The Sun's magnetic north pole
, which was in the northern hemisphere just a few months ago, now points south says NASA
"This always happens around the time of solar maximum
," says David Hathaway
, a solar physicist
at the Marshall Space Flight Center
. "The magnetic poles
exchange places at the peak of the sunspot cycle
. In fact, it's a good indication that Solar Max
is really here."
The Sun's magnetic poles will remain as they are now, with the north magnetic pole pointing through the Sun's southern hemisphere, until the year 2012
when they will reverse again. This transition happens, as far as we know, at the peak of every 11-year sunspot cycle
Earth’s magnetic field
also flips, but with less regularity. Consecutive reversals are spaced 5 thousand years to 50 million years apart. The last reversal happened 740,000 years ago. Some researchers think our planet is overdue for another one, but nobody knows exactly when the next reversal might occur.
When solar maximum arrives and sunspots
pepper the face of the Sun, our star's magnetic field
begins to change. Sunspots are places where intense magnetic loops -- hundreds of times stronger than the ambient dipole field
-- poke through the photosphere
"Meridional flows on the Sun's surface carry magnetic fields from mid-latitude sunspots to the Sun's poles," explains Hathaway. "The poles end up flipping because these flows transport south-pointing magnetic flux to the north magnetic pole, and north-pointing flux to the south magnetic pole." The dipole field steadily weakens as oppositely-directed flux accumulates at the Sun's poles until, at the height of solar maximum, the magnetic poles change polarity and begin to grow in a new direction.
Hathaway noticed the latest polar reversal in a "magnetic butterfly diagram." Using data collected by astronomers at the U.S. National Solar Observatory
on Kitt Peak
, he plotted the Sun's average magnetic field, day by day, as a function of solar latitude and time from 1975
through the present. The result is a sort of strip chart recording that reveals evolving magnetic patterns on the Sun's surface. "We call it a butterfly diagram
," he says, "because sunspots make a pattern in this plot that looks like the wings of a butterfly
"Changes in the Sun's magnetic field are carried outward through the heliosphere by the solar wind," explains Steve Suess
, another solar physicist at the Marshall Space Flight Center. "It takes about a year for disturbances to propagate all the way from the Sun to the outer bounds of the heliosphere
Because the Sun rotates (once every 27 days) solar magnetic fields corkscrew
outwards in the shape of an Archimedian spiral
. Far above the poles the magnetic fields twist around like a child's Slinky
Because of all the twists and turns, "the impact of the field reversal on the heliosphere is complicated," says Hathaway. Sunspots are sources of intense magnetic knots that spiral outwards even as the dipole field vanishes. The heliosphere doesn't simply wink out of existence when the poles flip -- there are plenty of complex magnetic structures to fill the void.
Or so the theory goes.... Researchers have never seen the magnetic flip happen from the best possible point of view --that is, from the top down.
But now, the unique Ulysses
spacecraft may give scientists a reality check. Ulysses, an international joint venture of the European Space Agency
and NASA, was launched in 1990 to observe the solar system from very high solar latitudes. Every six years the spacecraft flies 2.2 AU over the Sun's poles. No other probe travels so far above the orbital plane of the planets.
To learn more about the Sun's changing magnetic field and how it is generated, please visit "The Solar Dynamo
," a web page prepared by the NASA/Marshall solar research group
. Updates from the Ulysses spacecraft may be found on the Internet from JPL at http://ulysses.jpl.nasa.gov.
Quotes and technical info taken from NASA.gov