Welcome, all, to Solar Cycle 25

Solar Cycle 24 has officially ended, solar physicists say, and Solar Cycle 25 is expected to be another mild one, with sunspots peaking at roughly the same level.

That’s good news for the continued safety of the world’s power grid, as well as for telecommunications, air traffic controllers and GPS users, because a weak solar cycle means fewer solar storms to fling energetic particles into space and wreak havoc with the Earth’s ionosphere.

Solar Cycle 24 was the weakest one in the past 100 years, says Lisa Upton, a solar scientist with the Space Systems Research Corporation and co-chair of an international panel that monitors the solar cycle.

And the new solar cycle, will be “almost identical” to it, says the other co-chair, Doug Biesecker, a solar physicist at the US National Oceanic and Atmospheric Administration’s Space Weather Prediction Centre. Both scientists spoke today at a NASA-sponsored virtual press conference.

Solar cycles are measured by changes in sunspot activity, with a new cycle starting when the previous one reaches its minimum and activity starts to rebound.

Cycles usually last about 11 years, and are numbered from the time they were first observed in the 1750s and 1760s.

The new cycle began in December 2019, but was not confirmed until now, Upton says, because sunspot numbers are variable. To avoid error, scientists monitor a 13-month window of sunspot counts to be sure that they are seeing a true minimum and not just a statistical fluctuation.

Sunspots are dark patches on the Sun’s surface that mark the locations of magnetic storms. At sunspot minimum, there may be only a handful per year; at maximum there can be more than 100.

But while they are used to mark the beginnings and endings of solar cycles, prediction of the intensity of the next solar cycle depends largely on factors other than the number of sunspots at minimum.

Primarily, Upton says, it is based on the strength of the Sun’s magnetic field at its poles.

But the number of days with zero sunspots is also an indicator, she says, along with the latitudes at which the first ones emerge. (Closer to the solar equator is better.) “Since we’re seeing these at low latitudes, that is another indicator that we should see a weak cycle,” she says.

Weak solar cycles aren’t without their hazards, however; even when the Sun is inactive there is still a risk of a large eruption. “There is always reason to be wary,” Biesecker says. “The Sun is always capable of sending weather our way.”

In fact, he says, in July 2012, during the otherwise-weak Solar Cycle 24, the Sun had one of its biggest eruptions in a long time.  “We’re saying Cycle 25 is going to be like Cycle 24 in terms of sunspots, but Cycle 24 produced an epic kind of 100-year storm.”

Fortunately, it missed Earth. But solar storms are like hurricanes, he says. Most don’t make landfall. “But the few that do really matter.”

Inactive solar cycles also allow space debris to accumulate at a faster rate in Earth orbit, increasing the risk to satellites, astronauts and future space launches.

That’s because energy from solar storms makes the Earth’s upper atmosphere expand. This causes it to exert more drag on such debris and cause it to spiral more quickly back to Earth and burn up in the atmosphere. “A small solar cycle means we’re not going to be clearing out the orbital debris that a big cycle would,” Biesecker says.

Likewise, says Lika Guhathakurta, program scientist in NASA’s Heliophysics Division, a weak solar cycle allows cosmic rays to penetrate deeper into the Earth’s atmosphere, creating radiation showers that increase exposure in high-flying jetliners. This affects not only frequent fliers, but aeroplane electronics, she says, increasing the rate at which they degrade.

Also, the next maximum is likely to coincide with NASA’s planned Artemis mission to the Moon – a concern because Moon-bound astronauts will be outside the protection of the Earth’s magnetic field and at greater risk from solar storms than are astronauts in low Earth orbit.

But it also presents an opportunity for research, says Jake Bleacher, chief exploration scientist in NASA’s Human Exploration and Operations Mission Directorate.

That’s because part of the Artemis project involves the construction of the Gateway station, which will orbit the Moon and serve as a waystation for astronauts.

One plan, he says, is put samples of foods and pharmaceuticals on it to see how, or if, they degrade when exposed to the type of conditions in which astronauts’ supplies are likely to be stored during long deep-space missions.

“We need to know that we can depend on it,” Bleacher says, referring both to the potency of medicines and to the nutritional content of food. “Gateway is a perfect platform for us to conduct those types of studies.”