Every 11 years or so, our sun suddenly becomes a much busier place, with sunspots, flares, and all manner of activity bursting from its surface. But, although the results are clear enough, no one was quite sure why. Now, researchers think they've finally find an answer to this 4-centuries old scientific mystery.
Top Image: Progression of solar activity, Judith Lean at the US Naval Research Laboratory
Instead of looking at just the sunspots for an answer, a team led by Scott McIntosh of the National Center for Atmospheric Research looked instead to the sun's "brightpoints" (just like it sounds, points of unusual brightness on the sun), which they used to get a better idea of what was going on beneath the sun's surface.
Using the brightpoints, they were able to come up with a model of how magnetic fields moved around the sun, converging on its equator, like so:
Image: Scott McIntosh / National Center for Atmospheric Research
Even more intriguingly, they say that what they've observed about the motion of the fields shows us that what we're actually seeing every 11 years (give or take a year or two) is not one single cycle happening on the sun. It's actually the effect of multiple cycles overlapping:
While the field lines remain relatively short like this, the sun's magnetic system is calmer, producing fewer sunspots and fewer eruptions. This is solar minimum. But once the two low-latitude marching bands reach the equator their polarities essentially cancel each other out. Abruptly they disappear. This process, from migratory start to finish at the equator takes 19 years on average, but is seen to vary from 16 to about 21 years.
Following the equatorial battle and cancellation, the sun is left with just two large bands that have migrated to about 30 degrees latitude. The magnetic field lines from these bands are much longer and so the bands in each hemisphere feel less of each other. At this point, the sunspots begin to grow rapidly on the bands, beginning the ramp-up to solar max. The growth only lasts so long, however, because the process of generating a new band of opposite polarity has already begun at high latitudes. When that new band begins to appear, the complex four-band connection starts over and the number of sunspots starts to decrease on the low-latitude bands. In this scenario, it is the magnetic band's cycle – the lifetime of each band as it marches toward the equator – that truly defines the entire solar cycle.
Of course, the theory is still in its working stages, but scientists won't have to wait to long to find out if their model is accurate. Using the new model, they were able to predict when we would see the next solar minimum, in 2017, and solar maximum, in late 2019 to early 2020.
You can read the full study over in Astrophysical Journal.