An occasional series
Jon TurneyGuardian
Sunspots, and associated solar storms, are expected to reach a peak this year. Explaining why, and where the spots come from in the first place, rests on having some idea of what the outer regions of the Sun are actually like.We cannot be completely certain - it is not an inviting place to visit - but distant observation and informed guesswork suggest the Sun, or any star, can be divided crudely into two parts. The ultra-hot core is an enormous nuclear furnace, in which atoms fuse together at 15m degrees centigrade. The vast heat of half a billion tons of hydrogen a second turning into the next heaviest element, helium, maintains the pressure needed to stop the whole lot collapsing under its own gravity. It also eventually finds its way to the surface, so the Sun shines.
The energy produced starts its journey to the surface as radiation - and is constantly absorbed and re-emitted in the incredibly dense gas it gradually passes through. That works for the first 600,000 kilometres, then the gas atoms get just cool enough to hold on to some of their electrons.
They block the radiation, and the only way all that energy can get the rest of the way to the surface - about the same distance again - is by convection. Huge masses of hot gas rise to the surface, emit light and heat, cool down, and sink back toward the core. So the Sun's outer parts are composed of a sphere of hot gas in motion.
The temperature at the surface is a mere 5,000 degrees, so things have cooled off a lot compared with the core, but it is still a pretty violent place by our Earthly standards. To account for why it is spotty as well, add two more features: magnetic fields and rotation. Like the Earth the Sun has a magnetic field, which can be "trapped" by the electrically charged gas. That gas is rotating as well as convecting, and the movement is faster at the Sun's equator than at the poles. So instead of just pointing north or south, the Sun's magnetic field lines get slowly wrapped around the star.
The general turbulence of the whole mass of gas also contributes to magnetic entanglements. Gradually, these magnetic tangles, like all the hot gas, rise to the surface, and when they get there the magnetic fields pass beyond the surface. This has a cooling effect on the solar gas - visible to us as a sunspot. The spots, which can be as big as the Earth, look dark against the solar glare because they are about 1,000 degrees cooler.
They come in north and south pairs, one spot where the field lines rise above the surface, one where they re-enter the Sun. Each pair lasts for a few weeks. The sunspot cycle varies in length for reasons which are not clear, but presumably have something to do with the build-up and release of magnetic tensions inside.
When the strong local magnetic fields associated with active sunspots come together, they can generate the solar flares which disrupt radio signals for terrestrials. A huge wave of charged particles is launched into space, and when it hits our atmosphere we can hear sunspots, as radio interference, as well as see them.