On a visit to the newly renovated and expanded Griffith Observatory in Los Angeles I heard Jay M. Pasachoff, an astronomer from Williams College in Massachusetts, tell about his plans to travel to China to observe the total solar eclipse of July 22. Having witnessed twenty-eight total eclipses before that one, he had already seen more than anyone else. But what really got my attention was his remark that lately the Sun has been unusually devoid of sunspots. Those relatively cool blemishes on the Sun’s surface normally wax and wane in an eleven-year cycle, but this is the deepest “solar minimum” since 1913, according to NASA.
New discoveries about the nature of sunspots abound, but first, what are they? The Exploratorium in San Francisco has a short primer on the enigmatic spots. According to George Fischer, a solar astronomer at the University of California who has a video interview on the site, “A sunspot is a dark part of the sun’s surface that is cooler than the surrounding area. It turns out it is cooler because of a strong magnetic field there that inhibits the transport of heat via convective motion in the sun. The magnetic field is formed below the sun’s surface, and extends out into the sun’s corona.” The site includes the fascinating history of sunspot science, which notes that Greek philosophers made references to sunspots in their writings as early as the fourth century B.C., and astronomers in China made systematic observations as early as 28 B.C. The fourth page of the history section shows you how you can safely use binoculars to project an image of the Sun on a piece of paper for viewing.
Looking directly at the Sun to check for spots can easily damage your eyes. In ancient times, astronomers could sneak a glance at the solar disk just before sunset, or when it was covered by a thin veil of clouds, but looking directly at the Sun is never a good idea. Galileo, one of the first astronomers to train a telescope on the Sun, used a method devised by his protégé Benedetto Castelli to project the Sun’s image from a telescope onto a surface to safely study sunspots. Visit The Galileo Project, hosted by Rice University in Houston, Texas, to learn more about early sunspot observations. At this site you can view a remarkable animation made from Galileo’s original drawings, showing the sunspots as they traveled across the solar disk during the summer of 1612.
A Website at Goddard Space Flight Center in Maryland has more on the “Magnetic Sun,” of which sunspots are just one manifestation. Here the Mount Wilson astronomer who discovered the true nature of sunspots is given his due: “The nature of sunspots remained unclear until 1908, when George Ellery Hale, using an instrument that observed the Sun in narrow ranges of color emitted by selected substances, reported that the light from sunspots was modified in ways that indicated it was produced in intense magnetic fields.”
The Internet is the safest (and cheapest) way to observe our star’s spots. One of my favorite sites, Astronomy Picture of the Day has three images to get you started. Zoom in over a gaping, planet-sized hole at The Sharpest View of the Sun. Go to An Active Sunspot Viewed Sideways to see a beautiful image formed by glowing gas as it traces the magnetic sweeping out from the sunspot’s interior. Another image, Sunspot Stack, shows how the magnetic disturbances look as seen from the photosphere, the chromosphere, and the corona.
For current images go to the homepage of the Solar and Heliospheric Observatory (SOHO), a joint mission of the European Space Agency (ESA) and NASA. The SOHO spacecraft is locked in an orbit between the Sun and the Earth at about four times the distance to the Moon from us. Launched in 1995, it is the first solar observatory to have a continuous view of our star. Click on the “sunspot” image on the top right to see the latest image of the solar disk. One of the best features of this site is that it allows you to generate time-lapse movies (much like the one using Galileo’s drawings) of the sunspot activity over a period of months. Go to the menu bar at the top and click on “data/archive” and then select “SOHO Movie Theatre.” Next, click on “MDI Continuum” for the image type and enter the dates for your sunspot movie to start and stop. Using it to watch the Sun for the last few months, I saw barely a blemish. In contrast, a movie from a few years ago showed spots galore on the rotating star.
Another site with current information, predictions, and links related to solar activity is spaceweather.com. Here I learned a technique called helioseismic holography is used to monitor sunspots on the far side of the Sun. Click here to learn how it is done. For an extensive list of links with real-time or near real-time solar images, go to this site at NOAA’s Space Weather Prediction Center. And, if viewing via the Web does not satisfy you, a good site for amateur astronomers interested in making their own solar observations is called Sungazer.
A host of recent observations from space- and ground-based observatories has begun to answer many of the remaining mysteries swirling around sunspots. This 2001 news release from Goddard Space Flight Center, The Sun’s Dark Secret: How Sunspots Pull Themselves Together, reports on scientists getting the first clear picture of what lies beneath sunspots. They observed the flow of electrified gas that creates a self-reinforcing cycle, holding the sunspots together. And recently supercomputers have been used to model the magnetic phenomenon. Go to the University Corporation for Atmospheric Research to view their supercomputer-generated animations of sunspots.
While delving into sunspots on the Internet, I came across a related news story that finally seems to answer a fundamental question of solar physics: How is the Sun’s outermost layer, the corona, heated to a few million degrees kelvin, while the underlying photosphere—where our visible light comes from—hovers at a relatively chilly 5,000 degrees Kelvin? See Giant Solar Twists Discovered to learn about the newly discovered Alfvén waves that heat the corona hot enough to glow in x-ray light. The magnetic twists are named after 1942 Nobel Prize-winner Hannes Alfvén, who had postulated the waves long before today’s proof.
And then there is the two-part question of the day: why so few sunspots, and what might an extended sunspot drought mean for us on Earth? Surprisingly, the first part seems to have an explanation. At NASA, read Mystery of the Missing Sunspots, Solved?, a recent article reporting on the discovery that “a jet stream deep inside the sun is migrating slower than usual through the star’s interior, giving rise to the current lack of sunspots. Rachel Howe and Frank Hill of the National Solar Observatory (NSO) in Tucson, Arizona, used a technique called helioseismology to detect and track the jet stream down to depths of 7,000 km below the surface of the sun. The sun generates new jet streams near its poles every 11 years, they explained to a room full of reporters and fellow scientists. The streams migrate slowly from the poles to the equator and when a jet stream reaches the critical latitude of 22 degrees, new-cycle sunspots begin to appear.”
The second part of the question is not as easy. The solar energy falling on the Earth drops by about a tenth of a percent from the solar cycle peaks to the minimums. (Sunspots, although relatively cool, are accompanied by bright patches called faculae; the result is an overall increase in the amount of radiation sent our way. Go to this NASA page to see the Sun during three different levels of activity.) During the last four centuries that sunspots numbers have been accurately recorded, a period of few spots, called the Maunder Minimum, coincided with what is called the “Little Ice Age” in the second half of the seventeenth century. This observation has lead some to suggest a dearth of sunspots might herald a period of global cooling, eliminating the need to worry about the greenhouse gases we are pumping into the atmosphere. But the consensus of climatologists is that variations in the Sun’s output are small compared with human influences. Read this 2007 article in New Scientist Climate Myths: Global Warming Is Down to the Sun, Not Humans.
While scientists agree that the eleven-year sunspot cycle and its accompanying small variation in solar output have a minimal influence on Earth’s climate compared with recent anthropogenic changes, it appears there may be some links to regional weather. Go the National Science Foundation’s 2007 news report, Link Between Sunspots, Rain Helps Predict Disease in East Africa to read how, statistically, the year before the peak of each cycle experiences unusually heavy rainfall.
In the near future, we are likely to learn more about the earthly effects of sunspots. Despite the relatively minor role solar variation may play in our climate, absolute measurements of total solar irradiance (as the Sun’s output is called) taken from satellites have only be available for three decades. Next April, NASA plans to launch its Glory mission, which will further refine measurements of the Sun’s radiation and how it affects Earth’s energy balance. And, on February 6, 2011—for the first time in human history—we will be able to image the entire Sun and all of its spots. For a press release on the two STEREO spacecraft that will make this possible, go to NASA Sees the ‘Dark Side’ of the Sun.