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May 8th, 2024. On the blazing surface of the Sun, a collection of sunspots have been growing
for days, ignored by most of us but watched with fascination and some confusion by solar
astronomers.These dark spots marked the presence of an invisible tangle of magnetic fields that
held enormous magnetic energy—energy that would be released in a series of eruptions,
sending a blast of plasma and magnetic field directly at the Earth. Two days later this coronal
mass ejection plowed into Earth’s own magnetic field, causing auroras that were visible in the
tropics. I was in upstate New York and the entire northern sky was ablaze, albeit dimly. And the Sun
is only getting started. Solar activity is still increasing in a sunspot cycle that is proving way
more intense than scientists predicted. Just how much stronger is it going to get?
The magnetic storm back in May was the strongest experienced by the Earth since 1989,
and we’re still at least a few months of the peak of activity in the current 11-year solar
cycle. Which is weird, because this—the 25th cycle since sunspot monitoring began—was supposed to be
a particularly weak one. In fact, the strength of the solar cycles had been decreasing for decades.
Instead, the Sun’s activity for this cycle is rising far more rapidly than predicted,
towards an imminent polarity reversal of the Sun’s entire magnetic field.
So what’s going on? Why does the Sun’s magnetic field act so crazy and why is
it acting so unpredictably crazy right now and how much more crazy is it going to get?
To answer these excellent questions we’re going to have to ask some more basic questions like:
What is the source of the Sun’s magnetic field? Why does solar activity change over
an 11 year cycle? What causes the magnetic field to flip direction,
resetting the cycle? And how can we predict the strength of a solar cycle in advance?
First though, a quick 101 on solar structure. The Sun’s interior has three main parts—the
core where hydrogen is fused into helium, a process that produces all of the Sun’s energy.
Then you have the radiative zone where fusion power is transported upwards by high-energy
photons bouncing their way through the dense plasma. Finally you have the convection zone,
where energy is transported by flows of plasma carrying heat up to the surface
The core and the radiative zone are fluids, but they rotate like solid spheres, with
all latitudes completing a rotation in the same amount of time. But the convective zone is much
more sloshy and fluid-like. Over most of the top 30% of the solar radius, the equatorial material
rotates once per 25 days, while the stuff at the poles takes a lot longer—34 days to rotate once.
Broadly speaking, the Sun’s magnetic field is generated in the same way as Earth’s—by
the dynamo effect. In the Earth, this happens when circular currents in Earth’s
liquid metal outer core amplify small magnetic fields, which amplify the circular currents,
and through a positive feedback process build a self-sustaining global dipole field,
sort of like a giant bar magnet. We did an episode on this.
The processes behind the Sun’s magnetic field are less well established because it’s very
hard to make measurements beneath the solar surface. What I’m about to describe represents
a set of current leading ideas, but this is subject to modification as we learn more.
In the Sun, it’s believed that interactions between the solid-like radiative zone and the
sloshy convective zone lead to various swirly currents, for example through turbulence and/or
the coriolis force. This kickstarts the dynamo effect just like in the Earth.
At first, the magnetic field looks much like the Earth’s simple dipole field. There’s a north pole,
a south pole, and great loops of magnetic force forming concentric shells outside
the Sun and in the solar interior passing through the plasma flows of the dynamo.
Earth’s magnetic field rotates at the same rate as its mostly solid interior,
and so that magnetic field is pretty stable. But that’s not the case for the
Sun. Its magnetic field is generated and supported in the convective zone, where,
if you recall, the equator rotates faster than the poles. This differential rotation drags
the nicely-aligned dynamo generators and their magnetic fields out of north-south alignment,
twisting these field lines around the Sun. The result is that the dipole field lines get slowly
transformed into a ring around the solar interior. The technical terms for these two directions are
poloidal—towards the poles for the original dipole field—and toroidal—like a torus for the ring-like
lines. This coiling of the poloidal into toroidal field lines is known as the omega process.
That was step one in breaking the Sun’s magnetic field. Now another cool bit of physics gets to
work—magnetic buoyancy. So when you have a tube of magnetic flux in a conducting fluid
also being in a gravitational field, the tube will tend to rise to the surface. At a given depth
beneath the surface of the Sun, the pressure has to be the same everywhere. In the magnetic flux
tube, the magnetic field contributes part of that pressure. That means the pressure of the plasma
(the gas pressue) in that tube has to be lower in order to have the correct pressure for that
depth. So we have lower pressure plasma that’s the same temperature as its surroundings. That
can only be true if the density of that plasma is also lower. Therefore the magnetic flux tube rises
towards the surface. The stronger the magnetic field, the stronger the magnetic buoyancy,
so as the toroidal field winds itself up, more and more field lines push to the surface.
And now those rising flux tubes experience yet another influence—the Sun’s coriolis force. This
is the same effect that generates hurricanes on Earth. The speed of the Sun’s plasma varies
with latitude due to the differential rotation, but also due to the fact that
the Sun is roughly spherical. The stuff at the equator has to travel a greater distance in one
rotation to make it all the way around the sphere compared to the poles. This leads to a further
twisting of the magnetic field lines, inducing kinks. These kinks can then protrude from the
Sun’s surface. This kinking of the toroidal magnetic field is called the alpha process.
The strong magnetism in these kinks can shut down convection where they protrude
from the Sun’s surface. With heat no longer being efficiently carried to the surface in that region,
it cools down and so looks a lot darker than the surrounding solar surface, leading to sunspots.
This whole story nicely explains some of the features of sunspots. They tend to appear in
pairs with opposite polarity where the one magnetic kink emerges from and reenters the
solar surface. Sunspot pairs are often duplicated above and below the equator
whether the same toroidal field line becomes kinked in both hemispheres.
Now as our magnetic field gets wound up, it increases in strength. The relatively
weak poloidal field during the solar minimum becomes a very strong toroidal field at the
solar maximum. At the beginning of this process the field lines pile up at mid-latitudes,
and that’s where sunspots first appear. As the winding intensifies the toroidal field
approaches the equator, and the sunspots follow, and the number of sunspots also increases.
This migration of sunspots from high to low latitudes
and their increase in number is seen in the so-called butterfly diagrams,
which show the number and location of sunspots over a series of solar cycles.
The magnetic kinks emerging from the solar surface have strengths several
hundred times higher than the baseline surface magnetic field. These kinks
are not very stable. There’s an enormous amount of energy stored in these tangled and twisted
magnetic flux tubes. They can release that energy when various tightly knotted or twisted flux tubes
reconnect into a more orderly configuration. The pent-up energy is released in this reconnection,
and the result is often an explosive ejection of plasma and magnetic field from the Sun’s surface:
a coronal mass ejection like the ones that caused massive auroras back in May.
OK, so we’ve wound up our magnetic field and then tangled it into a horrible,
chaotic mess. We know that this tangling happens in cycles,
but how does the Sun reverse this process? As the toroidal field winds up to maximum
intensity and the coriolis force rotates these field lines back into poloidal lines, we start
to get lots of north-south mini-loops. Loops beneath the surface of the sun
begin to connect to loops close to or above the surface to form much larger magnetic loops.
Because of the direction of the line twisting by the coriolis force, these loops have polarities
opposite that of the original poloidal field. Over time, the strength of these growing loops will
exceed that of the original field. At that point the magnetic field flips direction. In essence,
the little coriolis-induced kinks in the toroidal field become the new flipped global dipole field.
At the same time, that wound-up toroidal field is fragmented by all these magnetic reconnections
and it decays away. And so the whole process starts again with a nice, clean dipole field. Ok,
now that we know how the solar cycle works. Let’s see if we can answer some of our questions.
We know why the cycle happens and why it resets. But why does this take eleven years? Well, because
that’s how long it takes. That’s the amount of winding needed to turn the nice gentle dipole,
poloidal field into a tightly-wound and sufficiently unstable toroidal field.
That 11 years is also an average. Sometimes it takes a bit longer,
sometimes a bit shorter. There’s even evidence of double cycles in the past—two very short
cycles masquerading as a single longer one. We can actually look to the geological record to
track solar cycles back 11,000 years. For example, Beryllium-10 is a long-lived radioactive isotope
formed when cosmic rays hit the atmosphere. Stronger solar maxima mean more cosmic rays,
so by measuring its levels of beryllium-10 in Antarctic ice cores we can get both the
duration and strength of ancient solar cycles. And it turns out solar cycles have lasted on
average 11-ish years for at least as long as human civilization. There are fossil records
that suggest that stability goes back as much as 700 million years. Now the
average cycle length may be stable, but the intensity of cycles fluctuates a lot.
Over the geological record, solar activity appears to rise and fall through several peaks and valleys
that each last decades to centuries. The last deep valley was the Maunder minimum in the latter half
of the 1600’s, right near the beginning of our effort to actually count sunspots year after year.
At the depths of this minimum sunspot activity was around 1000 times lower than in the modern era.
The most recent several cycles—since the 1940s—have actually been of above average
strengths, leading to what we call the modern maximum. But over the past few
cycles that maximum appears to have been winding down—quite dramatically in fact,
leading some to wonder if we’re approaching a new Maunder-like minimum. That is,
until the current cycle got started, when its high intensity defied all of our predictions.
So how do we make these predictions in the first place? And why are they so hard to get right?
The strength of a given cycle seems to be correlated with a number of things. For example,
the strength of the current cycle plays some role in determining how strong the next cycle will be,
but there’s a lot of variation. A much better predictor for the strength of the
next solar maximum seems to be the strength of the poloidal or dipole field during the solar minimum.
And that makes sense. After all, that poloidal field is what gets wound up into a strong toroidal
field, leading to more kinks, more sunspots, etc. etc. But the reconstruction of a new poloidal
field from the chaotic fields at the previous solar maximum is much harder to predict because
it depends in detail on how well the kinks in the toroidal field happen to line up and reconnect.
During cycle 24 there was speculation that cycle intensity may continue to drop,
leading to a weak cycle 25 and perhaps continuing the trend towards a new
long-lasting period of low solar activity. But solar experts didn’t place much stock in these
predictions because at that stage we just didn’t know. But as cycle 24 bottomed out,
the newly rebuilt poloidal field was stronger than expected. In 2019, at the depths of this minimum,
experts convened and produced this projection for the number of sunspots and magnetic field
during cycle 25. It was supposed to be around the same strength as cycle 24, relatively weak
compared to the modern maximum, but not continuing the imagined fall towards a new Maunder minimum.
But this is what cycle 25 actually turned out to be—at least to date. A much steeper
rise in activity, indicating we’re in for a stronger and probably shorter
cycle. Scientists have been updating their models based on ongoing solar activity.
The previous study estimated a cycle peak in 2025, but now the peak and the
accompanying flip in the magnetic field looks scheduled for later this year.
So what went wrong? Well, nothing really. Currently we forecast the strength and
length of a solar cycle based on various measurables—like sunspot numbers and field
strengths and field properties at different times. But these are crude metrics for an
extraordinarily complex system of plasma and magnetic field dynamics. Scientists are also
doing some incredibly sophisticated simulations of this complex system. But these simulations
can’t make detailed predictions—they’re primary value is in exploring the deep physics at work,
which can allow us to come up with better heuristics to use in our predictions.
Solar cycle 25 is ramping up to be a good one. It probably won’t be as strong as those in the
peak of the modern maximum—although it’s too early to be entirely sure about even that.
But it’s likely that before this cycle is done we’ll get some more spectacular solar activity,
and perhaps auroral activity with it. So keep your eyes on the space weather forecasts and
maybe find a dark place to watch the sky next time the Sun’s twisted field snaps,
sending blasts of magnetic plasma to bombard our terrestrial space time.
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Twelve years ago on July 4th, 2012 the ATLAS and CMS collaborations announced the discovery
of a new particle to a packed auditorium at CERN. It was revealed to be the Higgs
Boson - the particle that makes the existence of matter and interactions as we know them
possible. To celebrate the fireworks of that collision we’ve designed our very own limited
edition UV glow print Higgs T-Shirt & Hoodie. Both the t-shirt and hoodie are UV charged,
so after being out in the sun they’ll begin to glow. Because of the UV printing process, we
only are able to print a limited number. If you’d like to receive your shirt before July 4th, then
you’ll need to put your order in by June 23rd. Check out pbsspacetime.com/shop to see them.
There’s a link the in the description.