Of all of the features associated with our sun, sunspots are by far the most conspicuous.
Long before the invention of the telescope, the Chinese Chronicles recorded numerous sightings of dark areas on the face of the sun, apparently seen when the sun's light was significantly dimmed by clouds and haze, especially around sunrise or sunset. The Chinese often referred to these dark areas as "flying birds."
When Galileo looked at the sun through his crude telescope in 1610, he was blinded for a week, and his eyesight remained permanently affected. But he came up with a solution: He darkened the room, placed his telescope at an opening in the window shutter, and projected the image of the sun onto a sheet of paper; thus, he was safely capturing the image of the sun, and in doing so was able to follow the path of the sunspots. Over several days they moved across the disk of the sun, vanishing around one edge and reappearing on the opposite side. Clearly, the sun was rotating on an axis just like Earth.
From observations of sunspots, scientists have now deduced that the sun rotates fastest at the equator, taking 25 days for one full turn compared to about 29 days near the poles.
A matter of contrast
Sunspots initially give the impression of being dark, sculptured holes on the face of the sun. On the surface of the sun (called the photosphere), the temperature is around 11,000 degrees Fahrenheit (6,100 degrees Celsius). But a sunspot is quite a bit cooler, averaging about 8,000 F (4,400 C). The spot therefore appears darker than the surrounding region because it's cooler and thus gives off less light compared to the rest of the surface.
But make no mistake, if you could somehow lift a sunspot off the sun's surface and place it in the sky on its own, it would appear dazzling — as bright as a hundred full moons!
The dark, irregularly shaped center, or umbra, of a sunspot can range from about 900 miles (1,400 kilometers) to over 50,000 miles (80,000 km) in diameter. This is surrounded by a less-dark area called a penumbra, which often more than doubles the size of the sunspot — into which more than 20 Earths could easily disappear. The spots frequently swim through the sea of incandescent gases in pairs or clusters; they grow rapidly and then slowly decline.
Although astronomers have been studying sunspots for hundreds of years, the exact cause of these spots is still unknown. They have strong magnetic fields and appear to be giant solar storms, which may be caused by deeper, periodic changes. On Sept. 1, 1859, English astronomer Richard C. Carrington was routinely charting sunspots. The solar image in his telescope was filtered to reduce its blinding brightness, but suddenly in one sunspot group two brilliant spots of light appeared.
At first, Carrington thought there was a gap in his filter that allowed full sunlight through, but the spots became even brighter. He was the first person to witness a solar flare; a geyser of hot gases on the surface of the sun caused by a sudden release of energy. Within minutes the magnetic needles in every observation station throughout the world gyrated in a wild dance.
Since then, the correlation between solar flares and sunspots has been well established. As a rule, the more spots there are, the more likely big flares are. Typically, shortly after a flare erupts, great streams of electrified particles rocket through space at speeds of 2 million mph (3.2 million km/h) or more, ultimately colliding with the rarefied gases of the upper atmosphere, which can blaze into a colorful battleground of diffuse shifting and glowing lights resembling arcs, streamers and rays: the aurora borealis, or northern lights.
Most auroral displays occur in the arctic regions around Earth's magnetic poles, but on rare occasions an exceptionally large sunspot or solar flare will push their zone of visibility through Canada and down into central or even the southern United States. (The flare witnessed by Carrington caused the northern lights to be seen as far south as the Caribbean!) But along with this spectacular sky show, such magnetic disturbances can send surges of electricity into Earth's atmosphere, which can black out radio communications and cause utility company circuit breakers to trip, cutting off power. One such outburst in March 1989 shut down power across Quebec, Canada, for nine hours.
The man who discovered the solar cycle was Heinrich Schwabe of Dessau, Germany. His original intention was to make daily observations of the sun, starting in 1825, in hopes of discovering a planet within the orbit of Mercury passing across the sun's disk. The planet didn't exist, but on every sunny day for 17 years Schwabe drew the disk of the sun, recording all the dots and spots he could see, from tiny points to vast, sprawling blemishes. Gradually Schwabe detected a distinct cycle — an 11-year period of increasing and decreasing sunspot activity. Ever since, everything imaginable has been examined for a relationship to the 11-year sunspot cycle: the stock market, wars, epidemics, the price of whiskey and even the fertility of fur-bearing animals in Canada!
Do sunspots affect our weather?
Some scientists believe they do. In the late 19th century, two astronomers, Edward Maunder and Gustav Spörer, published papers that pointed to a period between the years 1645 and 1715 when sunspots became exceedingly rare. Interestingly, this same 70-year interval roughly coincided with the middle part of the "Little Ice Age," during which Europe and North America experienced much colder-than-average temperatures.
Over the past decade, however, astronomers noticed the sun going through unusually long stretches without any sunspots; even at the most recent solar maximum in 2014, sunspot numbers were 36% below normal. Some think that we might be heading for the start of another prolonged sunspot minimum, similar to the one 375 years ago, and that another spell of unusually cold weather — some call it "global cooling" — will ultimately result.
Many environmental scientists, however, strongly disagree, stating that the Little Ice Age in the late 17th and early 18th centuries was more likely caused by an unusually high level of volcanic activity worldwide, spewing widespread clouds of ash and dust into the atmosphere. Such aerosol clouds acted like an atmospheric shade to reduce incoming sunlight, which was likely the main cause of the Little Ice Age, according to NASA.
Regardless, it is still a contentious issue.
A new tool for solar forecasts
In 2007, the Space Environment Center changed its name to the Space Weather Prediction Center, and it recently added a new, first-of-its-kind space weather forecast model to its suite of tools to improve forecast capabilities, further protecting the U.S. from space weather hazards.
The new tool, called the Whole Atmosphere Model-Ionosphere Plasmasphere Electrodynamics (WAM-IPE) space weather forecast model, will predict how Earth's upper atmosphere responds to solar and geomagnetic storms. It will also help predict total electron content, which is important for communication and navigation systems. The new neutral density field product will aid in orbit prediction and space situational awareness for satellite operators and ground tracking systems.
The model's latest output can be viewed through the Space Weather Prediction Center (SWPC) website.
"There is no bad weather, just bad preparation," said Jake Bleacher, chief scientist for NASA's Human Exploration and Operations Mission Directorate. "Space weather is what it is — our job is to prepare."
Solar activity bottomed out in December 2019, signaling the beginning of a new sunspot cycle — number 25. The current expectation is that we will reach a maximum around mid-2025. But even for this, not all solar scientists are on the same page regarding how strong it will ultimately become. The general consensus is that solar cycle 25 may have a slow start but will peak with a sunspot range of 95 to 130. This is well below the average number of sunspots, which typically ranges from 140 to 220 sunspots per solar cycle.
However, a forecast published in the journal Solar Physics in November 2020, was diametrically opposite, predicting that "Sunspot Solar Cycle 25 could have a magnitude that rivals the top few since records began."
So, it appears we'll just have to wait and see what transpires in the months and years to come. But even if it turns out to be a below-average solar cycle, that doesn't mean there is no risk of extreme space weather. "The sun's impact on our daily lives is real and is there. SWPC is staffed 24/7, 365 days a year because the sun is always capable of giving us something to forecast," Doug Biesecker, a solar physicist at the Space Weather Prediction Center, noted.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Follow us on Twitter @Spacedotcom and on Facebook.