While observing the recent total solar eclipse, many observers may have noticed a line of small, dark spots on the surface of the sun.
These dark sunspots are areas that usually appear fairly regularly in the sun’s atmosphere, although they have recently been suspiciously absent. Astronomers in ancient China recorded sunspots around the 12th century BCE, and referred to them as ‘stars within the solar disk.’ The Aztecs considered them blemishes on the face of Huitzilopochtli their sun god. In Europe, people had a more difficult time accepting their existence because they adhered to the Greek philosopher Aristotle’s idea that all the heavens were perfect and unchanging. In fact, when a large sunspot appeared for eight days in the year 807 CE, they dismissed the phenomenon as the passage of the planet Mercury across the sun. Sunspots have also been dismissed as birds in flight passing in front of the sun, while one Renaissance astronomer postulated they could be undiscovered planets.
Sunspots are large, highly magnetic storms on the surface of the sun, which usually appear in pairs or groups on either side of the sun’s equator. To an observer on Earth, sunspots appear relatively small. However, because the sun is 109 times larger in diameter than Earth, they are actually quite large. In fact, the average sunspot is bigger than the Earth, while others are quite huge. They can vary in size ranging from a few hundred miles to many times the Earth’s diameter. They can exist for less than an hour, while larger ones can last up to over a half year. Some sunspots are large enough to be viewed without optical means, such as the large sunspot group in 2002, which measured about 20 times the size of Earth, which was seen by Arizona Hot Shot David Malpas as he observed the solar disk through the smoke of the Rodeo-Chediski fire that acted like a solar filter.
Sunspots only appear dark to us because they are cooler than the surrounding areas on the sun’s visible surface, known as the photosphere, which has an absolute temperature of about 5,800 degrees (10,000 degrees Fahrenheit) while sunspot temperatures are about 3,800 degrees. The dark interior of a sunspot is called the umbra, which is surrounded by a larger, lighter area called the penumbra. Sunspots are cooler because they are intense magnetic storms, which inhibit the flow of hot gases from the sun’s interior to its surface. The interior and the exterior of the sun rotate at different rates, the outside rotating more quickly at the equator than at the solar north and south poles. (A point on the equator takes about 24.5 days to make one complete rotation, while a point near one of the poles takes 36 days.) Over time, all that uneven motion distorts the sun’s magnetic field. The spots, which are actually twists in the magnetic field lines, have so much magnetic power they push back the hot gases beneath them and prevent the heat from rising directly to the surface.
In a cycle of about every 11 years, the annual count of sunspots increases from only a few to more than 100, and then decreases to nearly zero again as a new cycle begins. The point at which sunspots appear and reach their peak of intensity is called the solar maximum, and when there is very little activity, the solar minimum. At the beginning of a cycle, sunspot activity rises quickly then declines gradually. Solar cycles are usually relatively regular. However, there was a period between 1650 and 1710, called the Maunder minimum (named after the astronomer who discovered it) when there was little or no sunspot activity. This event also coincided with a recorded period of cold temperatures and severe winters in both Europe and North America.
Sunspots are connected with other solar events like flares and coronal mass ejections. A solar flare is a sudden release of energy from the sun, while the latter are ejections of hot plasma from the sun’s surface into space. Though the precise mechanisms are still not yet fully understood, the larger the group of sunspots, the more intense such solar events tend to be. Although responsible for the Aurora Borealis, these flares and ejections can send great amounts of energy and charged particles into space which can then collide with the Earth’s atmosphere, resulting in magnetic storms that can sometimes disrupt radio and cell phone communication, as well as wreak havoc with our electrical grids. For example, during the 1989 solar maximum, a power surge triggered by solar energy damaged transformers that were part of the Hydro-Quebec power system leaving 6 million people in Canada and northeastern U.S. without electricity. As we develop, and become more and more reliant on, electrical modes of communication, transportation and similar life applications, it is inevitable that we will be more prone to sunspot events and other solar activity.