The Sun Has Seasons, Too

When storm season hits, you’re advised to stay the hell away.

Apr 8 2015, 4:05pm

​The Sun, being a total beast with solar flares and tsunamis erupting across its surface. Image: NASA/SDO/AIA

Solar flares—sudden, bright flashes across the Sun's surface—are actually monstrous eruptions, equivalent to billions of megatons of TNT. When their cosmic shrapnel hits the Earth, solar storms can disrupt our satellites, communications, and power grids.

But this ferocious Sun-wrath is actually the result of something that sounds quite ordinary: seasons. Yes, the Sun has seasons too, in a manner of speaking. According to a study that appears this week in Nature Communications, solar seasons are driven by toroidal magnetic bands that stretch and warp as they traverse the Sun's surface. And when it comes to storm season on the Sun—well, let's just say George R.R. Martin himself couldn't cook up anything half this hellish.

"What we're looking at here is a massive driver of solar storms," lead study author Scott McIntosh, director of National Center for Atmospheric Research's High Altitude Observatory, said in a statement. "By better understanding how these [magnetic] activity bands form in the Sun and cause seasonal instabilities, there's the potential to greatly improve forecasts of space weather events."

For over a century, astronomers have known that the Sun's activity—its total brightness, the number of dark sunspots and solar flares—swells and diminishes following an 11-year "solar cycle." But the mechanisms behind that pattern were something of a mystery, until last year, when McIntosh and colleagues discovered that the solar cycle is caused by the interactions of twisted, ring-shaped magnetic bands. These bands, fueled by the rotation of the Sun's deep interior, originate at the surface near the poles, and slowly migrate toward the equator, where they meet and annihilate one another over the course of 22 years.

Artist's depiction of the "heliospheric current sheet" caused by the Sun's rotating, ring-shaped magnetic field lines. Image: NASA

In their new paper, the researchers show how the Sun's magnetic bands also cause solar activity to wax and wane over an 11-month cycle, which can be as strong as its 11-year counterpart. These "seasonal" variations, which take place separately in the Sun's northern and southern hemispheres, are the result of very slow-moving waves within the bands that sometimes expand and warp the band at large. This is analogous to how Earth's jet stream—a fast river of wind that circumnavigates the northern hemisphere—can sometimes warp, influencing the air current's trajectory.

When the jet stream meanders, it can have a powerful effect on regional weather, as New Englanders were made acutely aware this past winter. On the surface of the Sun, wavy magnetic bands can dredge up other magnetic fields from the deep solar interior. This produces a surge of magnetic "fuel" that drives powerful solar storms.

"These surges, or 'whomps' as we have dubbed them, are responsible for over 95 percent of the large flares and coronal mass ejections—the ones that are really devastating," McIntosh said.

As our understanding of these seasonal patterns grows, so too will our ability to predict solar storms. Still, when it comes to forecasting, space weather scientists today are a bit like meteorologists in the 1950s: Stymied by a lack of satellite data. That's why McIntosh and his colleagues have recently begun discussing a new network of satellites that would provide 360 degree coverage of the Sun's corona.

"Think about how limited our view of Earth's weather would be without satellite observations that completely span the Earth all of the time," McIntosh told me. "If we can combine these pieces of information, forecast skill goes through the roof."

So, in the future, when our power cuts out because the Sun decided to belch fiery plasma all over the solar system, at least we might be able to see it coming, even if we can't do a damn thing about it.