Spaceflight Now: Breaking News

Ulysses solar explorer detects magnetic shift

Posted: January 28, 2001

An artist's concept of Ulysses with the north pole of the sun in the background. Photo: ESA
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."

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.