Death from the Skies! by Ph. D. Philip Plait

welling up from below can’t reach the surface, and regions where the lines are particularly dense begin to cool off. Since the brightness of the Sun is due to its temperature, a cooler region becomes dimmer, forming a dark area on the Sun called a sunspot. Because sunspots are inherently magnetic phenomena (they are really a cross section of the magnetic field lines where they intersect the surface of the Sun), they always come in pairs with reversed magnetic polarity: one is like a magnet’s north pole, and the other is the south pole.
    Sunspots can be small, barely visible to telescopes on Earth, and they can be huge, dwarfing the Earth itself, with some so large that they can be seen by the naked eye when the Sun is on the horizon. 10
    In fact, it was the observation of sunspots that first keyed astronomers into the Sun’s magnetic field. Heinrich Schwabe was a solar observer in the early nineteenth century who counted the number of sunspots every day for decades. He discovered that the number of spots waxes and wanes with a period of about eleven years from peak to peak—we now call this the sunspot cycle. At the time of the maximum, there can be well over a hundred sunspots on the Sun, but at the minimum that number drops to essentially zero.
    Schwabe decided to publish his results in 1859, and it was quickly determined that the times of peak sunspot number also corresponded to the times of peak magnetic activity on the Earth, indicating a connection between sunspots and magnetism. In 1908, the astronomer George Ellery Hale discovered that the magnetic fields in sunspots can be thousands of times stronger than the Earth’s, indicating the presence of intense energies being stored there.

    This is a typical sunspot, appearing darker than the surrounding solar surface because of its cooler material. This particular spot is far larger than the Earth. The graininess of the Sun’s surface around the spot is caused by convection, rising currents of hot material that cool and sink back down into the Sun.
     
    STANFORD-LOCKHEED INSTITUTE FOR SPACE RESEARCH AND BIG BEAR OBSERVATORY
    Which brings us back to balance. As the magnetic field lines tangle up, there is a balance struck between the pressure built up by the magnetic energy stored in them and the tension that exists in the lines. Imagine the magnetic field lines are like steel coil springs, all tangled together and interconnected. The springs are compressed and want to expand, but the tension of the intertwined mess keeps them from springing back. Now keep compressing them and adding more springs, again and again. The energy stored up would get pretty impressive.
    What happens if you take a bolt cutter and snip one of the springs?
    Right. Better stand back.
    The same thing happens in a sunspot—in fact, much of the physics is pretty similar to a convoluted mess of coiled springs, with the analogous tension and pressure. As the field lines get more entangled, and more are added, the pressure builds up. Sometimes the pressure is relieved early in the process, and not much happens. But other times it builds, and builds . . .

    Loops of extremely hot material flow up from the Sun’s surface, following along the magnetic field lines. When the loops get tangled or twisted, a flare or coronal mass ejection can be triggered.
     
    TRACE TEAM/NASA
    Something’s gotta give.
    Eventually, something does. The field lines emerge from the Sun in tall, graceful loops, with one footprint being the magnetic north pole and the other the south. If the gas flow zigs instead of zags, for example, the footprints can be brought together, or twisted past each other. The pressure in the coil goes up, but the tension can’t compensate. The line snaps.
    There is a lot of energy stored in the field line (just like the energy stored in a spring). When it snaps—what solar physicists call magnetic reconnection —the energy is released. A huge amount of energy. The explosion is titanic, but in