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.
And?
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.
1 Notes/ Hide
- lookfar liked this
- strictlyastronomy posted this