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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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