by Bjorn Carey

Stanford Report
November 11, 2013

from Stanford Website 




The effects of the event, which occurs every 11 years, will ripple throughout the solar system and be closely monitored by Stanford solar physicists.






Video by Kurt Hickman
The sun's magnetic field is poised to reverse its polarity.

The effects of the event will be closely monitored by Stanford solar physicists.


Every 11 years, the sun undergoes a complete makeover when the polarity of its magnetic field - its magnetic north and south - flips. The effects of this large-scale event ripple throughout the solar system.

Although the exact internal mechanism that drives the shift is not entirely understood, researchers at Stanford's Wilcox Solar Observatory have monitored the sun's magnetic field on a daily basis since 1975 and can identify the process as it occurs on the sun's surface. This will be the fourth shift the observatory has monitored.

New polarity builds up throughout the 11-year solar cycle as sunspots - areas of intense magnetic activity - appear as dark blotches near the equator of the sun's surface. Over the course of about a month, sunspots disintegrate, and gradually that magnetic field migrates from the equator to one of the sun's poles.

As the surviving polarity moves toward the pole, it erodes the existing, opposite polarity, said Todd Hoeksema, a solar physicist at Stanford since 1978 and director of the Wilcox Solar Observatory.


The magnetic field gradually reduces toward zero, and then rebounds with the opposite polarity.

"It's kind of like a tide coming in or going out," Hoeksema said. "Each little wave brings a little more water in, and eventually you get to the full reversal."

The effects of this event are widespread:

The area of space where the sun's magnetic field exerts its influence - called the heliosphere - stretches well beyond Pluto, out to NASA's Voyager probes at the edge of interstellar space.

The sun is also typically at the peak of its activity during a magnetic field reversal, which, in addition to an increased number of sunspots, is marked by a surge in solar flares and coronal mass ejections.

The sun's changing magnetic field and the bursts of charged particles can interact with Earth's own magnetic field, one manifestation of which is a noticeable uptick in the occurrence and range of auroras.


Earth's magnetic field can also affect major electronic systems, Hoeksema said, such as power distribution grids and GPS satellites, so scientists are keen to monitor the heliosphere.

"We also see the effects of this on other planets," Hoeksema said. "Jupiter has storms, Saturn has auroras, and this is all driven by activity of the sun."

Because Wilcox has been operating for so long and with minimal changes to its detecting devices, it has produced one of the most complete and detailed records of daily changes to the sun's global magnetic field.


An interesting feature of the current transition, Hoeksema said, is that the sun's hemispheres are changing at different rates. The northern hemisphere flipped this summer; the southern hemisphere should flip in the near future.

Hoeksema and his colleagues have also noticed that the strength of the magnetic fields at the poles measured two or three years ago were only half what they usually are at the solar minimum.


This indicated that the upcoming solar cycle would be weak as well, which is being borne out in current measurements.

"What happens in the next cycle will be interesting," Hoeksema said. "Technologically, it's something we need to pay more attention to as time goes on."



















Sun's Magnetic Field to Reverse

-   What It Means   -
Ker Than
August 7, 2013
from NationalGeographic Website



The flip could come

in the next three to four months, scientists say.
Magnetic forces draw out loops and strands

of hot plasma from the sun's surface.




An illustration of what it might look like

if magnetic forces draw out loops and strands

of hot plasma from the sun's surface.
Image courtesy SDO/NASA



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.