The Wind From The Sun

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. The first convincing proof of such a solar wind was when astronomers discovered that the tails of comets didn't point exactly away from the Sun. They were cocked at a 5-10 degree angle away from this direction. Because comet tails are composed of gases boiled off from the comet, they acted like million kilometer-long windsocks in the solar system. There had to be a wind from the Sun pushing them ever so slightly out of kilter. 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!

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 coronal temperature was over 1.2 million degrees Centigrade. There were several detailed theories about how this could be, but all of them needed data that was as yet unavailable to confirm them. This changed in the 1990s when a new generation of research satellites came into use. The secret of the corona's temperature seemed to lie in the complex and continuous formation of granule-sized magnetic loops all over the Sun's surface. As these loops short-circuit and reconnect with their neighbors, they heat the solar gases to over one million degrees. The gases then escape from the surface and evaporated into the corona. Some of the gases become trapped in the coronal magnetic field so that a spectacular solar atmosphere forms. You can see this during a total solar eclipse. Other gases flow out into the solar system on magnetic field lines that do not swing back to the surface of the Sun, and it is here that the solar wind owes much of its existence.

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.

The solar wind contains overlapping patterns of high-speed and slow-speed flows, as fast moving particles from so-called coronal holes have easy times escaping from the magnetic conditions near the solar surface. At any given time, there are sectors where the polarity reverses sometimes 5-10 times around the circumference of the Earth's orbit. 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.


Teacher Notes:

What direct evidence is there that demonstrates the interaction of the solar wind with Earth?

A system is a collection of things that have some influence on one another. Any part of a system may be considered a system. This is referred to as a sub-system, it has its own interactions.

(K-2) Most things are made of parts.

(3-5) In something that consists of many parts, the parts usually influence one another.

(6-8) Thinking about things as systems means looking for how every part relates to others. The output from one part of a system (which include material, energy or information) can become the input to other parts. Such feedback can serve to control what goes on in the system as a whole.

(9-12) Understanding how things work and designing solutions to problems of almost any kind can be facilitated by systems analysis. In defining a system, it is important to specify its boundaries and subsystems, indicate its relation to other systems, and identify what its input and its output are expected to be.


Science Nuggets

2000...TRACE discovers that reconnection in the small-scale solar magnetic field is the probable origin of coronal heating.

1999...SOHO discovers the association between solar wind flows and the magnetic fields in coronal holes.

1999...Ulysses confirms that comet tail kinks are caused by reconnection of the solar interplanetary magnetic field.

1998...ACE discovers that the interplanetary solar wind has very little turbulence, based on measurements of oxygen isotopes.

1998...Voyager 2 detects the heating, and slowing down, of the solar wind by its interaction with the interstellar medium beyond the orbit of Pluto..

1994...Ulysses spacecraft detects high-speed solar wind leaving the Sun's south polar regions at over 3 million kilometers per hour.