New sunspot cycle promises to be mild, but not harmless
Link between space weather and El Niño comes under the spotlight. Richard A Lovett reports.
The sun is nearing the low point of its current sunspot cycle and should start to see an increase in activity again in mid-2020, scientists report.
But the increase probably won’t be all that dramatic, they say, because the next sunspot cycle, like the one just ending, is expected to be unusually quiet.
In fact, says Irina Kitiashvili, a sunspot researcher at NASA Ames Research Centre in California, her models predict that the upcoming cycle will be the weakest in the past 200 years.
Alexander Kosovichev, a solar physicist at the US New Jersey Institute of Technology, agrees. His models, he says, estimate that the next cycle will produce only about half as many sunspots as the current one.
There are uncertainties in such estimates, he adds, “but there is a strong indication it will be weaker. How much is difficult to say.”
Not all solar scientist agree, however.
Loren Matilsky, a physicist at the University of Colorado, Boulder, thinks the next cycle will be roughly similar to the current one. Though, he adds, “that’s still low”.
Whatever the exact numbers, they are good news for those worried about the potential impacts of sunspot-associated solar flares, which can blast damaging radiation (sometimes known as “space weather”) toward astronauts, satellites, and earthly power grids.
Though, intriguingly, even apparently benign space weather can produce unusually bad Atlantic hurricane seasons, interfere with US wheat, corn, and oats productivity, and send other weather-related shockwaves around the globe.
Efforts to predict all of this begins with studying the solar dynamo, which Matilsky says, “is simply a fancy word solar physicists use to describe the sun’s magnetic field”.
Because researchers can’t dive into the sun to measure changes in its inner magnetic field, Matilsky says, it’s necessary to use computer models in an effort to figure out how what we see on its surface relates to what’s going on in its interior.
“So what we do is run massively parallel supercomputer simulations to try to figure out where the interior magnetic field is coming from,” he says.
The goal, he says, is not only to predict the strength of the next sunspot cycle, but to find clues to the processes that govern the internal clock that sets that cycle, which averages about 11 years, but can vary from as short as eight to as long as 14.
But that’s just the beginning. It’s also important to understand how this magnetic energy, once it erupts onto the surface in sunspots, produces the giant convulsions that produce devastating flares.
The current solar cycle was, thankfully, low in major flares, but a team led by Karin Muglach of NASA’s Goddard Space Flight Centre in Maryland was able to study the process in microcosm by looking at solar features called “coronal bright points”.
These features are too small to produce full-blown flares, but they have the advantage, Muglach says, of being much shorter-lived than sunspots, which means that their entire life cycle can be observed before the sun’s 24-day rotation carries them out of sight.
“A small active region only lives a few days,” she says.
Only a small number, she says, have been analysed to date, but one initial finding is that they only enter an eruptive phase late in their lives.
“In the emergent phase there’s not much going on in terms of eruptions,” she explains.
It’s a finding that might also apply to full-scale sunspots.
Ironically, however, dips in the sunspot cycle such as the one we are now approaching appear to be associated with major changes in the Earth’s weather, says Muglach’s NASA Goddard colleague Robert Leamon.
Leamon’s research finds that the end of one sunspot cycle and the beginning of another appears to be correlated with a switch in the Pacific Ocean’s El Niño/La Niña weather cycle, apparently due to a change in the number of cosmic rays reaching the Earth.
When the sun is active, the solar wind, composed of energetic particles streaming outward from it, is strong, reducing the number of intergalactic cosmic rays that can make their way through it.
When the sun is less active, the solar wind is weaker, and the cosmic ray flux increases. The effect is so powerful, Leamon says, that cosmic ray counts at the Earth’s surface can vary by a factor of nearly two.
The rays don’t carry an enormous amount of energy compared to that of the sun – only a few tenths of a watt per square metre, compared to more than 1000 watts per square metre from sunlight – but they pack an unusual type of punch.
“The working hypothesis is that it changes the electrical conductivity in the upper atmosphere,” Leamon says. An alternative explanation, he adds, is that they might also affect ozone and nitrogen oxide chemistry.
Either way, they shift the manner in which clouds form, a process that at the switch from one solar minimum into the upsurge for the next cycle appears to initiate a flip from El Niño to La Niña.
That, in turn, changes global weather in ways that does everything from strengthening Atlantic hurricanes to altering rainfall patters across important crop-growing regions.
It can even temporarily reduce the rate of global warming. 2019 is likely to be the warmest year on record, Leamon says, but if he’s right about the switch to La Niña, 2020 won’t be quite as bad.
“We will see soon enough,” he says. “I won’t make a prediction about how strong the La Niña will be next year, but I do say there will be a switch.”