Thursday, November 23, 2006

The Sun's Poles

Credits: JPL-ESA, 1994 ESA Space

Ulysses starts third passage over the Sun's south pole.

Launched in 1990, the European-built spacecraft is engaged in the exploration of the heliosphere, the bubble in space blown out by the solar wind. Given the capricious nature of the Sun, this third visit will undoubtedly reveal new and unexpected features of our star's environment.

The first polar passes in 1994 (south) and 1995 (north) took place near solar minimum, whereas the second set occurred at the height of solar activity in 2000 and 2001.

As Ulysses approaches the polar regions for the third time, the Sun has settled down once again and will be close to its minimum. Ulysses orbits the Sun once every 6.2 years, making it perfect for studying the 11-year solar activity cycle. One can really say that Ulysses is exploring the heliosphere in four dimensions - covering all three spatial dimensions as well as time.

Even though the Sun will be close to its activity minimum just as it was in 1994-95, there is one fundamental difference: the Sun's magnetic field has reversed its polarity. In addition to the 11-year activity cycle, the Sun has a magnetic cycle of 22 years, known as the Hale Cycle. Ulysses, now in its 17th year in orbit, is giving scientists the chance to observe the heliosphere from a unique, out-of-ecliptic vantage point and with the same set of instruments over almost a complete Hale Cycle.

The Ulysses science team is expecting to find that the change in polarity of the Sun's magnetic field will have a clear effect on the way cosmic ray particles reach our location in the inner heliosphere. During the last solar minimum, positively charged particles had a slightly easier time reaching the polar regions; this time, the negatively charged electrons should have the advantage.

But there could be surprises. In 1994, the pole-to-equator difference in the number of particles observed, although present, was much smaller than expected. This lead to several new models for the way charged particles move in the complex environment of interplanetary space. The new observations will test if these new theories are correct.

Another surprise from the first polar passes was the fact that the heliosphere is not as symmetric as previously believed. The Sun's magnetic field was found to be slightly stronger in the south than in the north. Scientists be watching out for this effect as Ulysses swings from the south pole to the north in 2007.

Ulysses returns to the Sun's polar cap NASA release 20th Nov 2006
Ulysses embarks on third set of polar passes 17th November 2006
Ulysses Starts New Journey Around The Sun's Poles 21st Nov 2006
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