A close-up of the Sun's corona. Image taken during a 1998 total solar eclipse over the Caribbean. ©The Exploratorium.

All you have to do is look at a picture of a total solar eclipse and it is pretty obvious that the Sun's influences do not stop at its surface. For much of the 20th century, astronomers have suspected that the outer atmosphere of the Sun, called its corona, is not the end of the line. By the 1950s, it was pretty clear from studying mathematical models of systems such as stars, that a corona is not stable. It has to be constantly leaking away into space like the steam rising from a pot of boiling water. Astronomers called this constant outflow of gas from the Sun the solar wind. It took the technological advances of the Space Age to confirm this flow of particles in the early 1960s. Since then, astronomers and space scientists have learned a lot about this wind, and the winds from other stars too!

Hot Corona
One of the biggest mysteries about the Sun has been why the corona is so hot. The surface of the Sun has a temperature of about 6000 Centigrade, but the temperature of coronal gases is over 1.2 million Centigrade. How is it possible that the Sun's atmosphere is hotter than its surface? This would be like the steam rising from a pot being hotter than the pot that is creating it. The answer seems to have to do with the constant creation and destruction of magnetic loops of energy all across the surface of the Sun. This magnetic energy, when released, seems to be more than enough to keep the corona hot. It also provides the energy that is needed to drive the solar wind and propel it into the depths of the solar system at speeds of 450 kilometers/second. At these speeds it takes about 150 days for it to reach the orbit of Pluto.

A computer simulation of the solar wind impacting the Earth's magnetosphere. Movie courtesy of University of Maryland Advanced Visualization Laboratory. (Click to launch movie.)

Blowing in the Wind
The solar wind travels at about 450 kilometers/second but can gust up to 1,700 kilometers/second or more if there is a powerful coronal mass ejection passing by. The solar wind is composed the same atoms that make up the Sun itself, and in nearly the same abundances. For example, out of 100 atoms in the wind, there are about 75 hydrogen atoms for every 23 helium atoms. Eventually, the solar wind collides with atoms from interstellar gases surrounding the solar system out beyond Pluto, forming a shock front and a vast, but invisible, bubble.

The solar wind is like a conveyor belt that transmits the outcomes of events on the solar surface into interplanetary space. It can carry both particles and magnetic fields that were formerly a part of the solar surface. When this wind encounters a distant planet, it causes changes in the electrical properties of the space around the planet that can have significant impacts on planetary atmospheres and especially on their own magnetic fields, if they have one. Venus and Mars bear the full brunt of the streaming wind, and lose some of their atmosphere into the wind. Other planets such as the Earth, Jupiter, Saturn, Uranus and Neptune have powerful magnetic fields of their own, which act as shields for much of the solar wind's forces.

Find out more about the Sun-Earth Connection at the Sun-Earth Connection Education Forum Web site.

Text adapted from the Sun-Earth Connection Tutorial courtesy of NASA, originally written by Dr. Sten Odenwald. Images and videos courtesy of NASA unless otherwised noted.