The Dynamic Sun

The Sun's almost constant supply of light and heat comes from the photosphere (or "sphere of light"). The Sun's layers are in ceaseless motion (structure of the Sun courtesy of the Space Physics Division/NASA). Hot tubes of plasma called prominences arc hundreds of thousands of miles above the Sun's surface following the invisible lines of magnetic force. (Photo courtesy of the Naval Research Laboratory)

Sunspots: Cool, Dark and Magnetic

Appearing in groups that last from several hours to several months, sunspots are darker, cooler regions of the Sun's photosphere, containing intense magnetic fields. Sunspots appear dark because they are cooler than the plasma surrounding them. (Photo at right courtesy of the Space Environment Center/NOAA)

An increase in the number of sunspots signals an increase in the Sun's magnetic activity. The graph at left shows that the number of sunspots waxes and wanes on an eleven-year cycle. The lack of recorded sunspots from 1645 to 1715 is called the Maunder Minimum, a period of exceptionally cold weather across the northern hemisphere of Earth. As can be seen in the graph, we are presently close to a solar minimum, and the next solar maximum is due in about the year 2001.

Solar Flares: The Most Explosive Solar Events

Solar flares erupt as bright flashes above or near cooler sunspot regions in the photosphere. Some flares extend upward more than 60,000 miles into the corona.

Though flares last from only a few minutes to a few hours, they are among the most powerful events in the solar system. (Photo courtesy of the National Optical Astronomy Observatories)

The Corona: The Sun's Crown

The Sun's corona is a vast halo of superheated plasma. When the photosphere is obscured by the Moon during a solar eclipse, tapered streamers reaching millions of miles into interplanetary space are visible from Earth. (Eclipse photo courtesy of Steve Albers)

The solar wind originates in areas of the corona called "coronal holes." These are regions of lower temperature and weak magnetic fields. Astronauts aboard Skylab took this X-ray image of a coronal hole shaped rather like the boot of Italy. (X-ray photo courtesy of the Harvard College Observatory)

What Causes Disturbances in the Solar Wind?

High speed coronal mass ejections (CMEs) produce major disturbances in the solar wind. Often loop-like in appearance, coronal mass ejections rise as massive clouds of material from the solar atmosphere. When directed towards Earth, they can cause large magnetic storms in the magnetosphere. (Photo sequence courtesy of Art Hundhausen)

The Heliosphere: Reaching Out to the Stars

Think of the heliosphere as a vast magnetic bubble containing the solar system, the solar wind, and the entire solar magnetic field.

At the outermost boundary of the heliosphere, called the heliopause, the solar wind meets the interstellar medium, a plasma that permeates our Milky Way galaxy. Sometime in the next century, Pioneer and Voyager spacecraft will finally cross this distant frontier. (Drawing courtesy of Tom Krimigis.)

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