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BY STEPHEN CLARK SPACEFLIGHT NOW
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An artist's concept of Ulysses with the north pole of
the sun in the background. Photo: ESA
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An intriguing
change in the Sun's magnetic field has been spotted by the European-led
solar probe Ulysses. Although the shift had been previously known
by scientists, this is the first time the event has been detected
by a spacecraft out of the elliptic plane of the solar system,
where all planets but Pluto orbit.
The shift
occurs once around every 11 years due to the solar cycle. Every
time the Sun enters period of maximum activity in this cycle --
called solar maximum -- the solar poles switch roles. 2001 is
one such year.
At the time
of the switch, Ulysses was above the south pole of the Sun in
its highly inclined solar orbit.
According
to Andre Balogh, the principal investigator for the Ulysses magnetometer,
the process is complex and fairly drawn out.
"In the
past few months, the direction of the magnetic field observed
by Ulysses fluctuated between the old and the new. Even now, there
are periods when the old polarity is still present," Balogh
said.
"Clearly,
a struggle is going on in the Sun's magnetic field, with freshly
emerging new polarity regions racing towards the polar regions,
encountering the slowly decaying older polarity regions. We know
that the new polarity will win through, but the battle is still
on for another few months."
However, terrestrial
observatories had already noticed the change late last year. Todd
Hoeksema of NASA, the person who made the announcement, explains
that Ulysses is in a much better position to observe the change.
"The
Earth is not the ideal vantage point to see what happens at high
heliolatitudes," he explained. "We are witnessing a
complex process in which different phenomena signal reversal processes
at different times and in different ways. This is why Ulysses,
flying over the polar regions, is much better placed to observe
the disappearance of the old magnetic polarities and the appearance
of the new."
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| Artist's impression of Ulysses in space. Photo: ESA/Dave
Hardy |
One sign
that a battle for magnetic control on the Sun was occurring stemmed
from a solar storm in November. Most solar activity occurs at
relatively low latitudes, but observations also indicated that
increased activity on the Sun's polar regions.
"The
highly energetic particles must cross magnetic field lines to
reach such high latitudes, which suggests that the field lines
must be very tangled up. We know that the magnetic field configuration
is completely different from how it was at solar minimum --- and
these particle observations will help us to understand these differences
in detail," said Richard Marsden, Ulysses project scientist.
"The
Sun's magnetism is very complex," adds Balogh, explaining
the significance of such a finding. "Given this unique chance,
to sit by the ringside as the two magnetic polarities fight it
out, Ulysses is once again able to make a significant step forward
in our understanding of the Sun and the heliosphere."
Launched in
1990, Ulysses is orbiting the Sun for the second time in its highly
inclined orbit. The probe is currently leaving the south pole
region of the Sun to head toward the north pole. The craft will
reach perihelion in May and it will begin observations of the
north pole in October 2001.
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