SERENE STATE An image taken in September by the STEREO spacecraft shows the sun in a state of relative serenity compared with past solar maxima.
StereO/NASA
Matt Penn is grateful for whatever the sun will give him. These days, that isn’t much. Penn’s job, as a solar astronomer, is to track the waxing and waning of sunspots on the solar surface. These dark blots mottle the face of the sun, increasing in number to a peak every 11 years and then falling off again in a rhythmic march choreographed by magnetic activity inside the star.
2013 marks the maximum of this solar cycle, yet Penn doesn’t have very much to look at. Atop Kitt Peak outside Tucson, Ariz., he often points his telescope at a barren orange orb. “Where are the sunspots?” he asks. “It’s amazing to see such low activity at the peak of our sunspot cycle.”
By almost any measure, this solar maximum has been pathetic. No more than 67 sunspots have appeared in a month so far; at the last peak, in 2000, that number was above 120. If the sun doesn’t pick up soon — which it probably won’t — the current solar cycle will be the wimpiest in a century.
But a quiet sun is not necessarily a boring sun. The current cycle is “the weakest in the space age, but it’s not so different from the turn of the 20th century,” says Giuliana de Toma, a solar physicist at the National Center for Atmospheric Research in Boulder, Colo. “It’s interesting for scientists because now we have instrumentation we didn’t have 100 years ago. We have not observed a weak cycle like this one.”
In fact, the sun’s slumbers are helping scientists better understand our nearest star. Among other things, solar physicists are learning how superheated gas, flowing from the sun’s equator to its poles, carries magnetic disturbances that help determine how strong the next solar cycle will be. That information may help researchers better predict what the sun’s future holds.
Today, that’s anyone’s guess. Some, like Penn, argue that the sun could be headed for a long-term decline, similar to a period in the late 17th century that saw hardly any sunspots and coincided with the “Little Ice Age” that froze rivers across Europe (though most scientists don’t think low solar activity caused the cold snap).
Other researchers say there’s little evidence for a slide into solar somnolence, and suggest that in this particular cycle the sun could still unleash monster storms of energetic particles before it’s done. That potential for sudden violent outbursts makes it important to learn about the sun even while it is in a quiescent phase.
However it ends, the current solar cycle will go down in the annals of astronomy as one of the most illuminating yet.
Seeing spots
Scientists want to understand the sun’s activity patterns because they can dramatically affect life on Earth. At or around solar maximum, the sun is more likely to hurl clouds of charged particles off its surface and occasionally toward Earth. When one of these solar eruptions hits the planet’s protective magnetic shield, most of the particles get funneled down toward Earth’s polar regions. There they collide with atmospheric particles and produce the eerie glow of the northern and southern lights. Occasionally, though, the magnetic storms are powerful enough to fry satellite electronics and interrupt electricity grids on the ground. In 1989, a solar storm famously zapped a power grid in Quebec and turned the lights out on 6 million people.
All of this solar action traces back to the magnetism roiling the sun’s guts. Electrical charges flowing through the star generate strong magnetic fields. Like Earth, the sun has a north magnetic pole and a south magnetic pole. But unlike Earth’s, the sun’s magnetic poles flip every 11 years or so, just as the sunspot number is peaking. Over a period of weeks to months, what was the north magnetic pole becomes the south
magnetic pole, and vice versa. The flip may be underway right now, says Todd Hoeksema, a solar physicist at Stanford University.
Magnetic fields also help explain the presence of sunspots. These are dark regions, sometimes as big across as Earth, where powerful magnetic fields loop from deep in the sun up through the surface and beyond. Sunspots look black or gray because they are cooler than the atmosphere around them, making them one of the easiest ways to observe changes in solar activity. Sunspots, solar eruptions and other solar phenomena generally act in concert; when there are more sunspots on the surface, the sun is more likely to spew out its particle blobs.
Galileo was among the first to observe sunspots through a telescope, in 1610, but it wasn’t until 1826 that a German amateur astronomer, Heinrich Schwabe, began systematically cataloging their rise and fall. Schwabe’s work caught the eye of professional scientists, including Rudolf Wolf of Switzerland, whose “Wolf sunspot number” calculation is still used to quantify how many sunspots are visible on any given day.
For centuries, sunspot numbers were just about the only scientific measure of solar activity. Astronomers used them to learn how each 11-year cycle differed from the last. During the “Maunder minimum,” between about 1645 and 1715, virtually no sunspots appeared on the sun’s disk. By the middle of the 18th century, solar activity picked up, for reasons nobody understood. It then dropped off again, less dramatically, during two periods around the years 1800 and 1900.
Based on these data, Wolfgang Gleissberg suggested in 1939 that there could be a roughly 100-year cycle superimposed on the 11-year activity cycle. If so, then scientists might expect another drop-off in the early 2000s. That could well be the pathetic solar cycle of today, says David Hathaway, a solar physicist at NASA’s Marshall Space Flight Center in Huntsville, Ala. “It’s almost certainly going to be the smallest sunspot cycle in 100 years,” he says.
Solar prognostication
Not everybody saw it coming. In 2007, a group of experts led by the National Oceanic and Atmospheric Administration took a stab at predicting what the current solar cycle — known as Solar Cycle 24, as it’s the 24th recorded cycle — might look like. The team used a number of statistical techniques to analyze sunspot numbers, polar magnetic field strength and other possible predictors. The group split pretty distinctly into two camps. One faction relied heavily on polar magnetic field strength and predicted a relatively moderate cycle, with about 90 sunspots at its peak. A second group thought other signals, which persisted over several past cycles as opposed to just one, would be more significant; those scientists predicted a much higher peak of 140 sunspots.
Six years later, the conservative group turns out to have been closer to right. “The prediction techniques I work with were saying this would be a weak solar cycle almost 10 years ago,” says Dean Pesnell, a solar physicist at NASA’s Goddard Space Flight Center in Greenbelt, Md. “That was not the most popular thing to say back then, but it turns out to have been correct.”
Polar magnetic fields are important because they serve as seeds for the upcoming solar cycle. They get their start in sunspots, where the churning solar plasma breaks magnetic fields apart into a morass of magnetic disturbances. Some of these fragments get caught up in a “meridional” flow of plasma that moves them away from the equator and toward the sun’s poles. There the fragments combine, build up strength and contribute to the flipping of the magnetic poles. Finally, after the magnetic reversal, the polar fields keep growing and help regulate how big the next solar cycle will be.
This, at least, is the scenario laid out by polar-field advocates like Hathaway. New discoveries seem to bear him and his colleagues out. In 2012, he reported new details on how this meridional flow might regulate magnetic fields at the sun’s poles.
For decades, scientists had assumed that the meridional flow traveled at a depth of about 200,000 kilometers below the sun’s surface. It was just too deep to see directly. But Hathaway reported spotting faint signs of this flow, based on shifts in light given off by the element nickel as it moved within the sun’s atmosphere. With these data, gathered by the European/U.S. Solar and Heliospheric Observatory during the last solar cycle, he calculated that the flows must be moving in much smaller cells — starting at about 50,000 kilometers deep. That shallower depth means that magnetic fragments can be carried to the poles faster than thought, since they don’t need to travel deep in order to move. That, in turn, suggests that the meridional flow can strongly influence how the polar fields build up and how active the sun is likely to be.
Not everyone believed Hathaway’s conclusions, in part because of the way he traced the flow. This August, though, Stanford University scientists announced that they, too, had found that the meridional flow was happening at a relatively shallow depth. The team, led by Junwei Zhao, used NASA’s latest and most sophisticated sun-watching satellite, the Solar Dynamics Observatory, to track sets of plasma waves moving across the sun’s surface. With those data, the scientists could calculate how material was moving inside the sun in greater detail than Hathaway could, says Zhao. They found the meridional flow starting at about 60,000 kilometers deep, the team wrote in Astrophysical Journal Letters.
Together the studies confirm that flows are happening within the sun quicker than thought. And in July, Hathaway and Lisa Upton of the Marshall center reported new data from the SOHO and SDO satellites that further support the importance of polar magnetic fields. Between 1996 and 2013, the scientists see the poleward flow getting weaker as it approaches the poles. Like one river current encountering another, the flow might be running into a second internal movement running in the opposite direction, from the poles to the equator.
During the last solar cycle, the counterflow was stronger than during the cycle before it, the scientists reported in July in Bozeman, Mont., at a meeting of the solar physics division of the American Astronomical Society. That could help explain why the current solar cycle is so weak: The meridional flow simply couldn’t carry enough magnetic fragments to reach the poles, there to combine and build up strength for the ongoing solar cycle. The team hopes to learn more about these possible counteracting flows and eventually be able to predict the polar fields a couple of years in advance.
No sunspots
Meanwhile, another hot dispute centers around sunspots themselves and whether they are fundamentally changing over time.
The idea first cropped up in 2006, when Penn and William Livingston, both at the National Solar Observatory, claimed to detect a long-term change in sunspot brightness. Using the McMath-Pierce solar telescope at Kitt Peak — one of the world’s biggest solar telescopes — Livingston has been measuring sunspot intensity and magnetism since 1998. He and Penn calculated that the maximum magnetic field in sunspots had been dropping as those solar blotches grew lighter by about 1.8 percent each year. That drop happens independently of the 11-year solar cycle.
If that trend continues, the scientists say, solar activity could decline to the point that there would be no sunspots at all by 2015. Other work seems to back up their general point; a new study looking at space-based measurements of sunspots’ magnetic fields also suggests that they have weakened over time. “That’s reassuring,” says Penn.
But the idea of changing sunspots remains controversial. In response to the Penn and Livingston work, de Toma and her colleagues have looked at sunspot measurements taken from 1986 to 2012 by the San Fernando Observatory near Los Angeles. This telescope has a much blurrier view than the Kitt Peak one, but has the advantage of covering a longer time period. In July in Astrophysical Journal Letters, de Toma’s team reported finding no dimming of sunspots. They point out that the Kitt Peak observations included only large sunspots early on, and added the smaller sunspots later. That change made the total sunspot trend look artificially lighter, says de Toma, because small sunspots are intrinsically fainter than big ones. “It’s a selection effect,” she says.
That would be good news for anyone worried about whether the sun is about to sink into another Maunder minimum, that 17th century slump that coincided with the Little Ice Age. While a drop in solar activity probably didn’t cause the cold snap (weather patterns and volcanic eruptions played a far bigger role), changes in the sun’s output do affect climate on Earth to a small degree.
Penn says he and Livingston have already corrected for any selection effects, and he questions whether smog from the 405 freeway might affect the San Fernando measurements. So the jury may remain out on a long-term sunspot dimming trend for a few years more.
For now, researchers are looking to see what tricks the sun may play in the waning years of the 24th solar cycle. “The fact that this one’s lower than the last one is no big shock,” says Scott McIntosh, a solar physicist at the National Center for Atmospheric Research. “The question is, where does it go from here? Will it rebound or continue to slide?”
To answer that, scientists will have to do what they always do: watch and wait. “We’re aware that the sun is at maximum conditions,” says Pesnell. “Everybody likes to assign a point for solar max, but it will rattle around for several years.” The sunspot number may rise again to match or surpass the 66.9 already recorded. Or it may continue to drop, in which case February 2012 will go down in history books as the maximum.
Either way, the sun isn’t necessarily done yet. The largest solar storms sometimes come after solar maximum, when the sun is on its downhill slide. The powerful Halloween storms of October 2003 hit several years after solar maximum, and still managed to blast satellites and deep-space communications.
As for what the next solar cycle will bring, it’s far too early to tell. Hathaway says he won’t have the nerve to even think about predicting solar cycle 25 until at least 2017. It all depends on how those polar fields build up, he says. “We’ve learned a lot this time around.”
