NOVA

(soft music) -

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 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 blinds. 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. 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 toward earth. Sometimes the energy released by these magnetic reconnections produces another type of solar storm., coronal mass ejections or CME's which can propel huge amounts of matter away from the sun 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.