Deep roots of solar wind help predict space weathe across the solarsystem

Bill Steigerwald
May 19, 2005
NASA Goddard Space Flight Center
(Phone: 301/286-5017)

Release 05-22

DEEP ROOTS OF SOLAR WIND HELP PREDICT SPACE WEATHER ACROSS THE SOLAR SYSTEM

A layer deep in the solar atmosphere can be used to estimate the speed of
the solar wind, a stream of electrified gas that constantly blows from the
Sun. Estimating the speed of the solar wind will improve space weather
forecasts, enhancing our ability to protect communications, navigation, and
other satellites from the effects of solar storms. We will also be able to
warn human explorers on their way to the planets of the severity of those
storms.

The solar wind flows from the Sun's hot, thin, outer atmosphere, the
"corona". The researchers were surprised to discover that the structure of
the Sun's cooler, dense lower atmosphere, called the chromosphere, could be
used to estimate the speed of the solar wind.

This was unexpected because the solar wind originates in the corona, and the
chromosphere is much deeper: it lies just above the Sun's visible surface.
"It's like discovering that the source of the river Nile is another 500
miles inland," said Dr. Scott McIntosh of the Southwest Research Institute,
Boulder, Colo., lead author of a paper on this research published May 10 in
the Astrophysical Journal.

The new work promises to increase the accuracy of space radiation forecasts.
The Sun occasionally launches billion-ton blasts of electrified gas, called
coronal mass ejections (CMEs), into space at millions of miles (kilometers)
per hour. If a fast CME is plowing through slow solar wind, a shock builds
up in front of the CME that accelerates the electrically charged solar wind
particles. These fast particles can disrupt satellites and are hazardous to
unprotected astronauts.

"Just as knowing more details about the atmosphere helps to predict the
intensity of a hurricane, knowing the speed of the solar wind helps to
determine the intensity of space radiation storms from CMEs," said co-author
Dr. Robert Leamon of L-3 Government Services at NASA's Goddard Space Flight
Center, Greenbelt, Md.

Like wind on Earth, the solar wind is gusty, ranging in speed from about
750,000 miles per hour (approximately 350 km/second) to 1.5 million miles
per hour (700 km/second).

Since the solar wind is made up of electrically-charged particles, it
responds to magnetic fields that permeate the solar atmosphere. Solar wind
particles flow along invisible lines of magnetic force like cars on a
highway. When the magnetic field lines bend straight out into space, as they
do in "coronal hole" regions, the solar wind acts like cars on a drag strip,
racing along at high speed. When the magnetic field lines bend sharply back
to the solar surface, like the pattern of iron filings around a bar magnet,
the solar wind acts like cars in city traffic and emerges relatively slowly.
Scientists have known this for over thirty years and used it to give a crude
estimate for the speed of the solar wind -- either fast or slow.

In the new work, the team has tied the speed of the solar wind as it blows
past Earth to variations deeper in the solar atmosphere than had previously
been detected (or even expected). By measuring the time taken for a sound
wave to travel between two heights in the chromosphere, they were able to
determine that the chromosphere is effectively "stretched thin" below
coronal holes with their open magnetic fields, but compressed below
magnetically closed regions.

The team used the observation to derive a continuous range of solar wind
speeds from the structure of the chromosphere. The wider the chromospheric
layer is, the more it is being allowed to expand by open magnetic fields and
the faster the solar wind will blow. This new method is more precise than
the old "fast or slow" estimate.

NASA's Transition Region and Coronal Explorer (TRACE) spacecraft was used to
measure the speed of sound waves in the chromosphere, and NASA's Advanced
Composition Explorer (ACE) spacecraft was used to take measurements of the
solar wind speed as it blew by the Earth. Comparing the data from the two
spacecraft gave the connection.

"Prior to this discovery, we could only determine solar wind speed from
spacecraft that were roughly in line between the Earth and the Sun, like
ACE, WIND, and the Solar and Heliospheric Observatory. This spacecraft fleet
was placed along the Earth-Sun line because we need to know about the space
weather coming our way. However, compared to the size of our solar system,
this is a very narrow range; it's like looking through a soda straw. With
this discovery, we can use TRACE to build up images that can predict the
solar wind speed throughout half the solar system," said Dr. Joe Gurman, a
solar researcher at NASA Goddard.

For images and more information, refer to:


http://www.nasa.gov/vision/universe/...ind_speed.html

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Jacques :-)

www.spacepatches.info