Solar Wind

Protons and electrons boil off the surface of stars, like the Sun, and stream out past any nearby planets, such as the Earth.

Here they meet the planet's magnetic field, and are forced by it to move to the poles. When they get down to the atmosphere they ionize the gases, which then emit radiation. This is the source of the aurorae.

When the solar wind meets a comet, it pushes the gases which the comet releases away from the star, so creating the comet's tail.