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Five spacecraft join to solve auroral puzzle

– A remarkable set of observations by five spacecraft orbiting the Earth has led to a breakthrough in understanding the origin of a peculiar and puzzling type of aurora.

Seen as bright spots in Earth's atmosphere and called "dayside proton auroral spots," they were first discovered by NASA's IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) spacecraft in 2000.

By combining observations made last year by the European Space Agency's (ESA) four Cluster spacecraft and IMAGE, a team lead by University of California, Berkeley, physicist Tai Phan has established that the auroras occur when fractures appear in the Earth's magnetic field, allowing particles emitted from the sun to pass through and collide with molecules in our atmosphere.

 Cluster spacecraft configuration
The configuration of IMAGE and the four Cluster spacecraft on March 18, 2002, when they made observations connecting dayside proton auroral spots (bottom image) and solar protons leaking through the Earth's magnetic shield in an area of magnetic reconnection. The solar protons were funneled into Earth's atmosphere along magnetic field lines to form the spot. (Courtesy of Geophysical Research Letters)

"Thanks to Cluster's observations, scientists can directly and firmly link for the first time a dayside proton auroral spot and a magnetic reconnection event," said Philippe Escoubet, ESA's Cluster project scientist.

"This result has opened up a new area of research," Phan said. "We can now watch dayside proton aurorae and use those observations to know where and how the cracks in the magnetic field are formed and how long the cracks remain open. That makes it a powerful tool to study the entry of the solar wind into the Earth's magnetosphere."

The results of the study are published May 21 in Geophysical Research Letters, a journal of the American Geophysical Union, in a paper by Phan of UC Berkeley's Space Sciences Laboratory and an international team of 24 colleagues.

On March 18, 2002, a jet of energetic solar protons collided with Earth's atmosphere and created a bright spot seen by IMAGE just as the four Cluster spacecraft passed overhead and straight through the proton jet.

Earth's magnetic field acts as a shield, protecting the planet from the constant stream of tiny particles, known as the solar wind, that is ejected by the sun. The solar wind itself is a stream of hydrogen atoms, separated into their constituent protons and electrons. When electrons find routes into our atmosphere, they collide with and excite the atoms in the air. When these atoms release their energy, it is emitted as light, creating the glowing "curtains" we see as the aurora borealis in the far north and the aurora australis in the far south. Dayside proton auroral spots are caused by protons "stealing" electrons from the atoms in our atmosphere.

An extensive analysis of the Cluster results has now shown that the region was experiencing a turbulent event known as magnetic reconnection. Such a phenomenon takes place when the Earth's usually impenetrable magnetic field fractures and has to find a new stable configuration. Until the field mends itself, solar protons leak through the gap and jet into Earth's atmosphere, creating the dayside proton aurora.

ESA's Cluster spacecraft were launched on two Russian rockets during the summer of 2000. They fly in formation around the Earth, relaying the most detailed information ever about how the solar wind affects the planet.

The principal investigators for the instruments in the current study were Henri Reme of CESR/Toulouse, France (Cluster Proton Detectors); Andre Balogh of Imperial College, London, United Kingdom (Cluster Magnetic Field Instrument); and Stephen Mende of UC Berkeley (IMAGE/FUV).

The current study was funded by the National Aeronautics and Space Administration and other organizations.

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