solar wind
The solar wind is a steady stream of electrically charged particles — mostly protons and electrons — that the Sun blows continuously out into space in every direction. It is not air or a literal wind, but a thin spray of plasma escaping from the Sun's hot corona at high speed.
The corona is so hot that the Sun's gravity cannot hold its outer gas down, so it leaks away outward, speeding up as it goes. Near the Earth the solar wind typically travels at about 400 to 800 km per second and is extraordinarily sparse — only a handful of particles per cubic centimetre, far thinner than the best laboratory vacuum. It carries a piece of the Sun's magnetic field with it, woven into a vast spiral as the Sun rotates. The wind blows outward past all the planets and eventually meets the gas between the stars, carving out a giant bubble around the whole solar system called the heliosphere.
The solar wind matters because it connects the Sun physically to the Earth and everything else in the solar system. It drives the shimmering auroras near the poles, shapes comet tails so they always point away from the Sun, and, when the Sun is stormy, can disturb satellites, power grids, and communications. Its outer boundary, the heliosphere, also shields the inner solar system from some of the harsher radiation coming from interstellar space.
A comet always grows two tails pointing away from the Sun — one pushed by sunlight, one swept back by the solar wind — no matter which way the comet is travelling.
Comet tails are a windsock for the solar wind, revealing the invisible flow.
The solar wind is the steady, ever-present outflow; it is not the same as a coronal mass ejection, which is a sudden, violent blast of plasma launched during a solar storm.