We are now just past the summer solstice, which occurred at 12:45am CDT on June 21. Anyone willing to brave the almost 100 degree heat can go out to our sundial and project a real image of the Sun – which would reveal if any sunspots are present. Lately, however, this activity has been slightly less exciting because in the past three years, the Sun has been largely without sunspots.
Sunspots are slightly cooler regions on the photosphere (the Sun’s surface). Most of the Sun is at about 10,000o Fahrenheit, while sunspots are only about 6,000oF. To understand why they form, let’s consider some basic facts about the Sun. First, the Sun is not solid but is instead made of burning, ionized gases, or plasma (the ‘surface’ of the Sun is the layer where this gas becomes opaque.) When a solid body, such as a ball, rotates, it does so rigidly; every part of the ball takes the same amount of time to rotate once. For example, no matter where you are on the Earth, one day is 24 hours. Since the Sun is fluid, however, it has whats called differential rotation; different parts of the sun rotate at different rates. At its equator, the Sun rotates once in about 24.5 days. Near the poles, this period is much longer: up to 35 days.
Secondly, the Sun has a magnetic field. Any current causes a magnetic field; any moving charge is a current. Among the layers of the Sun is a convective zone in which hotter gases from the center of the Sun rise while cooler gases fall back towards the Sun’s center. The Sun, as a plasma, is made of ionized gases. The motion of ionized gas particles in convective loops forms currents in the Sun; which in turn generate a magnetic field. Due to differential rotation, magnetic field lines can become distorted or twisted.
Where these twisted magnetic field lines puncture the Sun’s surface, the transport of heat via convection from inside the Sun is blocked. This results in a cooler, darker region on the Sun’s surface–a sunspot. The lower temperature of a sunspot means that it emits light at a much, much lower intensity than the rest of the Sun’s surface. Further, the cooler temperature shifts more of its radiation into the infrared portion of the spectrum. This is why the spot appears much dimmer than the rest of the Sun. Sunspots appear dark only because of the contrast with the much brighter solar disk. If you could separate a sunspot from the Sun, you would discover that it is quite bright in and of itself. When we observe the Sun at the H-alpha wavelength (a particular wavelength of red light), sunspots appear brighter than the Sun’s disk.
Once formed, sunspots exist for about two weeks. They also vary in size, with the biggest sunspots being up to 50,000 miles across. Compare that with the Earth, which is less than 8,000 miles across.
It turns out that the biggest sunspots are noticeable to the naked eye when the Sun is low to the horizon or seen through mist or clouds. You should never try this however; always observe the Sun by projecting its image or by looking through a filter expressly designed for this purpose. There is evidence, though, that before modern understanding and technology, early astronomers risked eye damage by looking at the Sun when it seemed dimmer than usual. For example, ancient Chinese astronomers may have made reference to sunspots in 28 BC. (This may have supported the legend that a raven lived in the Sun).
Early telescope users Galileo Galilei and Thomas Harriot were among the first western astronomers to observe sunspots. David Fabricius and his son Johannes were the first to publish a description of sunspots in June 1611. In 1843, German astronomer Heinrich Schwabe discovered that the number of sunspots varies in a cycle of about 11 years. Swiss astronomer Rudolf Wolfthen used data from Schwabe and others to reconstruct solar cycles back to about 1745. Wolf designated the cycle from 1755-1765 as Cycle 1, and we still use that count today. Accordingly, the last cycle which peaked in 2001 was Cycle 23, and the next cycle expected to begin now and peak in 2012 will be Cycle 24. Also, solar astronomers use the ‘Wolf number’ to describe the number of sunspots on the Sun. It was George Ellery Hale who first associated sunspots with magnetism.
This graph shows that not all solar cycles are the same. Peaks in the early 19th century were much smaller than those of the 20th century, for example. Towards the left of the graph, covering about 70 years including the last half of the 17th century, is a period which seems to have no peaks. This is the Maunder Minimum, noted by Edward R. Maunder. The decades of few sunspots coincided with decades of unusually cold winters in Europe and North America. This is also a time when few aurorae were observed. In fact, the 11 year cycle of minima and maxima continued in this time as well, it’s just that the peaks were very, very small compared to later periods.
For much of 2009, we’ve been past due for the start of the next solar cycle–Cycle 24. Since the peak of Cycle 23 occurred in 2001 and the next peak was expected in 2012, scientists expected to begin seeing many Cycle 24 sunspots in late 2007 and especially by 2008. Instead, 266 of the 366 days of 2008 were spotless. The dearth of sunspots continued into early 2009, where 134 days (78% of all days through June 22) have been spotless. Solar scientists were a bit baffled by the late start to the new cycle; a few wondered if the Maunder minimum might be recurring. On June 17, however, researchers Rachel Howe and Frank Hill of the National Solar Observatory in Tucson, Arizona, put forward an explanation. Far below the surface of the Sun is a stream of plasma analogous to the jet stream on Earth. The Sun generates these ‘jet streams’ once about every 11 years. Once formed, they then gradually shift from the polar region towards the equator. Using helioseismology, Howe and Hill were able to determine that this time around, the solar jet stream is shifting more slowly than usual, resulting in a delayed Cycle 24. However, they reported, that jet stream has just now gotten to a low enough latitude–22 degrees from the sun’s equator, to allow frequent sunspot formation. If this explanation holds, we should expect to see many more sunspots as we approach 2012.
As if on cue, two sunspots of the new cycle were visible on the Sun as of June 22. Does this mark the long awaited increase in sunspot activity? You can surf to http://www.spaceweather.com to view an image of the Sun each day. Or, if you can stand the heat, come out to our sundial and make your own image of the Sun. Near the solstice, the sun’s apparent height in our sky does not change all that much. For quite a few days after the solstice, the Sun will shine through the lenses in our gnomon.
Nature rarely allows us the comfort of feeling that we’ve got it all figured out. Even when we understand something well, such as the 11-year solar cycle in this case, it often turns out that we understand it only partially. Constant observation and discovery is always required. In this context, the spotless Sun of the past few years reminds us that scientific knowledge is not a series of decrees from on high, but is a process that we can all participate in.