Sunspots are one way we can track the activity of the Sun. There have been fewer sunspots than usual in recent years, and that may point toward an historic solar minimum. 

Sunspots are dark patches that occasionally appear on the surface of the Sun. They aren’t actually dark. If you could see a sunspot by itself it would appear bright red, but since sunspots are about a quarter as bright as the rest of the Sun, they appear as dark regions. Since the early 1600s astronomers have counted the number of sunspots over the years, and we’ve noticed a few patterns. One is that sunspot counts vary between maximum and minimum over an 11-year period. There are other patterns as well, such as the Gleisberg Cycle, which lasts 80 – 90 years.

Directly recorded sunspot counts over the years. Credit: Robert A. Rohde (CC BY-SA 3.0)

There are times when the pattern seems to break down, and the Sun can enter into an extended period of little sunspot activity. The most famous is the Maunder minimum of the 1600s. While we don’t have direct sunspot counts before the early 1600s, we can look at the levels of carbon-14 as measured from tree rings. Since carbon-14 levels have a good correlation to sunspot counts, we can get a handle on a much longer history of sunspots. It turns out there have been other periods of minimum activity, such as the Wolf minimum of the 1300s. In general, the sunspot activity of the Sun in recent centuries is somewhat higher than most, except for a period during the middle ages known as the Medieval maximum.

For the past couple of cycles the sunspot maximums have been lower than usual. The pattern is similar to the early stages of the Dalton minimum in the early 1800s, which has raised the question of whether we are entering a period of reduced sunspot activity. This may also have some effect on global temperatures. The Dalton minimum saw a brief period of colder temperatures, and the Maunder minimum was marked by the “little ice age” where Europe and North America experienced a colder period. It should be stressed that connections between sunspot activity and global temperatures is still not clear. The Dalton cold period for example, saw the explosion of Mount Tambora, which would also contribute to cooler temperatures.

What is clear is that periods of minimal sunspot activity are notoriously difficult to predict. While the pattern of the past few cycles has similarities with the early Dalton minimum, it could also be a small fluke before a return to cycles as normal.

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Yesterday I talked about how water can form on the Moon. It might seem a bit surprising that water exists on the Moon, but it doesn’t sound like a crazy idea. What does sound crazy is the idea that there is water on the Sun’s surface, and yet we know that there is.

The surface of the Sun (specifically the photosphere) has a temperature of about 6000 K. It is so hot that hydrogen atoms are ionized, and molecules can be ripped apart. It’s pretty inhospitable for a molecule such as water. Despite this, the potential for water is there. Oxygen is produced in stars through the CNO fusion cycle, and we have observed quantities of oxygen in the Sun’s spectra. Hydrogen is the most abundant element in the universe, and most of the Sun’s mass is hydrogen. All that’s needed is a cool enough temperature for the hydrogen and oxygen to come together to form water.

It turns out there is just such a place in sunspots. We normally think of sunspots as dark regions on the Sun. They aren’t actually dark, but they are cooler and dimmer than the rest of the Sun, which is why they appear dark in solar images. Within a large sunspot, the temperature can be as cool as 3500 K, which is cool enough for water to form. Naturally it only exists as water vapor, but there really is water on the Sun’s surface.

Spectra for hot water. The red line is the theoretical spectra for water at 3000K. The blue line is the observed curve in sunspots. Credit: Polyansky, et al.

The presence of water on the Sun has long been suspected, but proving it has been a real challenge. That’s because water has a complex spectra with millions of absorption lines. These lines also vary with temperature, making it even more challenging. Experimentally measuring the line spectra of water vapor at 3500 K isn’t feasible, so you need to calculate the expected spectra using computer simulations.

In 1997, a team did just that. They were able to calculate more than 6 million absorption lines for very hot water, and then compared the results to observed spectra deep within sunspots. They found a clear match, showing that water does indeed form within “cool” sunspots.

So yes, there really is water on the Sun’s surface. It’s a fact worth remembering if you ever want to win a bar bet.

 Paper: Polyansky et al. Water on the Sun: Line Assignments Based on Variational Calculations. Science 277 (5324): 346-348 (2014)

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We generally think of the Sun as a constant in our lives. It rises and sets regularly, and seems to be an unchanging sphere of brilliant light. In fact the Sun has a turbulent surface with prominences that fly off its surface, granules caused by convection in its upper layers, and even the appearance of slightly cooler regions known as sunspots.

Of all these features, sunspots are relatively easy to observe, and were the first evidence that the Sun is not a fixed constant. Since the early 1600s we’ve been able to make reasonably accurate counts of the number of sunspots over time. One of the most obvious patterns that appears is an oscillation in the amount of sunspot activity over a typical span of 11 years. This is known as the 11-year cycle due to its average rate, though the cycle is sometimes longer or shorter. There are other patterns as well, such as the Gleisberg Cycle, which lasts 80 – 90 years.

Sunspot cycles also have variations that don’t follow the pattern. During the period from 1645 to 1715 there was very little sunspot activity at all, known as the Maunder minimum. From about 1790 to 1830 there was a period when the oscillations were smaller, and the periods of maximum sunspot activity were less intense, known as the Dalton minimum. We’re not entirely sure why such minimums occur, but it is clear that sunspot activity can be quite varied.

Credit: David Hathaway

Recently there has been a lot of talk of the strange lack of sunspot activity at a time when sunspots should be pretty active. You can see this in the image here, which plots sunspot counts (the jagged line) compared to the prediction range (the curves) for the current period known as cycle 24. You can see that this maximum is about half what the last one was. This is a bit unusual, but it isn’t “puzzling scientists” as some articles have said. This kind of variability happens from time to time.

One thing that has been noticed is that cycle 24 has some similarity to the early stages of Dalton minimum. Before the Dalton minimum, cycles got successively smaller, with the same drop to about half the activity that had been typical. If we follow a similar pattern, then cycle 25 will be about the same as cycle 24, and we will have a several decades period of a relatively quiet Sun.

Because of this you may also hear talk about global cooling and the like in the popular press. The reason is that sunspot activity (or lack thereof) has a demonstrated connection to winter temperatures. During the Maunder minimum, Europe was in the middle of what is known as the “little ice age” when they experienced very cold winters. During the Dalton minimum European winters were about a degree celsius cooler than typical, including the “year without a summer” in 1816 (though that was largely due to the Tambora eruption in 1815).

If we are entering a Dalton-like minimum, then we can expect winter temperatures be slightly cooler than recent years. However this will not be an indication of an end to the global warming trend. A sunspot minimum could help to ease off the accelerator of rising temperatures, but it won’t stop the rise or reverse it. The Dalton minimum was followed by cycles of average to high activity, so after a few decades we’d be right back where we started.

It will be a few years before we are sure if the Sun is really in a minimum, or if cycle 24 is just an unusual fluke. Either way, we know eventually the Sun will wake up.

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Sunspots are dark regions that appear on the surface of the Sun. Despite their appearance, sunspots aren’t actually dark. They are cooler than the surrounding surface, which means they are less bright.  When an image of the Sun is made dim enough to view directly, either by viewing the Sun through a filter or by projecting the Sun’s image onto a surface, the cooler, dimmer sunspots appear dark.

Sunspots are actually depressions in the Sun’s surface. They are caused by magnetic fields, which interact with the ionized plasma of the Sun.  Because the Sun rotates differentially (that is, its polar regions rotate more slowly than its equatorial region), the magnetic field of the Sun winds around the star until it snaps back into alignment, which gives rise to sunspots.  We can actually measure the strength of magnetic fields near sunspots due to the Zeeman effect.

Sunspots also appear and disappear over time.  Their numbers rise and fall with a cycle of about 11 years, and was the first evidence of varying solar activity known as the solar cycle.  We’ve been tracking the sunspot cycles since the 1600s.  Of course now we can observe their formation in real time, as seen in the video. Each frame of this video is about a minute apart, and it tracks the formation of sunspots over about two weeks.

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We generally think of the Sun as a steady constant, but it’s actually quite variable.  The Sun’s magnetic field varies over time, which means the activity of the Sun varies.  The earliest observation of this cycle was seen in sunspots.  While sunspots were observed throughout history, in the early 1600s astronomers began making regular observations of sunspots, and soon discovered an 11-year cycle of high and low sunspot activity.  Over time similar variations in solar flare activity and brightness.  Thus the Sun cycles through active and quiet periods.
In the early 1800s Joseph von Fraunhofer discovered that sunlight was not a continuous range of colors, but rather had gaps at certain wavelengths.  These gaps are now called Fraunhofer lines, and we now know they are cause by the Sun’s atmosphere absorbing certain wavelengths of light the Sun produces.  The specific wavelengths that are absorbed depends on the type of atoms and molecules which are in the Sun’s atmosphere.  We see the same effect in the light of other stars, which is how we can determine the types of elements a particular star has.

Two of these absorption lines in the ultraviolet are known as the H and K lines.  They are due to the presence of calcium in the Sun’s atmosphere.  These particular lines are affected by magnetic activity, so when the Sun is active you can actually get H and K emission lines.  The greater the magnetic activity, the stronger these lines are.  This is useful because we can observe these spectral lines in other stars.

Directly measuring the magnetic activity of other stars is very difficult, since we can’t observe most stars as a disk.  That means we can’t observe sunspots on stars other than our Sun (starspots).  But since we can observe the H and K lines of stars, and these correlate to magnetic activity, we can observe these lines over time to study the activity cycles of other stars.

In 1966, Mount Wilson Observatory began a project to observe the H-K lines of about a hundred local stars.  The project has been ongoing continuously, and now monitors about 400 stars.  This means we have long-term observations of the activity cycles of hundreds of stars.  You can see examples of these in the figure below.  Some stars have short activity cycles (less than 3 years), while others have cycles of more than 20 years.  A few stars seem to be in a long inactive period, similar to the Maunder minimum our Sun had in the late 1600s.

Activity cycles of several stars. Credit: Mount Wilson Observatory

We now have other ways to observe stellar activity cycles.  For example, the Sun’s active periods also produce more x-rays.  X-ray telescopes such as XMM-Newton have observed similar variations in sun-like stars, and these cycles correlate to the H-K cycles of the stars.

Observations such as these continue to confirm that our Sun is just a star like many others throughout the galaxies.  There are billions of stars in the universe, but we think one of them is special, simply because it’s ours.

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We generally think of the Sun as a constant in our lives.  It rises and sets regularly, and it seems to be an unchanging sphere of brilliant light.  In fact, the Sun has a turbulent surface with prominences that fly off its surface, granules caused by convection in its upper layers, and even the appearance of slightly cooler regions known as sunspots.

Of all these features, sunspots are relatively easy to observe, and they were the first evidence that the Sun is not a fixed constant.  Since the early 1600s, we’ve been able to make reasonably accurate counts of the number of sunspots over time.  I’ve plotted some historical data in the figure below.

One of the most obvious patterns you can see is the wide variation in sunspot activity over a relatively short period of time.  This is known as the 11-year cycle due to its average rate, though the cycle is sometimes longer or shorter.  There are other patterns as well, such as the Gleisberg Cycle, which lasts 80 – 90 years. There have also been unusual periods such as the one from about 1645 to 1715 when there was very little sunspot activity at all, known as the Maunder minimum.

Sunspot activity now allow us to gain a better understanding of the upper regions of the sun, and that in turn gives us a better understanding of the sun as a whole.
But it all began by spotting a pattern.

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