The Sun turns once every 27 days as seen from Earth. So the wind spirals out like water from a turning garden sprinkler, and Earth passes through fast streams and slow ones in turn. When a fast stream catches up with slow wind ahead of it, the two pile up into a dense, strongly magnetic front. High-speed streams bring geomagnetic storms; slow wind brings calm space weather.

The wind doesn't stop at Earth. It blows past every planet and blows a vast bubble around the Sun, called the heliosphere. So Earth lives inside the Sun's own atmosphere.

Far out, the wind meets the thin gas between the stars and can push no further. That boundary is the heliopause. Voyager 1 crossed it on 25 August 2012, at 121.6 times Earth's distance from the Sun. Voyager 2 crossed in a different direction, at 119.0.

Beyond it is interstellar space. Both spacecraft are still sending back what they find there.