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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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