Our Changing Sun

Although the average distance from Earth to the Sun is a whopping 149,600,000 kilometers (93,000,000 miles), careful observation from Earth reveals a surprisingly large number of different visible features. The most obvious and best known feature is the sunspot. Typically moving in groups, these dark (in visible light), planet-sized features have been known to humankind for centuries. As sunspots form and disappear over periods of days or weeks, they also appear to move across the Sun's surface. Composed of strong magnetic fields, sunspots are shaped much like a horseshoe magnet that rises from below the Sun's surface. The rising hot gas is trapped by the sunspots' intense magnetic field that cools the sunspots from 6000 °C to about 4200 °C. The cool area appears dark compared to the area around it. Thus, from Earth, we see spots on the Sun. In some photographs, we can also see light colored areas around groups of sunspots that resemble tufts of cotton candy. We call these fluffy looking fringes plages.

Sunspots are the source of massive releases of energy called solar flares, the most violent events in the solar system. In a matter of minutes to several hours, a solar flare releases about 10,000 times the annual energy consumption of the U.S. Solar flares give off radiation that includes X-rays and ultraviolet rays, and charged particles called protons and electrons. This sudden surge in radiation can damage spacecraft and even give a dose or radiation to travelers flying in airplanes over Earth's polar regions.

One of the most spectacular features of the Sun are solar prominences. They appear to stream, loop and arch away from the Sun. The most recognizable prominences appear as huge arching columns of gas above the limb (edge) of the Sun. However, when prominences are photographed on the surface of the Sun, they appear as long, dark, threadlike objects and are called filaments. Like sunspots, prominences are cooler (about 10,000 °C) in relation to the much hotter background of the Sun's outer atmosphere (about 1,500,000 °C). Prominences can also erupt from the Sun with a tremendous burst of energy.

If you have seen photographs of a solar eclipse, then you have probably noticed a bright halo around the Sun, called the corona. Sometimes parts of the corona appear to be missing. Logically, we call this area a coronal hole. Scientists believe that the solar wind, a million mile per hour gale that blows away from the Sun, originates in coronal holes. Unlike wind on Earth, the solar wind is a stream of ionized (electrically charged) particles speeding away from the Sun.

The Sun's corona changes with sunspot activity. When there are more sunspots, the corona appears to be held closely to the Sun; when there are fewer sunspots, the corona streams out into space in a shape that resembles the spike on a warlike, peaked helmet called helmet streamers. While helmet streamers are long-lived, their demise often occurs abruptly through a massive and powerful eruption called a coronal mass ejection (CMEs).

These huge clouds of hot solar gas and magnetic field are often associated with solar flares. They can cause magnetic storms when they hit Earth's magnetic field and damage electrical equipment in space and on the ground. For example, in 1989, the Quebec province in Canada suffered an electrical blackout because many transformers were destroyed by a large magnetic storm. That one storm caused many millions of dollars worth of damage. A powerful solar flare erupted from the Sun about three days before the start of the storm at Earth. Even when the Sun is not too active, solar storms can cause problems. A magnetic storm on January 11, 1997 was blamed for the loss of a $270 million dollar AT&T communications satellite. This moderate storm was caused by a coronal mass ejection that erupted from the Sun even though there were no noticeable sunspots.