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A Public Outreach Module:
Solar Wind, Genesis, and the Planets

What is solar wind?

The solar wind generated by our sun is quite different from Earth's surface winds, which are created by differences in atmospheric pressures. This "wind" carries about one million tons of hot plasma, electrically-charged gas particles, at a temperature of about 100,000 Kelvins, away from the sun every second. Solar wind plasma is a mixture of mostly protons (H+) with some helium nuclei (He2+) and electrons to electrically balance these positive ions. About 0.1% of the solar wind is made up of ions of other elements, including carbon, nitrogen, oxygen, neon, magnesium, silicon, and iron as well as almost all of the other known elements. The Genesis mission hopes to analyze nearly all elements from lithium (atomic number 3) to uranium (atomic number 92).

The solar wind forms from systems of electrical currents within the sun's corona, the outermost layer of the sun's atmosphere, and behaves like an electrically conducting fluid. These incredibly dilute, superheated solar wind gases reach millions of miles into space. Voyager 1 observed evidence of solar wind at a distance greater than 85 AU, well past the orbit of Pluto.

The corona's motion is synchronized with the solar activity cycle, changing shape from a jagged ring around the sun during the peak of the cycle to wispy plumes and streamers that reach millions of miles into space at the end of the cycle. These plasma-containing streamers in the corona are the origin of the dense, lower-speed component of the solar wind. During periods of low solar activity, a very fast type of solar wind flows from relatively stationary "coronal holes" over the north and south poles of the sun.

These solar wind streams are carried at different speeds varying from 300 to 1000 km/sec, independent of the wind's distance from the sun. Because they are carried at different speeds, the streams collide and rebound, producing low magnetic regions and regions in which the magnetic field is amplified.

The plasma particles that have enough kinetic energy to escape the sun carry its magnetic field into interplanetary space. Its magnetic field strength is about 10-4 of Earth's and its strength decreases as the distance between the plasma and the sun increases.

The density of the solar wind varies between 1 and 10 particles/cm3 and it, also, decreases as it moves farther from the sun. Solar Max activity can cause large increases in density, however.

After escaping from the sun's gravitational field, the solar wind flows outward radially, like water from a rotating garden sprinkler. Each drop moves straight out from the source, but the pattern rotates. When it reaches the Earth, the spiral makes an angle of about 45 degrees with a radial line from the sun and the solar wind is traveling over one million miles per hour.

For a more detailed research data of solar wind properties, click on http://www.gps.caltech.edu/genesis/DocumentG.html

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For a more technical description, take a Closer Look at
The Structured Sun and Solar Max: At the Core of the Matter.