Sun Does a Flip
NASA scientists who monitor the Sun say
that our star's awesome magnetic field is flipping
-- a sure sign that solar maximum is here.
can't tell by looking, but scientists say the
Sun has just undergone an important change. Our
star's magnetic field has flipped.
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."
Above: Sunspot counts, plotted
here against an x-ray image of the Sun, are nearing
their maximum for the current solar cycle.
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.
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."
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
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.
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
"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."
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 toy.
Left: Steve Suess (NASA/MSFC)
prepared this figure, which shows the Sun's spiraling
magnetic fields from a vantage point ~100 AU from
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.
just passed under the Sun's south pole,"
says Suess, a mission co-Investigator. "Now
it will loop back and fly over the north pole
in the fall."
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.
"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.