|Subj:||Ulysses Forecasts Weather at Sun's North Pole|
|Date:||9/1/01 1:13:35 AM Pacific Daylight Time|
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Contact: Martha J. Heil (818) 354-0850
FOR IMMEDIATE RELEASE Aug. 31, 2001
ULYSSES FORECASTS WEATHER AT SUN'S NORTH POLE
Space physicists predict gusty winds for the next few
months at the Sun's north pole, an area that will be observed
when the Ulysses spacecraft passes over it starting on Aug.
This pass over the pole occurs at a time of solar
maximum, when the Sun is more active. "The solar wind is
patchier -- gustier -- during solar maximum, because the
sources of the wind are more irregular," said Dr. Edward
Smith, the project scientist at NASA's Jet Propulsion
Laboratory on Ulysses, which is managed jointly by JPL and the
European Space Agency.
This will be Ulysses' second pass over the Sun's north
pole. It completed a circuit of the Sun in 1996 when the Sun
was less active, then passed by the Sun's south pole last fall
when the Sun was much more active. Now scientists will use
what they've learned from previous passes to analyze data in
the coming months.
Earth's magnetic field reverses itself only once every
200,000 years or so, but the Sun's internal magnet flips every
11 years, associated with a rise in solar activity. As it
flips, the magnetic field becomes disordered, and the surface
of the Sun becomes more active, shooting out bubbles of gas
and energy called coronal mass ejections in every direction.
The reversal of the magnetic field has a particularly dramatic
effect on the poles. The Sun's magnetic field will be weak and
X-ray coronal holes at the north pole should have shrunk
during this solar maximum, because the magnetic field is
unevenly spread around the surface of the Sun.
In 1995, Ulysses saw strong and simple magnetic fields at
both poles of the Sun. Large, dark X-ray coronal holes formed
in the solar atmosphere over the polar caps. The solar wind is
fast and hot when coronal holes in the Sun remain open over
long periods of time, as they do at solar minimum.
As Ulysses passed by the south pole of the Sun a few
months ago, scientists expected to find that magnetic lines
were pointing outward, because observations from Earth show
that the magnetic field has already reversed at the Sun's
surface. Instead, they found that the magnetic lines were
still pointing inward, just as they had been throughout solar
minimum. They theorize that this could be caused by the
magnetic lines forming small, closed loops next to the pole
instead of escaping into space. For now, the magnetic field
and solar wind seen over the pole appear to originate near the
Sun's equator rather than from the polar cap.
The Ulysses science team will also study energetic
charged particles, which speed away from the Sun's surface and
escape into space. These are a regular feature of solar
maximum. "These energetic solar particles can be as hot as
100 billion Kelvin (180 billion Fahrenheit) and can cause
damage to spacecraft," said Dr. Bruce Goldstein, deputy
project scientist on Ulysses at JPL. "However, they still
follow the magnetic field lines."
At solar minimum, Ulysses saw very few particles coming
from the poles. This was because there were fewer particle
events occurring on the Sun and also because the particles
follow magnetic lines of force that did not reach into the
Sun's polar regions.
As it studied the south pole during solar maximum,
Ulysses saw energetic particles from many of the past year's
solar flares, though they originated at the Sun's equator.
Those observations are further evidence that the magnetic
field and solar wind seen at the polar caps really originate
close to the equator.
Ulysses, launched in 1990, is a joint venture of NASA and
the European Space Agency. JPL manages Ulysses for NASA's
Office of Space Science, Washington, D.C. More information on
the Ulysses mission is available at the JPL Ulysses web site:
http://ulysses.jpl.nasa.gov and the ESA Ulysses web site,