Death from the Skies! by Ph. D. Philip Plait Read Free Book Online Page B

heat from the core makes gas above it rise, 9 generating towering conveyor belts of gas over 100,000 miles high, moving up and down inside the Sun. Other rivers of gas move around it like the jet stream does on Earth, and yet another set of streams flows north and south as well. When taken all together, the Sun more closely resembles a ball of writhing worms than a simple sphere of gas. It’s like a street map of Tokyo, but in three dimensions and changing with time as well. Because of this, the magnetic field of the Sun is a nightmare as well, making it ferociously difficult to understand. On the positive side, though, it also keeps a lot of solar physicists off the streets.
    All of this together is what creates the Sun’s dynamo. The charged particles inside the Sun are moving in currents. These currents move inside a magnetic field, so the currents themselves generate a magnetic field, and the whole thing is self-reinforcing. The crank, in this case, is the Sun itself, with its own rotation providing the mechanical input energy needed to generate the dynamo. The Sun is huge and massive, so there is a vast amount of rotational energy to tap into. The solar magnetic field is created at the cost of the Sun’s spin, but it will take billions of years for the energy loss to result in a noticeably slower spin.
    The Sun’s magnetic field is complicated and interesting, and by interesting, of course, I mean dangerous.
    Or had you forgotten the title of this book?

MAGNETIC BUBBLE, COIL AND TROUBLE
    Earlier, I mentioned that a star can be defined as an object with fusion in its center, whose tendency to expand due to energy production is balanced by its gravity.
    Stars are a study in balance in this way. If gravity were weaker, they’d expand or explode. If their energy generation were a little weaker, they’d shrink or collapse (more about both of these in later chapters). Their rotation, chemical composition, gravity, heat, pressure, and yes, magnetic field all combine in exquisite balance to produce a stable star.
    But sometimes things get out of whack.
    When a simple magnetic field is illustrated, you usually see a set of lines emerging from the poles of the magnet, connecting one pole to the other. The field lines of a bar magnet, for example, look something like a doughnut. These magnetic field lines are useful to visualize the strength of a magnet: where the lines are bunched up together (like near the poles of a bar magnet), the magnetic field is stronger; where they are spaced out the field is weaker. If you place a small bar magnet inside the magnetic field of a larger magnet, the smaller one will align itself along the larger’s field lines. That’s why a compass points north; the needle is a magnet, and it aligns itself along the Earth’s magnetic field lines.
    Things get complicated if the magnet is not a simple shape. If you bend a bar magnet, the field lines will bend as well. If you take a dozen magnets, a hundred, and throw them together, the field lines can get very distorted, because each bit of the magnetic field is attached to the object generating it. Mess with one and you affect the other.
    The magnetic field of the Sun is generated by moving currents of gas—currents that get twisted, distorted, and bent around just like rivers on the Earth. These field lines may be generated beneath the surface of the Sun, but they don’t stay down there; they pierce through the surface, looping upward and back down into the interior in an incredibly complex, interwoven, and interconnected way. These magnetic field lines can really get their knickers in a twist, becoming entwined and entangled. When this happens, there are profound changes on the surface of the Sun.
    For one thing, since the field lines and the gas are coupled, when the lines get tangled and compressed, the gas has a harder time moving around as well. It’s like a giant net is thrown over the gas, preventing it from moving freely. Hotter gas