The Sun Does a Flip
The Sun's magnetic north pole, which was in the northern hemisphere just a few months ago, now points south. It's a topsy-turvy situation, but not an unexpected one.
"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."
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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 -- like clockwork.
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.
Although solar and terrestrial magnetic fields behave differently, they do have something in common: their shape. During solar minimum the Sun's field, like Earth's, resembles that of an iron bar magnet, with great closed loops near the equator and open field lines near the poles. Scientists call such a field a "dipole." The Sun's dipolar field is about as strong as a refrigerator magnet, or 50 gauss (a unit of magnetic intensity). Earth's magnetic field is 100 times weaker.
Below: The Sun's basic magnetic field, like Earth's, resembles that of a bar magnet.
"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."
In the butterfly diagram, pictured below, the Sun's polar fields appear as strips of uniform color near 90 degrees latitude. When the colors change (in this case from blue to yellow or vice versa) it means the polar fields have switched signs.
Above: In this "magnetic butterfly diagram," yellow regions are occupied by south-pointing magnetic fields; blue denotes north. At mid-latitudes the diagram is dominated by intense magnetic fields above sunspots. During the sunspot cycle, sunspots drift, on average, toward the equator -- hence the butterfly wings. The uniform blue and yellow regions near the poles reveal the orientation of the Sun's underlying dipole magnetic field. [more information]
The ongoing changes are not confined to the space immediately around our star, Hathaway added. The Sun's magnetic field envelops the entire solar system in a bubble that scientists call the "heliosphere." The heliosphere extends 50 to 100 astronomical units (AU) beyond the orbit of Pluto. Inside it is the solar system -- outside is interstellar space.
"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."
Left: Steve Suess (NASA/MSFC) prepared this figure, which shows the Sun's spiraling magnetic fields from a vantage point ~100 AU from the Sun.
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.
Right: Following an encounter with Jupiter in 1992, the Ulysses spacecraft went into a high polar orbit. It's maximum solar latitude is 80.2 degrees south. [more]
"This is the most important part of our mission," he says. Ulysses last flew over the Sun's poles in 1994 and 1996, during solar minimum, and the craft made several important discoveries about cosmic rays, the solar wind, and more. "Now we get to see the Sun's poles during the other extreme: Solar Max. Our data will cover a complete solar cycle."
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.
Ulysses Home Page -- learn more about NASA's exploration of the inner heliosphere from JPL
Magnetic Reversals and Moving Continents -- How do we know Earth's magnetic field flips? This excellent web site from NASA/Goddard reveals the answer.
Magnetism - the Key to Understanding the Sun -- an introduction to solar magnetism presented by the NASA/Marshall Space Flight Center solar research group
The Dynamo Process -- a nice tutorial on magnetic dynamos
The Sun's Magnetic Cycle -- from the Goddard Space Flight Center
The Spiral of the Interplanetary Magnetic Field -- includes an eye-catching lawn sprinkler animation that illustrates the Sun's spiraling magnetic field.
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