Active regions on the sun are the sites of spectacular explosive events, including solar flares, and coronal mass ejections, which are associated with eruptions of plasma. The radiation and plasma from these events sweep past the earth and can disrupt spacecraft, radio communications and power systems. Scientists watch closely for these eruptions because modern systems are increasingly sensitive to solar disturbances. But we can still be taken by surprise as the sun rotates, bringing hidden active regions into view. For example, an active region can emerge from the interior of the Sun to a place on the surface facing away from us. Two weeks later, the rotation of the Sun will bring the active region around until it faces us. At that time, we are vulnerable to eruptions of flares and coronal mass ejections.

To locate these regions in advance, scientists developed a technique of using waves on the sun's surface to make images of the far surface of the. Sun. They use the sound waves that come from the far side of the Sun (facing away from the Earth) and reach the near side (facing the earth) after bouncing once from the solar surface. An active region reveals itself because it contains strong magnetic fields that cause a dent in the solar surface. Sound waves which pass through the active region take less time to reach the near side that if there were no active region present.

Using observations of the Sun taken with the SOHO satellite, scientists were able to make images of the far side of the Sun. The image below shows three views of an active region as the spinning of the Sun carried it from the near side to the far side and back again.

The areas in black are active regions, which include sunspots and places with strong magnetic fields. Notice that the images of the far side, made with sound waves, are fuzzier than the images of the near side, which are images made with visible light through telescopes on Earth. This movie shows a comparison of what was imaged on the solar far side with what was present on the near side at the same time: The ability to image the solar far side is an important new tool in predicting space weather. Just as weather forecasters can tell us when a storm is coming on Earth, space weather forecasters can tell us when a solar storm is on its way. Imaging active regions on the far side of the Sun is gives us a very early warning system for telling us when such storms are more likely to occur over the next two weeks.

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