Do we Finally Know the Causes of Sunspots?

James Maynard
Sep 20, 2019 · 5 min read

Sunspots have been studied for 400 years. Although the effects of their 11-year cycle is well understood, underlying questions remained about how they form. Now, we may have those answers.

Between the end of 1610 and early 1611, several astronomers, including Galileo, independently saw sunspots in their primitive telescopes. Since that time, the features have been closely examined by astronomers around the world.

Sunspots are relatively cool, dark regions on the Sun which form roughly 30 degrees from the stellar equator, and slowly drift closer to the equator over time. As the cycle progresses, sunspots become less common, fading out to a minimum. This cycle repeats every 11 years, although the magnetic orientation of the sunspots flips back and forth each cycle.

A sunspot seen from space by NASA’s Solar Dynamics Observatory (SDO). Image credit: NASA’s Goddard Space Flight Center/SDO

The origin of sunspots has remained a mystery for four centuries, as well as the nature of the regular 11-year cycle. A new model suggests a process by which these features form, and it could also explain why the magnetic field of sunspots flips every cycle. The presence of such a thin layer could explain the appearance and behavior of sunspots (and their magnetic reversals), as well as other mysteries of our nearest star.

“Our model is completely different from a normal picture of the sun. I really think we’re the first people that are telling you the nature and source of solar magnetic phenomena — how the sun works,” stated Thomas Jarboe, professor of aeronautics and astronautics at University of Washington.

That’s a Fine Twist

Sunspots appear dark because they are cool compared to the photosphere (visible layer) of the Sun. These features have average temperatures of 3,500 degrees Celsius (6,300 Fahrenheit), compared to the surface of the sun, which hovers around 5,500 degrees Celsius (9,900 Fahrenheit). They are highly magnetized, and can erupt in solar flares, magnetic eruptions which can bathe Earth in extreme ultraviolet and X-ray radiation.

Examining the results of research on nuclear fusion reactors, investigators found sunspots could be explained by the behavior of a thin layer, or circuit, of magnetic flux and plasma (swarming with free electrons) just beneath the surface of Sun.

Sunspots appear with little warning, suggesting they are formed just beneath the outermost layers of the Sun before they make their dramatic appearance. Most astronomers hold the idea the sunspots formed inside the Sun, roughly 30 percent of the way to the core. There, a rope of plasma was thought to rise up, eventually popping above the surface, creating an effect we see as sun spots.

This new study suggests sunspots exist within supergranules that form within layers of plasma, just 150–450 km (100 to 300 miles) thick, sitting just beneath the surface of the Sun. The electromagnetic field of this thin layer is thought to break down, eventually shedding its outer layer to space. What had been the inner side of the plasma layer (with a charge opposite to its now-departed partner) develops a new inner layer, before the cycles repeats.

As the layer moves at different speeds at varying parts of the Sun, the flow creates twists in magnetic fields, known as magnetic helicity. Similar to effects expected to occur in fusion reactors.

This chart, known as the “Butterfly Diagram,” shows how sunspots form in bands roughly 30 degrees from the solar equator (EQ) at the start of cycles, but closer to the equator as cycles die out. Image credit: Hathaway 2019/

When circuits in each hemisphere of the Sun are moving at the same speed as each other, a large number of sunspots are produced. Periods featuring few sunspots occur when the circuits are traveling at different speeds.

“If the two hemispheres rotate at different speeds, then the sunspots near the equator won’t match up, and the whole thing will die,” Jarboe states.

Such conditions may have fueled the best-known of all periods with few sunspots, the Maunder Minimum which took place between 1650 to 1710.

Take Your Plants in Before the First Frost

Even during solar minimum, the Sun remains active, and it is during this time that astronomers are able to study long-live coronal holes. These enormous holes in the outer atmosphere (corona) of the Sun are formed by the Sun’s magnetic field. Once created, they can allow streams of solar particles to escape our stellar companion as high-velocity solar wind, which can leave our parent star at speeds up to 2.9 million kilometers per hour (1,800,000 MPH).

The solar wind released from coronal holes can cause space weather effects near the Earth. The results could include geomagnetic storms (temporary disturbances of the Earth’s magnetosphere), auroras, and disruptions to satellite communications and navigation systems.

“We see these holes throughout the solar cycle, but during solar minimum, they can last for a long time — six months or more,” Dean Pesnell of NASA’s Goddard Space Flight Center explains.

The Sun is currently experiencing a solar minimum, which could last through at least the year 2020.

Sports on a Frozen River by Aert van der Neer shows people walking on rivers that would normally never freeze. Image credit: The Metropolitan Museum of Art.

During the Maunder Minimum, the overall brightness of the Sun was reduced, and temperatures across Earth dropped, causing a mini-Ice Age. Although the total output of the Sun was reduced by just around one-quarter of one percent, the effects on the climate resulted in temperatures plunging during those decades. Although few temperature readings were taken during the late 16th century, temperatures can be deduced from ice cores, tree rings, and other physical data.

“[T]he reduced brightness of the Sun during the Maunder Minimum cause[d] global average surface temperature changes of only a few tenths of a degree, in line with the small change in solar output. However, regional cooling over Europe and North America is 5–10 times larger due to a shift in atmospheric winds,” Drew Shindell explains in a NASA Science Brief.

In Europe, these lower global temperatures resulted in waterways which normally never iced over, becoming frozen. Groups of Native people in North America banded together into the League of the Iroquois to fend off famine caused by crop failures.

A NASA video from 2017, explaining then-upcoming solar minimum. Video credit: Science@NASA

“When the sun is shining I can do anything; no mountain is too high, no trouble too difficult to overcome.” — Wilma Rudolph

The 2019–20 minimum is not expected to be as severe, or last as long, as the event which began 370 years ago. However, the magnetic fields near the poles of the Sun are believed to play a critical role in seeding solar cycles, which determines the strength of the upcoming sunspot cycle.

Weak polar fields established after the maximum seen in 2000–2002 (Cycle 23), was only half the strength of the previous two cycles, creating the weakest solar cycle in more than a century.

If sunspot cycles do not return to normal soon, our planet could be headed toward a repeat of the Maunder Minimum, potentially posing significant challenges to modern society.

Analysis of the new study on sunspots was published in the journal Physics of Plasmas.

Did you like this article? Subscribe to The Cosmic Companion Newsletter!

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

James Maynard

Written by

Writing about space since I was 10, still not Carl Sagan. Mailing List/Podcast:

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

Welcome to a place where words matter. On Medium, smart voices and original ideas take center stage - with no ads in sight. Watch
Follow all the topics you care about, and we’ll deliver the best stories for you to your homepage and inbox. Explore
Get unlimited access to the best stories on Medium — and support writers while you’re at it. Just $5/month. Upgrade