Solar Activity Cycle
Solar Activity Cycle
Discovery of the activity cycle
The solar activity cycle is the periodic, typically 11-year-long variation in the number of active features (for example, sunspots) visible on the Sun’s apparent surface or in its atmosphere. Over a period of 11 years, the number of sunspots gradually rises from a low level, reaches a maximum near the midpoint of the cycle, and then declines to a minimum. Solar activity is governed by the Sun’s magnetic field, and one of the unsolved problems in astronomy is the origin of the regular changes in the magnetic field that drive the activity cycle.
Discovery of the activity cycle
The most easily observed solar active features are sunspots, which are relatively cool regions on the Sun’s surface that appear as dark areas to viewers on the Earth. Italian astronomer and physicist Galileo Galilei (1564–1642) made some of the first telescopic observations of sunspots in 1610, but it was not until 1843 that German astronomer Samuel Heinrich Schwabe (1789–1875) noticed that the number of sunspots rose and fell in a cyclic fashion. One of the chief ways that scientists today track solar activity is by monitoring sunspots.
The overall sunspot record appears in Figure 1. The horizontal axes of these graphs show time, beginning in 1610 and continuing to 1980, and the vertical axes show the sunspot number. From one minimum to the next is usually about 11 years, but this is not always the case. From 1645 to 1715, the cycle disappeared. This period is called the Maunder minimum after British solar astronomer Edward Walter Maunder (1851–1928). (In the early 1800s, the cycles were very long—nearly 14 years rather than 11.)
Between 1645 and 1715, when no sunspots were observed, the Northern Hemisphere experienced a mini ice age. Indirect evidence suggests that the Sun was also inactive around 1300—the same time that there is evidence for severe drought in western North America and long, cold winters in Europe. Although other minima are believed to have occurred in the past, no sunspot records exist prior to 1610. There has also been speculation that a Maunder maximum might someday occur. Solar maximums are accompanied by many sunspots, solar flares, and coronal mass ejections, all with the potential of disrupting communications and weather on Earth.
Accompanying the variations in sunspot number are corresponding changes in other types of solar activity. Prominences appear as large regions of glowing gas suspended in magnetic field loops arching far above the solar surface. Sometimes there are violent flares, which are eruptions in the solar atmosphere that usually occur near sunspots. Matter ejected from the Sun by flares sometimes streams into Earth’s atmosphere, where it can interfere with radio communications and cause aurorae (the so-called northern lights and southern lights). The radiation accompanying solar flares has on occasion subjected airline passengers to doses of x rays comparable to a medical x-ray examination.
Cause of the activity cycle
No one has yet fully explained the origin of the solar activity cycle. Astronomers have developed several possible scenarios, or models, that reproduce the general characteristics of the cycle, but the details remain elusive. One of the most well-known of these models was developed in the early 1960s by American astronomer Horace Welcome Babcock (1912–2003).
Unlike Earth, the Sun is made totally of gas, and this makes a big difference in how these two bodies rotate. To see how Earth rotates, look at a spinning compact disc. Every part of the disc completes one rotation in the same amount of time. To see how the Sun rotates, study the surface of a freshly made cup of instant coffee. The foam on the surface rotates at different speeds: the inner parts rotate faster, so that spiral patterns form on the surface of the coffee. This is called differential rotation, and it is how any liquid or gaseous body rotates. Therefore the Sun, being gaseous, rotates differentially: the equator completes one rotation every 26 days, while regions near the poles rotate once every 36 days.
This is important because the Sun’s magnetic field, like the Earth’s magnetic field, gets carried along with the rotating material. When the magnetic field at the Sun’s equator has been carried through one complete rotation, the more slowly rotating field at higher latitudes has fallen behind. Over the course of many rotations, the field gets more and more twisted and tangled. Therefore, solar active features, like sunspots, are associated with regions of strong and complex magnetic fields. So, the more twisted the magnetic field gets, the more activity there is. Finally, when the magnetic field gets tangled to a critical level, it rearranges itself into a simpler configuration, just as when one twists a rubber band too many times, it snaps. As the magnetic field’s complexity decreases, so does the activity, and soon the cycle is complete.
None of this happens on Earth, because Miami, Florida, and Fairbanks, Alaska, for example, both rotate once every 24 hours. There is no differential rotation on Earth to tangle its magnetic field.
Coronal mass ejections (CMEs) are solar bursts that are as powerful as billions of nuclear explosions. These ejections are the largest explosions in the solar system, typically hurling up to 11 billion tons of ionized gas into space. CMEs produce geomagnetic storms that reach Earth in about four days. These storms can damage satellites, disrupt communication networks, and cause power outages. The 1989 power blackout in the northeast portion of the United States and Canada was triggered by a geomagnetic storm that overloaded part of the power grid and caused a blackout to propagate through the system. Satellites have been disrupted and, on occasion, destroyed by the radiation accompanying CMEs. For these reasons, operators of satellites, power systems, pipelines, and other sensitive systems follow solar-terrestrial activities by monitoring data from ground and orbiting solar telescopes, magnetometers, and other instruments.
In 1999, scientists reported a strong correlation between an S-shaped pattern that is sometimes observed on the Sun’s surface and the probability that a coronal mass ejection will occur from that region within several days. These S-shaped regions are thought to be produced by the twisted solar magnetic fields. If the correlation holds up under closer examination, it may be possible to predict CMEs as routinely as meteorologists predict weather patterns. To help this prediction ability, NASA launched the Solar Terrestrial Relations Observatory (STEREO), two spacecraft, in October 2006. They were positioned far from one another so they could produce stereoscopic images of CMEs and other solar activity measurements.
The poles of the Sun’s magnetic field change places each 11-year activity cycle. The north pole becomes the south magnetic pole, and vice versa. Thus, the 11-year cycle of sunspot frequency is actually half of a 22-year solar cycle in which the magnetic field reverses itself repeatedly. Actually, the length of the activity cycle is not exactly 11 years; that is just an average value. May 1996 saw the start of Cycle 23, i.e., the 23rd cycle since reliable data first became available. Cycle 23 was predicted to be of the same size as Cycle 20. However, it peaked prematurely in April 2000 at a smooth sunspot number (SSN) of about 120.8, with a second, smaller peak in November 2001. Cycle 23 should end in 2007, with Cycle 24 picking up where Cycle 23 ended—and Cycle 24 ending sometime around 2018.
In the course of each 11-year cycle, an increasing number of sunspots appear at high latitudes and then drift towards the equator. As already noted, sunspots are actually regions of intense magnetic activity where the solar atmosphere is slightly cooler than the surroundings. This is the reason sunspot regions appear black when viewed through viewing filters. Sunspots are formed when the magnetic field lines just below the Sun’s surface become twisted, and poke though the solar photosphere, i.e., the region of the Sun’s surface that can be seen by viewers on Earth. The twisted magnetic field above sunspots are frequently found in the same places that solar flares appear.
Sunspots pump x rays, high-energy protons, and electrified gases into space. That is the reason sunspots can affect satellites and power and communications systems on Earth.
In 1998, scientists reported finding giant convective cells (red and blue blotches) on the face of the Sun. Although evidence of the existence of these structures had been sought for more than 30 years, they had not been seen before because their movements were buried in the more violent, small-scale activities on the Sun. These blue and red shifts are believed to correspond to the rising and falling of gases and their spreading out across the solar surface. Now, in the 2000s, this data is still being studied by scientists.
The flow of solar gases is more powerful than the solar magnetic fields, so the gases can carry magnetic structures with them. The eruption of these magnetic structures from the surface and their looping into space and back coincides closely with the appearance of sunspots.
KEY TERMS
Differential rotation —Describes how a nonsolid object, like the Sun, rotates. Different parts of the object rotate at different rates; the Sun’s equator, for example, completes one rotation faster (26 days) than its poles (36 days).
Maunder minimum —The period of time from 1645 to 1715 when the solar activity cycle disappeared entirely. This period also corresponds to a time of unusually severe winters in Europe, suggesting that the solar cycle may be somehow connected to dramatic variations in Earth’s climate.
Sunspot number —An international estimate of the total level of sunspot activity on the side of the Sun facing Earth, tabulated at the Zurich Observatory (Germany). Observations from around the world are sent to Zurich, where they are converted into an official sunspot number. Since the Sun rotates, the sunspot number changes daily.
See also Global climate; Solar flare; Total solar irradiance.
Resources
BOOKS
Arny, Thomas. Explorations: An Introduction to Astronomy. Boston, MA: McGraw-Hill, 2006.
Bhatnagar, Aravind. Fundamentals of Solar Astronomy. Hackensack, NJ: World Scientific, 2005.
Chaisson, Eric. Astronomy: A Beginner’s Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004.
Eddy, J.A. The Ancient Sun. ed. R.O. Pepin, J.A. Eddy, & R.B. Merrill. New York: Pergamon, 1980.
Hill, Steele, and Michael Carlowicz. The Sun. New York: Abrams, 2006.
Mendillo, Michael, Andrew Nagy, and J.H. Waite, eds. Atmospheres in the Solar System: Comparative Aeronomy. Washington, DC: American Geophysical Union, 2002.
Jeffrey C. Hall
Randall Frost