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.