While the the sun might appear like a still disk in the sky, our star is actually a dynamic object that goes through cycles of activity. But what exactly are solar cycles and what are they characterized by?
The sun is a giant, fiery ball of electrically charged, superheated matter—mostly hydrogen and helium—that lies at the center of our solar system, the mass of which it makes up 99.86 percent.
The motion of this superheated, gas-like charged matter, known as plasma, throughout the star produces a powerful magnetic field. And this magnetic field flips in polarity—meaning the sun's north and south poles switch places—at roughly regular intervals. It is this phenomenon that gives rise to the cycles in solar activity that we can observe.
What Happens During a Solar Cycle?
According to Rachel Howe, a researcher from Birmingham University in the United Kingdom who studies the sun, solar cycles are a repeating pattern of changing "solar activity"—a term that refers to phenomena caused by magnetic fields emerging at our star's surface.
Among these phenomena are sunspots, which are cooler areas where magnetic field lines come up through the surface and appear as (relatively) small dark patches on the solar disk.
According to Sandra Chapman, astrophysicist and director of the Centre for Fusion, Space and Astrophysics at the University of Warwick, also in the U.K., sunspots are magnetically active regions. "So more sunspots equals a more active sun," she told Newsweek.
Scientists characterize solar cycles—which are defined by periods of high and low activity called solar maximums and minimums, respectively—by counting the number of sunspots that appear.
"At solar minimum there are very few sunspots, sometimes none at all for days on end," Howe told Newsweek. "When the spots for a new cycle start to appear they are at higher latitudes (about 30 degrees from the equator) and over time they become more common and appear closer and closer to the equator."
"At solar maximum there are many sunspots over a wide range of latitudes, and phenomena such as flares—where the sun throws out bursts of radiation—and coronal mass ejections where it throws part of its outer atmosphere into space—are more common," she said.
When these clouds of solar material interact with Earth's own magnetic field they may produce space weather phenomena, which include aurorae—such as the Northern Lights—or solar storms, which have the potential to affect power grids, satellites and other technological systems.
"The strongest solar storms are likely to occur during solar maximum conditions," Piyush Mehta, an assistant professor of mechanical and aerospace engineering at West Virginia University, previously told Newsweek. "While there can be storms even during solar minimum conditions, the strongest ones are likely to occur when the sun is very active."
According to Howe, as the sunspot belts move down closer to the equator, they eventually fade away as the solar minimum approaches. Then a new cycle begins.
Why Is It Now Solar Cycle 25?
The average length of the solar cycle is roughly 11 years, but they can be longer or shorter than this.
"It's a chaotic system which is why the cycle is never exactly the same," Chapman said. "There is still a lot that we don't understand about how this works, and part of the problem is that 25 cycles is not many for a chaotic oscillator—we haven't seen everything that the sun can do! Hence the predictions for the upcoming maximum have a very wide spread."
Currently we are in solar cycle number 25, which scientists determined to have begun in 2019.
The first solar cycle began in 1755 and the numbering system we use today was introduced by a scientist, known as Rudolf Wolf, in the mid-19th century. Sunspots had actually been observed in the early 1600s but observational difficulties meant experts at the time were unable to properly track cycles.
"Sunspots were observed from the early 17th century when Galileo started using a telescope, but shortly after they were discovered, there was a long period called the Maunder Minimum when very few sunspots were seen at all and it was difficult to count cycles," Howe said.
Since, 1755, there have been 24 complete cycles, hence, why we are now going through Cycle 25. So far, the current cycle seems to be more active than the last one, according to Howe.
"The prediction was that it would peak in 2025 at about the same strength as Cycle 24, which was a weak cycle compared with most that have been seen since the early 20th century but is considered 'average' overall," she said.
"However, Cycle 25 seems to have been rising faster than the consensus prediction by the NOAA[National Oceanic and Atmospheric Administration]/NASA international panel, who looked at a lot of predictions by scientists using different methods."
One 2020 study suggested that Solar Cycle 25 could have "a magnitude that rivals the top few since records began."
But while the cycle could still prove to be particularly strong, Mark Miesch, a space weather observer at the University of Colorado Boulder, told space.com in April, that on current trends, the sun might have around 125 sunspots at its peak, which would be more than the peak of Cycle 24, but lower than that of 23.
Nevertheless, these predictions are difficult to make and the answer won't be known until the end of this cycle.
"There's a few predictions that [Cycle 25] might be strong, but we'll wait and see," Miesch said.
When Was the Sun's Most Active Cycle Ever Recorded?
The most active cycle ever recorded—as measured by the highest sunspot count at solar maximum—was Cycle 19, which spanned the period between April 1954 and October 1964.
The maximum number of sunspots observed during the cycle was a record 285 at one time, which was observed in March 1958.
A number of significant space weather events occurred during this cycle, including the so-called Acheron submarine geomagnetic storm, which occurred on February 24, 1956.
While this storm was extremely powerful, it apparently caused little worldwide disruption. It is notable because the storm disrupted radio communications with a British submarine on patrol in the Arctic known as Acheron. Officials thought it had disappeared due to a loss of contact. An emergency rescue operation was launched but the "missing" submarine was located several hours later before it was found.
The New York Times also reported at the time that a "rare aurora" in the shape of a pure red arc was seen in Fairbanks, Alaska.
And on February 11, 1958, a geomagnetic storm caused radio blackouts in North America and produced intense, red aurorae that were visible in many locations on the continent—including relatively southern locations such as Los Angeles—in addition to Europe.
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