Solar 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 Earth. Galileo Galilei (1564-1642) made some of the first telescopic observations of sunspots in 1610, but it was not until 1843 that the amateur astronomer Heinrich Schwabe 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 the British solar astronomer E. Walter Maunder. (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 almost always occur near sunspots. Matter ejected from the sun by flares sometimes streams into the earth's atmosphere, where it can interfere with radio communications and cause aurorae (the so-called "northern lights" or" 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