Two active regions on the sun with opposite magnetic fields: 2760 matches solar cycle 24 while 2761 matches solar cycle 25. (Image credit: Tony Phillips/SpaceWeather.com)
The sun offers plenty of brainteasers: Right now, for instance, it’s sporting magnetic knots formed by two different cycles — simultaneously.
This ambiguity is a common characteristic of the period called the solar minimum, when the sun is least active. The solar minimum falls at the dip between two solar maximums, with a full minimum-to-maximum-to-minimum cycle lasting about 11 years. As the solar activity cycle rolls, magnetic activity on the sun first rises, then falls, and each cycle has an opposite magnetic flavor to the last.
Solar activity is characterized by the sun’s surface being pockmarked by cooler, dark regions called sunspots, some of which release massive bursts of light or charged particles. In between maximum periods, like now, the sun is mostly quiet, but what activity there is can resemble that of either the previous or the upcoming solar cycle.
“During the time that the one cycle fades and the other cycle begins, you kind of see an overlap,” W. Dean Pesnell, project scientist of the Solar Dynamics Observatory, told Space.com. “At the current time, we are seeing that with two active regions.”
One of those active regions, dubbed 2760, is from solar cycle 24, which is wrapping up. Because scientists have been studying solar cycle 24 for about a decade now, that spot is easy to identify based on which side of the active region’s magnetic field is streaming out of the sun and which side is streaming back into it.
“When we look at the magnetic field, we can see that there’s information there that you don’t see in just looking at sunspots,” Pesnell said. “During this time of ambiguity and solar minimum, that’s how we can tell that the sunspot is an old-cycle sunspot.”
But for the second active region, 2761, that direction is flipped — a clear signal that it’s in the vanguard of activity to come from solar cycle 25. “The magnetic field is huge, it’s a global thing, it is all over the sun,” Pesnell said. “The parts of the magnetic field for one cycle kind of have their own identity … It has these distinct features that are associated with one solar cycle and then another.”
This isn’t the first patch of sun to be governed by solar cycle 25: The sun’s new cycle has been coming on for years, and scientists identified a half dozen visible sunspots in new-cycle active regions by January of this year, Pesnell said.
The two spots herald a pattern that will play out across the sun as the cycle switches fully, Pesnell said. Over a solar cycle, sunspots migrate from middle latitudes toward the equator, which, if plotted like a map, form paired diagonal smudges that look like butterfly wings lined up along the sun’s equator.
Solar scientists call this a butterfly diagram, and watch each butterfly fill out beyond the previous cycle’s sunspots. “The butterfly diagram from one cycle kind of nests inside the butterfly diagram of the next cycle, so you can see that they overlap by several years,” Pesnell said. That’s where the sun’s activity is right now, putting the final touches on last cycle’s butterfly while just beginning the next.
But for Pesnell and his colleagues, solar minimum isn’t just of academic interest: It’s also the time when solar scientists venture their predictions of what the next solar cycle will be like.
“This is actually the best time to study the sun because what’s going to happen at solar maximum is they’re going to prove us wrong, because our predictions will not be extremely accurate,” Pesnell said. “They’re pretty good but they could be better. So this is the best time because we just get to make our forecasts.”
Those forecasts are based on the strength of the magnetic fields at the sun’s poles — a tricky observation to make because most instruments for studying the sun are located more or less along the plane that slices through the sun’s equator, just as Earth is.
A recently launched mission called Solar Orbiter, a partnership between the European Space Agency and NASA, will change that during the second phase of its mission, when it will leave that plane to focus on studying the sun’s poles.
In the meantime, Pesnell and his colleagues have to make do with the observations they do have. They forecast that the new solar cycle, 25, will be fairly similar in strength to the most recent one, which was relatively quiet.
“We thought it might be a little stronger, but the polar magnetic fields have actually weakened a bit since then,” Pesnell said. “Now we’re saying it’s probably going to be more or less the same as cycle 24, maybe a little stronger. So not an extremely active cycle.”
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