The Sun, Spinning For A Flip
(An artist’s rendering of the sun’s poles, which will change when its magnetic field flips. Credit: NASA)
"Something big is about to happen on the Sun," intones the narrator of a NASA-produced video. The viewer waits for the big news, only to find out that (1) there’s no visible manifestation of some kind of event that’s billed as important, and (2) there’s actually nothing unusual about the event at all. Science journalism competes with various other media for consumers and let’s face it — physics doesn’t usually excite people like, say, the selection of the 12th Doctor. But that’s okay, it’s about spin, right?
The funny thing is that, in this case, it really is about spin, but not the way you might think.
Stories have appeared in various media outlets in the last week about the pending magnetic polarity reversal expected to occur in the outer reaches of the Sun sometime in the next few months; the NASA video says that we’re “three to four months away” from a “complete field reversal”. It sounds a little scary: what does that mean for us? Not a whole lot, as it turns out, and far less than the fate that might befall us if the Earth’s magnetic field reversed polarity, which some think is overdue. Digging a little further, it turns out that the polarity of the Sun’s magnetic field changes pretty reliably every eleven years at the peak of its magnetic activity cycle. This plot from the video shows the number of sunspots over time since 1900:
There’s a clear, rhythmic pattern to those numbers; sometimes the peak sunspot number is bigger, sometimes smaller. The numbers in the gray circles above the dots refers to the “cycle number” of each maximum. The maximum of Cycle 19, for example, was the strongest of the 20th century, but the next maximum in 1969-70 was comparatively weak. We’re right at the peak of Cycle 24.
How do we know that? The changing orientation of the Sun’s magnetic field betrays the coming change. Measurements of the strength and polarity (represented with the North and South poles of a bar magnet, or + and - on the plot below) show the polarity reversals when the plotted lines cross the horizontal line in the middle of the plot. The circled, shaded areas show the observed reversals since the mid-70’s in data from Stanford University’s Wilcox Solar Observatory:
Okay, great: we’re in one of the little circled regions now. What’s going to happen? All the norths are going to become souths, and vice versa.
Well that’s kind of it. The media reporting on this talked about current sheets and the heliosphere; in short, these magnetic changes on the Sun will ripple outward to the very edge of the Solar System, where the Sun’s magnetic field becomes indistinguishable from interstellar fields threading through the Milky Way. Which is still a good thing, because the Sun’s magnetic field helps shield the inner Solar System form harmful cosmic rays. But none of the stories I read this week about all this sign flipping said anything about why this is all happening in the first place.
And the short answer to that why, somewhat startlingly, is we don’t really know. The Sun has a magnetic field in the first place because of something called the dynamo effect: rotate charges in a circle somehow, and a magnetic field appears inside the encircled area. As the Sun is a big ball of rotating, electrically-charged gas, it can keep that going in a self-sustaining way; left to its own devices, the field lines should emerge at one pole of the Sun (where the spinning effect is the least) and terminate at the other pole, making complete loops inside our star. But there’s a problem: the Sun keeps spinning, and as it spins, it has a tendency to pull the field lines along with it, and they get twisted up. Magnetic field lines are kind of like rubber bands: stretch, pull, and twist them hard enough and they’ll break. But before they do, they’ll protrude out of the Sun’s interior. Where they break the surface, we see the visible phenomenon of sunspots.
Why does this break down, and why do the sunspots go away only to return several years later. That’s the part we don’t really understand. But it seems to have something to do with this polarity flip that happens at the peak of every solar cycle. Right as the magnetic fields are twisted up the most, suddenly the flip happens. New sunspots form, but for the next 2-3 years, they are seen in fewer numbers. After a low point, suddenly new spots appear at high latitudes on the Sun, and the process starts all over again.
Here’s the kicker: The next solar maximum (Cycle 25) could be the weakest in centuries. The peak of Cycle 24 has already come in below expectations. A provocative 2010 paper by Matt Penn and Bill Livingston extrapolated recent trends to conclude that Cycle 25 may have “virtually no sunspots”. That judgment comes not from counting sunspot numbers themselves but the strength of the magnetic fields in the sunspots that are seen. Here’s a plot from that paper:
In essence, the suggestion is that once the magnetic field strength drops below about 1500 Gauss, we would not see such a feature as a sunspot.
Why is the overall strength of the Sun’s surface magnetic field evidently dropping? Again, no one knows. We also don’t yet know when it might rebound. As nearby as our local star is, there is still an awful lot we don’t know about how it works.
Web-Based Tools For Tracking Solar Activity
As we head into the peak of solar Cycle 24 this summer, I thought it would be a good idea to list some websites that are useful for keeping an eye on what the Sun is up to.
Why care about this? Solar activity is rapidly ramping up and significantly affects the geophysical environment of our planet. Bursts of high-energy particles emitted from solar flares can damage Earth-orbiting satellites, disrupt terrestrial radio communications, interfere with GPS navigation, and even threaten the integrity of electrical power grids.
Here are a summary of some sites and what they have to offer:
☀ Current Solar Data (convenient, one-page summary of plots from NOAA)
☀ Spaceweather (the go-to website for information about current solar activity and space weather forecast information)
☀ SolarSoft ”Latest Events” (a single page, graphical, ‘at-a-glance’ summary of recent solar imagery and activity levels)
☀ SolarHam (lots of graphical information about solar activity mainly pertinent to terrestrial, long-wave radio propagation; of interest to amateur radio operators and others)
☀ Solar Terrestrial Activity Report (oodles of information about current solar activity levels and events in the geomagnetic environment compiled by Jan Alvestad)
☀ Planetary K-Index (a numerical representation of the current level of solar-geomagnetic activity near the Earth; when K ≥ 7, auroral activity at mid-latitudes is likely)
☀ Helioviewer (an interactive tool for visualizing solar activity data, mostly spacecraft imagery)
☀ POES Auroral Oval (a realtime depiction of the “auroral oval” around the Earth’s poles from orbiting satellites; useful for predicting where aurora may be seen on the ground)
☀ The Watchers Solar Activities Archive (blog consisting of short news-style updates about solar goings-on)
This list is by no means exhaustive, but should serve as a good jumping-off point for those interested in tracking the activity of our dynamic Sun.
(Image credit: NASA/Steele Hill)
Magnetic fields on the sun’s northeastern limb erupted around 17:45 UT on April 16th, producing one of the most visually-spectacular explosions in years. NASA’s Solar Dynamics Observatory (SDO) recorded the blast at extreme ultraviolet wavelengths (above). Gas heated to nearly a million degrees was blown off the solar surface, following the twisted loops of the Sun’s invisible magnetic field lines. This happened near the limb, or edge, of the Sun; luckily this means the Earth was shielded from the bulk of the harmful radiation emitted in the event. On the other hand, it also means it’s unlikely we will see any auroral activity in the next few days as a result.
Space weather in our solar system has been rather unsettled of late, and some of our tenacious robotic interplanetary explorers have been feeling the impact of the sun’s temper tantrums. The European Space Agency’s Venus Express orbiter suffered a particularly nasty solar sucker-punch, temporarily blinding one of its navigational systems.
The solar radiation hit Venus’ orbit on March 7 (Tuesday) after the sun had belched out a series of solar flares and coronal mass ejections (CMEs). This radiation uptick knocked-out Venus Express’ startracker cameras (including the backup camera), causing them to lose sight of stars the spacecraft uses to orient itself.
Waiting For The CME To Arrive
"Active sunspot 1401 erupted on Jan. 19th around 16:30 UT, producing an M3-class solar flare and a full-halo coronal mass ejection (CME). The Solar and Heliospheric Observatory recorded the cloud expanding almost directly toward Earth.” (animated image, above)
"Analysts at the Goddard Space Weather Lab say strong geomagnetic storms are possible when the cloud arrives this weekend. Their animated forecast track predicts an impact on Jan. 21st at 22:30 UT (+/- 7 hrs).”
"The cloud is also heading for Mars, due to hit the Red Planet on Jan. 24th. NASA’s Curiosity rover, en route to Mars now, is equipped to study solar storms and might be able to detect a change in the energetic particle environment when the CME passes by.”