Satellites unravel a spot of mystery. (Earth Science).

Satellites in the right places at the right time may have solved the puzzle of a strange phenomenon high in Earth's atmosphere.

The so-called proton auroral spots, which glow brightest at ultraviolet wavelengths, occur at altitudes of about 120 kilometers, says Tai Phan, a space physicist at the University of California, Berkeley. The spots occur when protons in the solar wind--the torrent of charged particles streaming from the sun--slam into Earth's atmosphere. Scientists have suggested that when the planet's protective magnetic field is disrupted, these high-energy protons break through and can reach the atmosphere. The satellite data gathered last year bolster that scenario.

On March 18, 2002, a 5.5-hour gust of solar wind struck Earth, says Phan. During that time, NASA's IMAGE spacecraft was about 33,000 km above the North Pole and spied a proton auroral spot over Greenland. Simultaneously, the four spacecraft of the European Space Agency's Cluster mission, which orbit Earth in close formation and monitor the solar wind's effects, passed through a stream of high-energy protons over the same region. Other Cluster instruments indicated that a portion of Earth's magnetic field had temporarily fractured and that protons were accelerating through the gap.

Phan and his colleagues report their findings in the May 15 Geophysical Research Letters.

Last year's lucky harvest of satellite data now enables scientists to infer changes in Earth's magnetic field from IMAGE observations alone. They also may shed light on how charged particles are accelerated by strong magnetic fields, such as those on the sun that cause solar flares, Phan notes.--S.P.