By Meghan Bartels for Scientific American
The poles of the sun’s magnetic field are fading away. But don’t panic: it’s all part of our host star’s usual 11-year cycle of activity.
Over the past couple of years, solar activity—as measured by the number of dark spots on the sun’s face—has been increasing, with solar outbursts such as flares of electromagnetic radiation and ejections of blobs of plasma on the rise. The sun storms have delivered stunning auroral displays and the occasional radio outage. Less obvious to Earthlings: this solar activity has also been eating away at the sun’s surprisingly fluid magnetic field, and our home star’s poles have nearly lost their charge as a result. As the months progress, the magnetic field will reverse and then gradually strengthen as solar activity fades, scientists say.
“Right now it looks like the sun’s polar fields are fairly well in sync,” says Lisa Upton, a solar scientist at the Southwest Research Institute in Boulder, Colo. “They’re getting really close to zero, so they’re getting very, very weak, but we haven’t quite hit the reversal yet.”
A polar reversal would mark the midpoint of a process that began around December 2019, when the sun was at its quietest, with hardly a sunspot to be seen. At this point, the star’s magnetic field was arranged as a relatively tidy dipole, where one pole is positively charged, and the other pole negatively charged.
But unlike Earth’s magnetic field or that of a bar magnet, the sun’s magnetism is patchy and highly fluid, even during its dipole stage. “It’s not uniformly positive,” says Todd Hoeksema, a solar scientist at Stanford University. “It’s made up of a bunch of small flux regions, most of which are one polarity and not the other, and that’s sort of a dynamic thing—it changes.”
And the dipole stage is fleeting. As the sun rotates, the seemingly orderly magnetic field warps and strengthens. The magnetic field also rises toward the sun’s surface, typically near the sun’s equator, where it manifests as sunspots. A sunspot appears dark because the increased magnetism blocks heat transport to the area, producing a cooler region that glows less fiercely than the rest of the sun’s surface.
Each sunspot comes in a pair; one is magnetically positive, and the other is negative. These magnetic pairs mostly—but not entirely—dissipate as the sunspots decay away, leaving a little leftover magnetic flux of one charge or the other. This leftover magnetism is typically the opposite polarity of that of the solar hemisphere it appears on. And as material moves around the sun, these leftovers typically migrate toward the pole of that hemisphere, which usually cancels out a little of the existing magnetic field there.
The leftover magnetic flux from one sunspot pair alone doesn’t make much of a difference, but during the solar cycle’s more active period, the sun can easily top 100 sunspots at any given moment. As all these sunspots form and fizzle, the tiny leftover charges gradually build up at the poles and cancel out their polarity. “You can chew away at the magnetic polarity in that pole,” Hoeksema says.
That said, the process can be bumpy, depending on the sun’s activity and on aspects of the magnetic field that scientists aren’t yet able to predict. “This doesn’t happen in an ordered fashion; it’s not a smooth function,” Upton says of the sun’s changing magnetism.
But at this point, with several years of sunspot activity having nearly eliminated the star’s magnetic poles, a reversal is on the horizon. “The sun is quite active right now,” says Sanjay Gosain, a solar scientist at the National Solar Observatory. “If it continues like that, my guess would be that in six months or so, we will see that the polarity completely flips.”
Scientists are eagerly waiting to see how the reversal process unfolds. “It’s not an instantaneous thing, and it doesn’t happen everywhere all at the same time,” Hoeksema says. In the last solar cycle, for example, the polarity of the sun’s northern hemisphere started to reverse in early June 2012 and then wavered around the neutral point until late 2014, even though the southern hemisphere transitioned smoothly to the opposite polarity in mid-2013. This year the poles seem to be transitioning more evenly. “I don’t know which one is going to go first; it’s kind of a horse race,” Hoeksema says.
The reversal of the sun’s magnetic poles typically signals that solar maximum is nearing, and sunspot tallies will begin to wane, scientists say. That fits previous predictions that this solar cycle would be relatively weak, although perhaps a little stronger than the previous one, which peaked in April 2014.
“It’s looking like the polar fields are probably going to be reversing in 2024. It’s looking like solar cycle maximum is probably going to be in 2024,” Upton says. “All of this is really lining up in a very standard, typical fashion. The sun is actually kind of behaving pretty well this cycle.”
In the coming years, sunspots will continue to add their leftover magnetism to the new charge pool growing at each of the sun’s poles, strengthening the new fields and re-creating the dipole state last seen in 2019. This time the dipole state will occur around the turn of the 2030s. Around solar minimum, scientists will also set about predicting what may happen during the next solar cycle, which is due to peak in the mid-2030s.
But for now, scientists are content to see how this pole reversal unfolds. “It’s always interesting to see how it’s actually going to go,” Hoeksema says. “It’s never the same twice.”
(Published 20 October 2023, 11:35 IST)