What are sunspots and why do they appear dark?

Why does the Sun have spots?

The Sun is not only a turbulent mass of hydrogen and helium gas. Enormous electric currents generate vast magnetic fields, which come and go as dark spots on the Sun’s surface. These spots form in groups, grow in intensity and number, reaching a peak every eleven years.

Sunspots were first seen in ancient times. But it was not until the nineteenth century that their cyclical nature was first discovered. A German amateur astronomer, Heinrich Schwabe, hoping to see a new planet pass in front of the Sun’s shining disc, became interested instead in some dark spots on its surface. For about seventeen years, he sketched the position and density of these spots and, by 1843, had established their pattern. He noted that their peak coincided with a brilliant display by the aurora borealis and aurora australis (northern and southern lights).

Early this century, American astronomer George Hale added to Schwabe’s work. He discovered that sunspots produce intense magnetic activity and that those with strongest force are about 8000 times as powerful as the Earth’s magnetic field at its surface.

Subsequent work has shown that sunspots, many of which are several times the size of the Earth, are part of vast solar storms. From time to time, magnetic fields reach the Sun’s surface from far below. They block the flow of heat and light from the core so effectively that, where they appear marked by a dark patch the surface is at least 1000°C (1800°F) cooler than adjacent areas.

If these flare ups occur near the centre of the Sun’s disc and facing the Earth, they can produce a magnetic storm in our polar regions. Radio reception is crackly, compasses go awry, and our weather may suddenly change.

Solar flares may also upset migratory birds, which sometimes navigate by the Earth’s magnetic field. In 1988, a solar storm sent clouds of electrically charged protons and other subatomic particles into space. At the time, some 3000 homing pigeons were racing from France to England. Bad weather obscured the Sun and the stars, by which the birds usually steer. They switched to their in built emergency direction finders only to find them jammed by solar fallout. Few birds reached their home lofts. Race organisers and pigeon owners were puzzled about the disappearance until scientists explained how the pigeons were deceived.

High energy particles from solar flares pose a serious risk also for astronauts, threatening them with radiation sickness and cancer. Manned space missions are usually postponed if intense solar storms are anticipated.