The solar wind is a continuous outward flow of plasma (ionized gas) from the Sun 's corona, its outer atmosphere.

The Sun's corona extends millions of kilometers above the surface of the Sun (if a ball of gas can be said to have a "surface"). At these extreme altitudes, the gases, which have temperatures of millions of degrees Kelvin, are not contained by the Sun's gravity. The wind, composed primarily of protons and electrons, streams far beyond Earth's orbit and into interstellar space.

The existence of the solar wind had been suspected since the sixteenth century. It was noticed that as a comet neared the Sun, its tail formed. This tail did not always follow behind the comet; often it preceded it. One thing was certain: no matter where the comet was in its orbit, the tail always pointed away from the Sun. Isaac Newton thought...