Image courtesy SDO/NASA
Published August 7, 2013
The sun’s magnetic field, which spans the solar system, is just months away from flipping, observatory measurements show.
“This change will have ripple effects throughout the solar system,” solar physicist Todd Hoeksema of Stanford University said in a statement.
Hoeksema is the director of Stanford’s Wilcox Solar Observatory, one of just a few observatories around the world that monitors the sun’s polar magnetic fields.
The sun’s magnetic field changes polarity approximately every 11 years during the peak of each solar cycle as the sun’s inner dynamo reorganizes itself.
This next reversal—which will be only the fourth observed since tracking began in 1976—will mark the midpoint of Solar Cycle 24.
During a magnetic field reversal, “the sun’s polar magnetic fields weaken, go to zero and then emerge again with the opposite polarity,” explained solar physicist Phil Scherrer, also at Stanford, in the statement.
Scientists are already seeing signs of the reversal happening, and this time there’s a twist: Data from Wilcox show that the sun’s two hemispheres are oddly out of sync, with the North Pole already beginning to change and the South Pole racing to catch up. That means that for now, at least, the sun effectively has two South Poles.
Soon both poles should be completely reversed. “It looks like we’re no more than three to four months away from a complete field reversal,” Hoeksema said.
The Wilcox Solar Observatory has been observing the sun’s magnetic field since 1975, and its scientists have been converting those numerical measurements into a map that can now be viewed online.
“What we’re really happy about is we’ve never changed or upgraded [the observatory],” Scherrer said in an interview Wednesday. “It’s the only instrument in the world where you can look back over 40 years and know you’re measuring the same thing. That allows us to compare fields from one cycle to the next.”
What Does a Reversal Mean?
A reversal of the sun’s magnetic field will have consequences throughout the solar system since the domain of the sun’s magnetic influence—called the heliosphere—extends far beyond Pluto. Changes to the field’s polarity ripple all the way out to the Voyager probes, which are racing toward interstellar space.
Playing a central role in solar field reversals is the “current sheet,” a sprawling surface that juts out of the sun’s equator where the sun’s slowly rotating magnetic field induces an electric current.
The current itself is small—only one ten-billionth of an amp per square meter—but there’s a lot of it, and the entire heliosphere is organized around it.
During field reversals, the current sheet becomes very wavy. Scherrer likens the undulations to the seams on a baseball.
As the Earth orbits the sun, our planet dips in and out of the wavy current sheet, and the transitions can stir up stormy space weather around us.
The geometry of the current sheet can also affect Earth’s exposure to cosmic rays, which are high-energy particles accelerated to the speed of light by supernova explosions and other violent events in the galaxy.
Cosmic rays pose a threat to astronauts and space probes, and some researchers say they might also affect the cloudiness and climate of Earth.
The sun’s current sheet functions as a barrier to cosmic rays, preventing them from penetrating into the inner solar system. And a wavy, crinkly current sheet appears to create a better shield against these energetic particles.
Earth’s Flip-Flop Coming
The sun isn’t the only body in the solar system with a magnetic field that reverses. Earth has a magnetic field as well, and it has flipped many times over the last billion years.
This isn’t surprising, Scherrer said, because the magnetic fields of both the sun and the Earth are thought to be generated by similar “dynamo” processes that involve rotating and convecting electrically conducting fluids—molten iron in the case of the Earth and hot, ionized gases for the sun.
The difference, however, is that Earth’s magnetic field reversals happen much less frequently—only once every 200,000 to 300,000 years on average, although the actual time can vary widely—and over much longer timescales.
An analysis of centuries-old ship logs performed in 2006, for example, found that the Earth’s magnetic field weakens in staggered steps, and that its strength has declined by a few percentage points since 1840.
If this decline is continuous, scientists predict the Earth’s magnetic field could reverse sometime in the next 2,000 years.
When it does happen, Scherrer thinks that the flip will happen gradually—as is the case with the sun—and won’t be marked by any kind of calamitous drop of the Earth’s magnetic field strength to zero.
“It won’t just disappear and come back again,” Scherrer said.