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For Immediate Release:
January 17, 1997

Contact: Dave Thomas
at (202) 777-7507

Solar Wind Heavy Ions Driving Force in Comet Hyakutake's X-ray Emissions

The surprising discovery of x-ray emissions from comet Hyakutake in 1996 left scientists scrambling for an explanation for their presence. A new theory involving often overlooked heavy ions in the solar wind has been published in the January 1 issue of Geophysical Research Letters of the American Geophysical Union.

Thomas Cravens, of the University of Kansas, says that the solar wind is known to interact with comets and the result of that is the visible comet tail we see from Earth. However, this interaction has not been known, until now, to produce x-ray emissions. Cravens' work suggests that ions comprising 0.1% of the solar wind, such as oxygen, carbon, and nitrogen, appear to be the driving force behind these newly discovered x-ray emissions.

These ions are highly stripped, meaning their electrons have been removed due to the intense heat of the sun's corona, which is where the solar wind originates. Once the solar wind is released into the solar system, its composition is frozen with these highly charged heavy ions intact.

When the solar wind approaches the comet, it finds the comet spewing out neutrals, which are atoms or molecules with equal numbers of protons and electrons. These molecules have no net electrical charge. In an attempt to become more stable, the solar wind ions grab electrons from these neutrals. This is called charge transfer. The grabbed electron is now part of the ion but is far from its nucleus, meaning it is still in a highly excited state. The electron will move closer to the nucleus, but in order to do so, the ion will have to rid itself of energy, called de-excitation. Depending on the level of energy released in this process, the result will be ultraviolet emissions (lower energy required) or x-ray emissions (higher energy required). Thus the energy created and stored in the sun's corona is eventually used up at the comet to produce observable energy.

Cravens' theory is not the only one to explain the mechanism of x-ray emission. The discovery of x-ray emissions from comet Hyakutake has stimulated several papers on the topic, which will be the subject of a special session at the 1997 Spring Meeting of the American Geophysical Union in Baltimore, Maryland, May 27-30.

Note: Reporters seeking additional information may contact Dr. Cravens at 913-864-4739.

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