David Aguilar
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Christine Pulliam
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CfA Press Release

Release No.: 98-07

For Release: Friday, January 9, 1998

New Understanding of the Solar Wind:
The Impact of Ulysses and SOHO Measurements
AAS Invited Talk
Shadia Rifai Habbal
Harvard-Smithsonian Center for Astrophysics

Talk Summary

WASHINGTON, DC---In late 1990, the Ulysses spacecraft set off on an unprecedented journey to orbit the Sun in an elliptical trajectory over its southern and northern poles. Bathed in the stream of particles known as the "solar wind" that rushes outward from the Sun's surface, Ulysses confirmed that the solar wind fills the entireheliosphere, that is, the vast volume of interplanetary space stretching from the Sun to the farthest edge of the Solar System.

The spacecraft also confirmed that the solar wind flows across this space in two remarkably distinct states: as a steady, fast wind that blows out at speeds up to 850 km/s, and as a more capricious, more turbulent, slower component puffing out in gusts of 300 to 400 km/s. The chemical composition of the two types of wind is also clearly different.

Surprises also awaited the mission. The fast wind occupied a significantly larger fraction of the heliosphere, extending to about 30 degrees in latitude above and below the equator. And, around the equator, a pattern of alternating fast and slow winds was present.

While generally steady in nature, the fast wind also showed some signs of fine structure, mainly in the form of microstreams coexisting alongside each other but flowing at slightly different speeds.

The puzzle regarding the origin of the fast and slow solar windat the Sun was eventually unraveled by two other spacecraft:the Solar and Heliospheric Observatory (SOHO) launched in 1995,which observes the Sun from near Earth; and the Galileo space probe, whose radio signal was transmitted through the solar wind while in orbit around Jupiter.

These spacecraft observed the slow wind originating along narrow structures called "stalks," which cap the arched magnetic fields of streamers. The measurements also hinted that the fast wind emerges not only from near the poles but from patches of open magnetic field lines all over the Sun.

Images from the Large Angle Spectroscopic Coronagraph (LASCO) on SOHO also revealed that the magnetic structure of the corona extending out to the edge of the field of view of 30 solar radii is highly filamentary, thus tracing the microstreams observed in interplanetary space by Ulysses.

Further stunning discoveries about the natureof the solar wind very close to its origin at the Sun were made by the Utraviolet Coronagraph Spectrometer (UVCS) on SOHO. For example, oxygen ions in the solar wind were found to have temperatures of over 100 million degrees Celsius, many times hotter than the protons and electrons which are the wind's dominant species. The oxygen ions were also found to be traveling faster than the protons and electrons, suggesting a vital role in the origin and acceleration of this supercharged stream.

With the Sun holding many more secrets, additional surprises may be expected from the continuing journeys of bothUlysses and SOHO into the solar wind.

For more information, contact:

Shadia Habbal, shabbal@cfa.harvard.edu, 617-495-7348
James Cornell, jcornell@cfa.harvard.edu, 617-495-7462

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