Surprises from the Sun's South Pole (Forwarded)

ESA News
http://www.esa.int

19 February 2007

Surprises from the Sun's South Pole

Although very close to the minimum of its 11-year sunspot cycle, the Sun
showed that it is still capable of producing a series of remarkably
energetic outbursts, ESA-NASA Ulysses mission revealed.

In keeping with the first and second south polar passes (in 1994 and
2000), the latest high-latitude excursion of the joint ESA-NASA Ulysses
mission has already produced some surprises. In mid-December 2006,
although very close to the minimum of its 11-year sunspot cycle, the Sun
showed that it is still capable of producing a series of remarkably
energetic outbursts.

The solar storms, which were confined to the equatorial regions, produced
quite intense bursts of particle radiation that were clearly observed by
near-Earth satellites. Surprisingly, similar increases in radiation were
detected by the instruments on board Ulysses, even though it was three
times as far away and almost over the south solar pole. "Particle events
of this kind were seen during the second polar passes in 2000 and 2001, at
solar maximum," said Richard Marsden, ESA's Ulysses Project Scientist and
Mission Manager. "We certainly didn't expect to see them at high latitudes
at solar minimum!"

Scientists are busy trying to understand how the charged particles made it
all the way to the poles. "Charged particles have to follow magnetic field
lines, and the magnetic field pattern of the Sun near solar minimum ought
to make it much more difficult for the particles to move in latitude,"
said Marsden.

One of the puzzles remaining from the first high-latitude passes in 1994
and 1995 has to do with the temperature of the Sun's poles. When Ulysses
first passed over the south and then the north solar pole near solar
minimum, it measured the temperatures of the large polar coronal holes.

"Surprisingly, the temperature in the north polar coronal hole was about 7
to 8 percent lower compared with the south polar coronal hole," said
Professor George Gloeckler, Principal Investigator for the Solar Wind Ion
Composition Spectrometer (SWICS) on board Ulysses.

"We couldn't tell then whether this was simply due to progressive cooling
of both polar coronal holes as the Sun was approaching its minimum level
of activity in 1996, or whether this was an indication of a permanently
cooler north pole."

Now, as Ulysses again passes over the large polar coronal holes of the Sun
at solar minimum we will finally have the answer. Recent SWICS
observations show that the average temperature of the southern polar
coronal hole at the current solar minimum is as low as it was 10 years ago
in the northern polar coronal hole. "This implies that the asymmetry
between north and south has switched with the change of the magnetic
polarity of the Sun," said Gloeckler. The definitive proof will come when
Ulysses measures the temperature of the north polar coronal during the
next 15 months.

For more information:

George Gloeckler
Ulysses SWICS Principal Investigator, University of Maryland, USA
Email: gg10 @ umail.umd.edu

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