Astronomy Picture of the Day 12-04-03

Explanation: Headed for the first close-up exploration of the Pluto-Charon system and the icy denizens of the Kuiper belt, NASA's New Horizons spacecraft is pictured here in an artist's vision of the robot probe outward bound. The dramatic scene depicts the 465 kilogram spacecraft about one year after a planned 2006 launch, following a flyby of gas giant Jupiter. While the Jupiter flyby will be used as a gravity assist maneuver to save fuel and cut travel time to the outer reaches of the Solar System, it will also provide an opportunity to test instruments and study the giant planet, its moons, and magnetic fields. The Sun is seen from eight hundred million kilometers away, with inner planets Earth, Venus, and Mercury aligned on the left. A dim crescent of outermost Galilean moon Callisto, orbiting Jupiter just inside of the spacecraft's trajectory, appears to the upper right of the fading Sun. Left of Jupiter itself is Europa and in the distant background are the faint, unresolved stars and dust clouds of the Milky Way. New Horizons' planned arrival at Pluto-Charon is in the summer of 2015.

Stormy space weather slips through cracks
NASA NEWS RELEASE
Posted: December 3, 2003

Immense cracks in Earth's magnetic field remain open for hours, allowing the solar wind to gush through and power stormy space weather, according to new observations from NASA's IMAGE spacecraft and the joint NASA/European Space Agency (ESA) Cluster satellites.

An artist's concept of the solar wind and the Earth's magnetic field as represented by red lines in the image. Credit: NASA

The cracks were previously detected, but researchers now know they can remain open for long periods, rather than opening and closing for just very brief intervals. This new discovery about how the Earth's magnetic shield is breached is expected to help space physicists give better estimates of the effects of severe space weather.

"We discovered that our magnetic shield is drafty, like a house with a window stuck open during a storm," said Dr. Harald Frey of the University of California (UC), Berkeley, lead author of a paper on this research published Dec. 4 in Nature. "The house deflects most of the storm, but the couch is ruined. Similarly, our magnetic shield takes the brunt of space storms, but some energy continually slips through its cracks, sometimes enough to cause problems with satellites, radio communication and power systems," he said.

"The new knowledge that the cracks are open for long periods, instead of opening and closing sporadically, can be incorporated into our space weather-forecasting computer models to more accurately predict how our space weather is influenced by violent events on the sun," said Dr. Tai Phan, also of UC Berkeley, co-author of the Nature paper.

The solar wind is a stream of electrically charged particles (electrons and ions) blown constantly from the sun. The solar wind transfers energy from the sun to the Earth through the magnetic fields it carries and its high speed (hundreds of miles/kilometers per second). It can get gusty during violent solar events, like Coronal Mass Ejections (CMEs), which can shoot a billion tons of electrified gas into space at millions of miles per hour.

Earth has a magnetic field that extends into space for tens of thousands of miles, surrounding the planet and forming a protective barrier to the particles and snarled magnetic fields the sun blasts toward it during CMEs. However, space storms, which can dump 1,000 billion watts, more than America's total electric-generating capacity, into the Earth's magnetic field, indicated the shield was not impenetrable.

In 1961, Dr. Jim Dungey of the Imperial College, United Kingdom, predicted cracks might form in the magnetic shield when the solar wind contained a magnetic field oriented in the opposite direction to a portion of the Earth's field. In these regions, the two magnetic fields would interconnect through a process known as "magnetic reconnection," forming a crack in the shield through which the electrically charged particles of the solar wind could flow. In 1979, Dr. Goetz Paschmann, of the Max Planck Institute for Extraterrestrial Physics, Germany, detected the cracks using the International Sun Earth Explorer spacecraft. However, since this spacecraft only briefly passed through the cracks during its orbit, it was unknown if the cracks were temporary features or if they were stable for long periods.

An artist's concept of the solar wind and the Earth's magnetic field as represented by red lines in the image. Credit: NASA

In the new observations, the Imager for Magnetopause to Aurora Global Exploration (IMAGE) satellite revealed an area almost the size of California in the arctic upper atmosphere (ionosphere) where a 75-megawatt "proton" aurora flared for hours. This aurora, energetic enough to power 75,000 homes, was different from the visible aurora known as the Northern and Southern lights. It was generated by heavy particles (ions) hitting the upper atmosphere and causing it to emit ultraviolet light, which is invisible to the human eye but detectable by the Far Ultraviolet Imager on IMAGE.

While the aurora was being recorded by IMAGE, the four- satellite Cluster constellation flew far above IMAGE, directly through the crack, and detected solar wind ions streaming through. Normally, these solar wind ions would be deflected by Earth's shield, so Cluster's observation showed a crack was present. This stream of solar wind ions bombarded our atmosphere in precisely the same region where IMAGE saw the proton aurora. The fact that IMAGE was able to view the proton aurora for more than nine hours, until IMAGE progressed in its orbit to where it could not observe the aurora, implies the crack remained continuously open. Estimating from the IMAGE and Cluster data, the crack was twice the size of the Earth at the boundary of our magnetic shield, about 38,000 miles (60,000 km) above the planet's surface. Since the magnetic field converges as it enters the Earth in the polar regions, the crack narrowed to about the size of California down near the upper atmosphere.

IMAGE was launched March 25, 2000 to provide a global view of the space around Earth influenced by the Earth's magnetic field. The Cluster satellites, built by ESA, and launched July 16, 2000, are making a three-dimensional map of the Earth's magnetic field.

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