A major solar flare, right, erupts last month from a sunspot about the size of Jupiter. Sunspots look dark because they are cooler than the area surrounding them. (NASA)

The biggest sunspot to grace the face of the sun in more than two decades just rotated out of Earth’s view, but it was responsible for kicking up some truly amazing solar activity last month.

The sunspot, called AR 2192, recently shot off four powerful flares in four days, along with many more smaller flares. The sunspot region, about the size of the planet Jupiter, is the largest observed in 24 years.

AR 2192 was actually one of the biggest observed sunspots of all time, according to NASA scientists C. Alex Young and Dean Pesnell. But how does a sunspot grow this big?

“The simple answer is: We really don’t know,” Young was quoted as saying. “Being close to solar maximum [the peak in the sun’s 11-year cycle of activity] means there is more concentrated magnetic field and magnetic energy under the solar surface waiting to bubble up. But the question of why it comes up as one spot instead of two or more is really still unknown, a mystery.

“I guess a good analogy is when you twist a rubber band or piece of string,” Young added. “Why do, say, three knots or bunches form instead of two or four? The physics is probably too complicated for us at this point, but we can get a handle on, say, when the knots will start to form once we better understand the properties of the rubber band or string and how much twist we put into them. We are not to that point with the sun, but we may get there eventually.”

This movie from NASA shows eight days – from Oct. 19-27, 2014 — in the life of the largest active region seen on the sun since 1990, including five X-class flares that erupted during that time. (NASA Goddard via YouTube)

Sunspots are active areas on the sun. They generally form when magnetic field lines are warped and become twisted. Sunspots look dark because they are cooler than the area surrounding them.

AR 2192 is particularly special because of the somewhat strange way scientists have seen it behave. Instead of shooting out huge bursts of plasma — called coronal mass ejections (CMEs) — with powerful flares, the giant sunspot hasn’t produced significant CMEs during its time rotating in view of Earth, according to Young.

“What’s really curious about [the large sunspot] is that it has produced so many flares of pretty good size, but little or no coronal mass ejections,” Young said. “It’s not that it’s never happened before, but it tends to be the case that when you have a big flare, you generally get a big CME.”

CMEs that head toward Earth are responsible for geomagnetic storms that can harm orbiting satellites or even knock out power grids. A CME produced by a sunspot larger than AR 2192 knocked out power in Quebec in 1989, Young said.

AR 2192 just rotated out of view of Earth, according to Spaceweather.com, but that doesn’t mean that it won’t make another appearance.

Huge sunspots tend to stick around for a while, Young said, so AR 2192 could survive its two week-journey around the sun, out of view of Earth.

“We don’t know exactly if it’s going to make it on its way around, but there’s a good chance,” said Young, who also runs The Sun Today blog. “It’s pretty big, and certainly there have been plenty of examples of other spots that have gone around. Some have gone around a couple times.”

AR 2192 was visible using eclipse glasses, and if the sunspot does return to the Earth-facing side of the sun, people may be able to spot it. But experts say observers should never look at the sun with the naked eye; doing so can cause serious eye damage.

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— Live Science