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Northern Lights
May 31, 2000

Auroras in action.

If you prefer to view it with RealPlayer, click here.

video courtesy NASA
edited by Jed Boyar

While beautiful to look at, auroras—those captivating displays of light that can be seen in the night sky over the Earth’s polar regions—have confounded scientists trying to explain what causes them. But after 50 years of debate, there finally is an answer—auroras are visible effects of space storms.

Using data collected from spacecraft, scientists from the International Solar-Terrestrial Physics (ISTP) program have shown how energy from the solar wind gets into the magnetic space around the Earth. The phenomenon, called reconnection, is responsible for both auroras and magnetic storms in space that can affect radio transmissions, satellite operations and electric power systems worldwide. The findings were presented today at a meeting of the American Geophysical Union in Washington, D.C.

Let there be light

Until now, it was widely believed that auroras, named for the Roman goddess of dawn, were caused by electrically charged particles from the sun plunging directly into the Earth’s atmosphere. But now scientists have learned that the process is more complicated.

This animation illustrates reconnection.

If you prefer to view it with a RealPlayer, click here.

video courtesy NASA

The magnetosphere—the magnetic field that surrounds our planet and extends tens of thousands of miles into space—normally deflects solar wind and other radiation away from the Earth, acting like a giant cocoon. In this way, it protects the Earth from the ravages of space weather caused by the much larger and much more powerful magnetic field of our nearest star: the sun.

Occasionally, however, the Earth’s magnetosphere and the solar wind become aligned, and if the solar wind is strong enough, this alignment can cause the magnetic field lines of the sun and the Earth to link together. This process, a little like two magnets being drawn together, has been dubbed "reconnection".

When reconnection occurs, the solar wind can tear through the magnetosphere, allowing the energy from the sun to enter it. "And that, as it were, opens up a trap door, a valve in the magnetosphere that allows energy to come in," explains Jeffrey Hughes, professor of astronomy and director of The Center for Space Physics at Boston University.

Sun (in green)
The Sun
image: NASA

The solar energy excites particles already trapped around the Earth, which are then guided toward the polar regions by Earth’s magnetic field. These excited particles are then released at the poles as the Earth’s magnetic field dives toward them. Because the particles energized by reconnection all end up at the poles, their energy is released explosively, says Hughes. "Those explosions create a lot of energetic particles that can knock out spacecraft, cause communication disruptions and even cause electrical power grids to fuse out," he says.

Auroras, a milder effect of reconnection, are formed when atoms of oxygen and nitrogen in the atmosphere are hit by high-energy particles from the magnetosphere. This impact causes the atoms to give off light causing the eerie shifting glow of the Northern and Southern Lights.

Seeing is believing

Aurora as seen from space.
image: NASA

Previously, evidence of reconnection had been indirect because scientists could only detect signs of it after the fact. But data collected from two spacecraft — NASA’s Polar and Japan’s Geotail—have allowed space physicists to see the process in action over different parts of the Earth.

By flying through a region on the sunlit side of the Earth while reconnection was in progress, the Polar spacecraft, first launched in early 1996, has helped scientists understand how solar wind energy gets into the Earth’s magnetosphere. On the night side, Geotail, first launched in 1992, has made dozens of passes through Earth’s magnetic tail (the long, stretched-out part of the magnetosphere), allowing scientists to pinpoint where reconnection occurs there: approximately 140,000 to 160,000 kilometers downwind of our planet.

Bad weather ahead

Debris in space (?)
There is more "space junk" to be damaged than ever before.
image: NASA
Now that scientists have discovered reconnection and how it works, they’re in a better position to understand solar events that cause damaging space storms around the Earth. Space weather can distort radio signals and navigation devices (like the Global Positioning System), disrupt satellite function and wreak havoc with our power systems, causing blackouts and fuel leaks, or even prove deadly to astronauts if they happen to get caught in it.

What’s more, every 11 years the solar maximum (the peak of solar activity) arrives, bringing with it everything from increased sunspots and flares to increased magnetic storms and auroras. In 1989, during the last solar maximum, a power grid collapsed in Canada leaving six million people without electricity.

The next solar maximum is due to take place sometime in the next 12 months. Since we are more dependent than ever on technology that could be affected by space storms, the potential for damage is much higher than in the past—there were only 150 geostationary satellites in orbit around the Earth during the last solar maximum; this time around there are about 600 in place. But, ironically, that’s exactly why we may be in a better position to predict bad space weather.

Elsewhere in the Universe

Earth isn’t the only planet with a magnetosphere. Jupiter, Saturn, Uranus, and Neptune all have magnetic fields. Jupiter’s is about 20,000 times stronger than the Earth’s and has many modes of radio emissions that scientists still don’t really understand. Even the moon has patches of magnetized rocks and may have had a magnetic field at some point.

While the Earth’s magnetic field probably originates in its molten iron core, the same doesn’t hold true for other planets. In the case of Jupiter and Saturn, the magnetic fields may originate in their metallic hydrogen cores, while the fields of Uranus and Neptune may be carried somehow in methane ices.

In addition, NASA’s recently launched spacecraft IMAGE uses an advanced imaging system to provide global images of the previously invisible magnetosphere. "We’ve got more of an idea of what’s going on and we hope we can predict," says Hughes. "I think the long-term view is to predict even better for 2010 when the next

Elsewhere on the web:

Aurora Facts from the Geophysical Institute

NASA’s Mission to Geospace

Today’s space weather

Exploring the Earth’s magnetosphere

The Sun Earth Connection



by Jill Max


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