The poles of the sun's magnetic field are gradually disappearing. But don't panic: This is all part of our host star's usual 11-year cycle of activity. Solar activity, measured by the number of dark spots on the sun's surface, has been increasing over the past few years, as have solar outbursts such as electromagnetic radiation flares and plasma jets. Solar storms bring stunning auroras and occasional radio blackouts.
A schematic diagram of the sun's magnetic field lines depicted in an image of the sun taken by NASA's Solar Dynamics Observatory on March 12, 2016, when solar activity was waning. Image source: NASA/SDO/AIA/LMSAL
But to Earthlings, these solar events are less obvious: They are also eroding the Sun's amazingly fluid magnetic field, causing our star's poles to lose almost all charge. Scientists say the magnetic field will reverse over time, then gradually strengthen as solar activity weakens.
"Right now, it appears that the Sun's polar magnetic fields are pretty much in sync," said Lisa Upton, a solar scientist at the Southwest Research Institute in Boulder, Colorado. "They're very close to zero, so they're getting very, very weak, but we're not quite at the point of reversal yet."
The polar reversal will mark the midpoint of a process that began around December 2019, when the Sun is at its quietest and barely a single sunspot is visible. At this time, the star's magnetic field is arranged into a relatively neat dipole, with one pole being positively charged and the other being negatively charged.
But unlike the Earth's magnetic field or that of a bar magnet, the Sun's magnetism is patchy and highly unstable, even during the dipole phase. "It's not a uniform positive magnetic field," said Stanford University solar scientist Todd Hoeksema. "It's made up of many small regions of magnetic flux, most of which are one polarity and not the other. It's a dynamic change." "
The dipole phase is fleeting. As the Sun spins, the seemingly orderly magnetic field twists and intensifies. Magnetic fields also rise toward the sun's surface, usually near the sun's equator, where they appear as sunspots. Sunspots appear dark because increased magnetism blocks the transport of heat to the area, creating a cooler area where the sunspots glow less brightly than the rest of the sun's surface.
Each sunspot is a pair, one with positive magnetism and the other with negative magnetism. As the sunspot decays, most (but not all) of these magnetic pairs dissipate, leaving only a little residual magnetic flux. This residual magnetism is usually opposite to the magnetic pole present in the solar hemisphere. As material moves around the Sun, this residual magnetic flux typically moves toward that hemisphere's magnetic poles, which often cancels out some of the existing magnetic field there.
The residual magnetic flux produced by a pair of sunspots doesn't have much of an impact on its own, but during more active periods in the solar cycle, the Sun can easily produce 100 sunspots at any one time. As all these sunspots form and recede, the remaining tiny charges gradually build up at the poles and cancel out their polarity.
Still, the process could stumble, depending on the sun's activity and aspects of its magnetic field that scientists can't yet predict. "It doesn't happen in an orderly manner, it's not a smooth function," Upton said of the changes in the sun's magnetic field.
But at this point, several years of sunspot activity have all but eliminated the star's magnetic poles, and a reversal is about to occur. "The sun is pretty active right now," said Sanjay Gossain, a solar scientist at the National Solar Observatory. "If it continues like this, I guess within six months or so, we'll see a complete flip of the magnetic pole."
Scientists are eagerly waiting to see how the reversal process unfolds. "It's not an instantaneous thing, and it doesn't happen everywhere at the same time. For example, during the last solar cycle, the polarity of the sun's northern hemisphere reversed starting in early June 2012 and then vacillated around neutral until late 2014, even though the southern hemisphere made a smooth transition to the opposite polarity in mid-2013. This year, the transition to the poles seems to be more even. I don't know which one will go first; it's a bit like a horse race."
Scientists say the reversal of the sun's magnetic poles usually heralds the approaching solar maximum and the number of sunspots will begin to decrease. This is in line with previous predictions that this solar cycle will be relatively weak, although probably slightly stronger than the last solar cycle that peaked in April 2014.
"It now appears that the polar magnetic field may reverse in 2024. The maximum of the solar cycle may occur in 2024," Upton said. "All of these are pretty standard, typical arrangements. The Sun actually does pretty well in this cycle."
Over the next few years, sunspots will continue to add their remaining magnetism to the pool of new charge growing at the Sun's poles, strengthening the new magnetic field and recreating the dipole state last seen in 2019. This time the dipole state will occur at the turn of the 2030s. Around the time of solar nadir, scientists will also begin to predict what may happen during the next solar cycle, which will peak in the mid-1930s.
But now, scientists are waiting to see how this extreme reversal develops. "It's always interesting to see how it develops, it won't happen twice," Hoeksma said.