Our Sun is always too bright to view with the naked eye, but it is far from unchanging. It experiences cycles of magnetic activity. Areas of strong activity manifest as visible spots—sunspots—on the Sun’s surface. The year 2008, however, earned the designation as the Sun’s “blankest year” of the space age. Our Sun experienced fewer spots in 2008 than it had since the 1957 launch of Sputnik. As of March 2009, the Sun was continuing its quiet pattern.
These images from the Solar and Heliospheric Observatory (SOHO) spacecraft compare sunspots on the Sun’s surface (top row) and ultraviolet light radiating from the solar atmosphere (bottom row) at the last solar maximum (2000, left column) and at the current solar minimum (2009, right column.) The sunspot images were captured by the Michelson Doppler Imager (MDI) using filtered visible light. On March 18, 2009, the face of the Sun was spotless.
The other set of images, acquired by the Extreme Ultraviolet Imaging Telescope (EIT), shows ultraviolet light radiating from the layer of the atmosphere just above the Sun’s surface. This part of the solar atmosphere is about 60,000 Kelvin—a thousand times hotter than the surface of the Sun itself. On July 19, 2000, the solar atmosphere was pulsating with activity: in addition to several extremely bright (hot) spots around the mid-latitudes, there were also numerous prominences around the edge of the disk. On March 18, 2009, however, our star was relatively subdued.
The long stretch of minimal solar activity in 2008 and early 2009 prompted some questions about whether the Sun’s quiescence was beginning to rival that of the Maunder Minimum in the late seventeenth and early eighteenth centuries. Of the 2008 minimum, solar physicist David Hathaway of the NASA Marshall Space Flight Center says, “It’s definitely been an exceptional minimum, but only compared to the past 50 years.” Citing human observations of the Sun extending back four centuries, he continues, “If we go back 100 years, we see that the 1913 minimum was at least as long and as deep as this one.” So although the minimal activity of the Sun in 2008-2009 is exceptional for the “modern” era, it does not yet rival the lowest levels of solar activity that have ever been observed.
Centuries of observations have shown that the number of sunspots waxes and wanes over a roughly 11-year period. Sunspots exhibit other predictable behavior. If you map the location of the spots on the Sun’s surface over the course of a solar cycle, the pattern they make is shaped like a butterfly. The reason for the butterfly pattern is that the first sunspots of each new solar cycle occur mostly at the Sun’s mid-latitudes, but as the solar cycle progresses, the area of maximum sunspot production shifts toward the (solar) equator. Since regular sunspot observations began, astronomers have documented 24 cycles of sunspot activity. The images acquired in July 2000 showed the Sun near the peak of Solar Cycle 23. That cycle waned in late 2007, and Solar Cycle 24 began in early 2008, but showed minimal activity through early 2009.
The small changes in solar irradiance that occur during the solar cycle exert a small influence on Earth’s climate, with periods of intense magnetic activity (the solar maximum) producing slightly higher temperatures, and solar minimum periods such as that seen in 2008 and early 2009 likely to have the opposite effect. Periods of intense magnetic activity on the Sun can spawn severe space weather that damages infrastructure in our high-tech society.
Roughly a million miles away from our planet, the SOHO spacecraft sits between Earth and the Sun, giving us an unobstructed view of the nearest star. Besides the vernal equinox, March 20 marks annual Sun-Earth day, on which NASA celebrates daytime astronomy.