Explore the driving force behind the Sun's variability: magnetism. Find out how the Sun's mixed-up magnetic field develops and what effect this has on solar activity.

Transcript & Credits [+] Expand

The Dynamic Sun

NARRATOR: From relatively minor interruptions that create sunspots, to massive surges that drive solar storms, change is one of the few constants on the Sun's surface.

So, what’s behind all this variability? The short answer is, magnetism. More precisely, it’s the shifting and twisting magnetic fields, seething below the Sun’s surface.

Like Earth, the Sun has north and south magnetic poles, with magnetic field lines connecting the two.

At times, these field lines follow neat vertical paths between the northern and southern hemispheres—but that’s the exception more than the rule.

More often, the motion of plasma inside the Sun mixes up the neatly organized field lines.

This is because plasma spins faster at the equator than near the poles, a difference that causes the lines to stretch.

At the same time, swirling currents in the convective zone twist the field lines and cause them to kink upward.

What’s more, all the stress and strain generated by these two forces actually strengthens the magnetic field, rather than weakening it.

SARAH GIBSON (High Altitude Observatory): Imagine this spring is a magnetic field line. The magnetic field inside the Sun is amplified, is strengthened by the rotating motions, and the shearing motions, and the churning motions inside the Sun. It wants to expand upwards, and it does, until it pokes out through the surface of the Sun.

NARRATOR: As magnetic field lines emerge, they form loops. Where they break through the surface, they temporarily divert the upward flow of hot plasma and create the relatively cool, dark regions that we know as sunspots.

But the effects can be more than cosmetic. Often extending far above the Sun's surface, the magnetic loops continue to be twisted by the currents beneath.

Sometimes, the field lines twist enough to cross. The resulting magnetic reconnection—similar to a short circuit—heats the surrounding plasma to tens of millions of degrees.

This unleashes a solar flare—a powerful burst of energy and particles that can have disastrous results if directed towards Earth.

Sometimes, the energy released by these magnetic reconnections produces another type of solar storm: coronal mass ejections, or CMEs, which can propel huge amounts of matter away from the Sun's surface.

A single CME might blast 10 billion tons of material out into the solar system.

While all this solar activity sounds chaotic, it actually follows a very regular cycle—once again, driven by magnetism. About every 11 years, the Sun’s magnetic field undergoes a dramatic shift.

Just when the twisting of the field lines reaches its peak—which we call the solar maximum—a realignment occurs, flipping magnetic north and south. Once again, the field forms neat vertical lines and solar activity drops to near zero.

That is, until movement of the plasma inside the Sun begins to act on them, and the march toward the next solar maximum resumes.


“Secrets of the Sun” National Geographic Television and WGBH Educational Foundation



Steven Bedard, Writer/Producer
Anna Rothschild, Animator/Editor/Narrator
Alistair Cameron / "Gentle Marimba"

[-] Collapse

Sun 101 3 of 3

The Dynamic Sun

What forces cause the Sun's magnetic field to become both stronger and more tangled? How do these changes influence solar activity and the potential for powerful solar storms? Write your answer in the space provided.