Ulysses witnesses the Sun's magnetic struggle
25 January 2001During 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 16 January, ESA's Ulysses spacecraft completed its four-month southern solar polar passage as solar activity reached its peak. Did it 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 intrepid probe continues to weather the effects of numerous solar storms churned up by the magnetic turmoil welling up from deep inside 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 8 November 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 was taking place around 20 north. The surprise is that we saw 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 16 January, 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 3 September, the activity should have begun to decline again.