Staying a Step Ahead of the Sun
The sun gives Earth light and heat. Its energy fuels Earth's weather and climate. Humans and animals -- nearly all living things -- would not exist without the star at the center of the solar system. Yet, from a very young age, children are told not to stare directly at the sun. The very object that gives people life can also hurt their eyes.
Image to left: Produced by SOHO's Large Angle and Spectrometric Coronagraph, this image shows a coronal mass ejection spinning off from the sun. A coronagraph is a telescope designed to block light coming from the solar disk (indicated by the white circle at the center of the image) in order to see the extremely faint emission coming from the corona, the outermost layer of the solar atmosphere. Credit: NASA
Fortunately, machines are less sensitive than people. NASA's Solar and Heliospheric Observatory not only looks right at the sun, but also sees inside of it. SOHO has watched the sun continuously since the spacecraft's 1995 launch into space. As the satellite orbits along a path that lies between the Earth and sun, a dozen onboard sensors gaze through blinding solar light to study the sun inside and out.
The more scientists know about the sun, the better they can anticipate its harmful impacts on Earth. Solar storms, for example, can cause power outages by damaging electricity distribution grids. These storms can disrupt radio signals, making it difficult for air traffic controllers to communicate with pilots. The storms can also bombard astronauts with dangerous radiation.
What is a solar storm? It's an intensification of the solar wind, a stream of charged particles that constantly flows off of and away from the sun. The solar wind rushes toward Earth at an average speed of about 400 km/s (about 1 million mph). Normally, this "electric breeze" is no match for Earth's magnetic field, which deflects the particles harmlessly away.
Occasionally, however, an explosion on the sun temporarily increases the density of the solar wind, overwhelming Earth's magnetic field and causing all sorts of problems. The frequency of these explosions, known as solar flares
and coronal mass ejections
, tends to go in cycles that last approximately 11 years. Understanding how gas flows deep inside the sun is critical to predicting the timing and intensity of these cycles, which alternate between quiet and active.
Image to left: An artist's conception shows Earth's magnetic field deflecting solar radiation away from the Earth. Credit: NASA
The sun is currently reaching the end of a quiet cycle. Observations by SOHO recently helped scientists at the National Center for Atmospheric Research make what they believe is a more accurate prediction of the onset and strength of the upcoming active cycle. The scientists estimate that solar activity will begin to escalate in late 2007 or early 2008, about one year later than previously thought. This information will help utility companies, satellite operators and others who manage sensitive systems prepare ahead of time for the expected 30 to 50 percent increase in solar storms over the current cycle.
A relatively new technique called helioseismology -- "helio" is Greek for "sun" -- made the improved forecast possible. Similar to the way a seismograph detects earthquakes caused by waves of energy traveling below and along the Earth's surface, SOHO has an instrument that measures the movement of sound waves inside the sun. Tracking these sound waves helps scientists create a picture of the sun's inner workings, including the flow of gas that controls solar cycles.
SOHO has almost reached the end of its lifetime. It is expected to continue monitoring the sun through early next year, after which it will be replaced by satellites such as the Solar Dynamics Observatory, scheduled to launch in 2008. SOHO is a collaborative mission between NASA and the European Space Agency.
Dan Stillman, Institute for Global Environmental Strategies