Solar Wind

Solar material flowing into interplanetary space. The Sun’s atmosphere is expanding radially outwards in all directions at supersonic speeds of hundreds of kilometers per second, filling interplanetary space with charged particles and Magnetic fields. This solar wind is made up of an equal number of electrons and protons, with lesser amounts of heavier ions. It carries solar material out into interstellar space at a rate of almost a million tons each second, flowing past the planets, which essentially orbit within the Sun’s outer atmosphere. The solar wind carves out a huge bubble in space with the Sun at its centre, known as the Heliosphere, extending out to about 100 AU from the Sun.

The existence of the solar wind was suggested from observations of comet ion tails by the German astronomer Ludwig Biermann in the 1950s. A comet’s ion tail always points away from the Sun. While the radiation pressure of sunlight is sufficient to push comets’ curved dust tails away from the Sun, it is not enough to create their ion tails. Biermann proposed that electrically charged particles pour out from the Sun at all times and in all directions, accelerating the ions to high speeds and pushing them radially away from the Sun in straight ion tails.

In 1958 Eugene Parker of the University of Chicago showed how a flow might work, dubbing it the solar wind. It would naturally result from expansion of the Sun’s Corona. At a critical distance of a few solar radii, the corona’s thermal energy overcomes the gravitational attraction of the Sun, allowing coronal plasma to expand supersonically into interplanetary space. Parker also demonstrated how the Sun’s magnetic fields would be pulled into interplanetary space, acquiring a spiral shape as a result of the combined effects of radial solar wind flow and the Sun’s rotation.

Spacecraft have been making in situ measurements of the solar wind since 1959. The average solar wind density near the Earth was shown in 1962–63 by Mariner 2 to be 5 x 10⁶ particles per cubic meter. Such a low density close to Earth’s orbit is a natural consequence of the wind’s expansion into an ever-greater volume. The Mariner 2 data also indicated that the solar wind has a slow and a fast component. The slow component travels at a mean speed of about 400 km/s (250 mi/s) and emanates from coronal streamers close to the solar equator; the fast component travels at a mean speed of about 7500 km/s (4700 mi/s) and originates from the coronal holes. Subsequent spacecraft showed that the interplanetary magnetic field has a strength of approximately 0.00006 Gauss at Earth’s orbit distance from the Sun.

The solar wind plays an important role in shaping Earth’s Magnetosphere and the magnetospheres of the other planets. Particles and magnetic fields carried by the solar wind drive Magnetic storms.

Questions to Ponder

• How does the solar wind affect Earth?
• Where does the solar wind occur?