This is the height of the 11-year solar cycle, the so-called solar maximum. The face of the sun should be pockmarked with sunspots, and cataclysmic explosions of X-rays and particles should be whizzing off every which way.
Instead, the sun has been tranquil, almost spotless.
As W. Dean Pesnell, the project scientist for NASA’s Solar Dynamics Observatory, dryly noted, “We’re not having much of a solar maximum.”
In mid-September, a solitary sunspot blemished an otherwise blank yellow disk. In the ensuing days, a few more specks appeared, but even a small explosion – or coronal mass ejection – a few days later seemed like the halfhearted effort of a slacker star.
“The truth of it is there isn’t a lot going on,” said Joseph M. Kunches, a space scientist at the Space Weather Prediction Center. “It’s been a bit of a dud. You look at the sun today and you say, ‘What?’”
For those who depend on Kunches’ work, such as satellite operators and power companies, that is actually good news. One of the worries in our highly technological 21st-century civilization is that a direct hit on Earth by a gargantuan solar storm could disable satellites and overwhelm wide swaths of power grids. A quiet sun makes that much less likely.
For scientists trying to understand the dynamics in the interior of the sun, it has been a humbling experience enlightening them about how much they do not know. “If there’s anyone who has figured it out, I haven’t heard, that’s for sure,” said Douglas Biesecker, a physicist at the Space Weather Prediction Center and the chairman of a panel that had issued predictions about the solar cycle.
They do have a basic understanding. Inside the sun, flows of electrons and protons generate magnetic fields that undulate on roughly an 11-year schedule. The roiling of the fields create regions that are cooler and darker – sunspots. The twisting magnetic fields within sunspots periodically snap, releasing enormous amounts of energy in solar flares and coronal mass ejections.
But some solar cycles are ferocious, while others remain calm. Why the cycle is 11 years is another mystery.
This cycle, No. 24 since scientists started keeping track, has been befuddling from the start. Some expected an active cycle, similar to the ones of the recent past. Others predicted that this one would be quieter than usual; those predictions looked perceptive as the lull of solar minimum stretched longer and deeper than expected. In 2008, the sun was spotless on 266 days – the blankest in half a century. The following year, when the percolating of sunspots should have picked up, the sun was blank for 260 days.
Solar activity picked up in 2010 and especially 2011. Then the number of sunspots started dropping again. That was not necessarily surprising. In some previous cycles, the sun’s Northern Hemisphere became active first, and scientists expected a second peak in sunspots as the Southern Hemisphere entered its active period.
The Southern Hemisphere indeed began to perk up, but then leveled off and has remained that way for the past year, leading to more head-scratching. “In all honesty, it really feels like the sun can’t make up its mind,” Biesecker said. “It’s just this flat mesa, and it’s not budging.”
If there is no second peak, and solar maximum actually occurred two years ago, then Solar Cycle 24 would be extremely odd – late to start and early to end. “What would surprise me is if it didn’t pick up over the next year,” Pesnell said.
Long quiet period
How far back do scientists have to look to find a solar maximum quite as weak? As far back as Chicago Cubs fans do for a World Series championship.
Cycle 14, in the early 1900s, was similarly quiet. (The Cubs won the 1908 World Series, about a year after the maximum of that solar cycle.) This time, solar scientists have sun-watching satellites providing reams of data for them to analyze. “For the first time, we'll be looking at a solar cycle that’s really different from the ones we’ve seen before,” Pesnell said.
Pesnell says it has already become apparent that the flow patterns within the sun are more complicated than had been supposed.
Despite the minimal sunspots, the sun is still going through the rest of its cycle as usual. Its magnetic field is on the cusp of flipping, as expected. At solar maximum, the magnetic fields at the poles essentially disappear for a brief time, and when they re-emerge, they are pointing in the opposite direction. If you had a compass on the sun’s North Pole and it were pointing north before solar maximum, it would be pointing south after solar maximum. (Actually, the compass would vaporize.) The North Pole has already flipped; the South Pole is behind, but last month, scientists at Stanford’s Wilcox Solar Observatory said they expected the transition to be complete soon.
“We do see indications that solar maximum is about now,” Pesnell said.
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