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Daily University Science News
During
solar maximum, when the Sun's activity is at a peak in its
11 year cycle, the polarity of its magnetic field changes:
the north pole takes on the polarity of the south pole and
vice versa.
Now,
for the first time ever, a spacecraft has witnessed this
process from a front-row seat high above the Sun's south
pole.
On January
16, the European Southern Observatory (ESA)'s Ulysses spacecraft
completed its four-month southern solar polar passage as
solar activity reached its peak.
Did
Ulysses see the Sun's polarity switch?
Andre
Balogh, from Imperial College, London, who is Principal
Investigator for the Ulysses magnetometer, says:
"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.
"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."
Viewed
from the Earth, the Sun's magnetic field seems to have already
switched. At the ESLAB symposium last October, Todd Hoeksema
from NASA headquarters reported that ground-based observatories
had already noticed the change.
Balogh
points out, however, that "the Earth is not the ideal
vantage point to see what happens at high heliolatitudes.
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."
The
Ulysses probe continues to weather the effects of numerous
solar storms churned up by the magnetic turmoil welling
up from deep within the Sun's interior. These storms release
large numbers of energetic particles that stream away from
the Sun.
One
particularly strong solar storm occurred around midnight
on November 8 last year. Spacecraft in orbit around the
Earth recorded large numbers of energetic particles generated
by it. The surprise was that Ulysses also detected the storm's
effects at about the same time.
"Most
of the activity on the Sun is taking place around 200 north.
The surprise is that we're seeing almost identical signatures
over the pole. 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," says Richard Marsden, Ulysses Project Scientist
from ESTEC, the Netherlands, who has been examining data
from the COSPIN experiment on board Ulysses.
"The
Sun's magnetism is very complex," adds Balogh. "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."
On January
16, Ulysses crossed the 70th solar parallel, marking the
end of its second passage above the south pole. The first
time Ulysses visited the south pole, in 1994, the Sun was
near its activity minimum. By the time the spacecraft begins
its north polar passage on September 3, the activity should
have begun to decline again.
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