A sunspot is an area on the Sun's surface (photosphere) that is marked by intense magnetic activity, which inhibits convection (heat transfer currents). This results in areas (spots) of reduced surface temperature that appear darker in color than their surroundings.
Sunspots tend to occur in pairs that have magnetic fields pointing in opposite directions. A typical spot consists of a dark region called the umbra, surrounded by a lighter region known as the penumbra. Sunspots appear darker than their surroundings because the surface of the Sun (the photosphere) is about 10,000 degrees F, while the umbra is about 6,300 degrees F. Sunspots are quite large as demonstrated by the fact that an average size sunspot is about the same size as the Earth.
Sunspot Numbers
Galileo and others made the first European observations of sunspots around 1610 and continuous daily observations of sunspots were first started at the Zurich Observatory in 1849. The sunspot number is calculated by first counting the number of sunspot groups and then the number of individual sunspots.
The “sunspot number” is then given by the sum of the number of individual sunspots times the number of groups. Since most sunspot groups have, on average, about ten spots, this formula for counting sunspots gives reliable numbers even when the observing conditions are less than ideal and small spots are hard to see.
The Solar Cycle
Sunspots increase and decrease through an average cycle of 11 years. Dating back to 1749, Earth has experienced 23 full solar cycles where the number of sunspots have gone from a minimum, to a maximum and back to the next minimum, through approximate 11 year cycles. In the 23rd cycle, the number of sunspots reached a peak in May, 2000 when the number of sunspots were measured at around 170.
A secondary sunspot maximum occurred near the beginning of 2002 where the sunspot number was about 150. The sunspot minimum for Cycle 23 occurred through mid 2007.
What has Happened to Sunspot Cycle 24?
It’s normal for the old and new sunspot cycles to overlap for a time before the old one completely fades away, but Solar Cycle 24 has been very slow to ramp up. Solar Cycle 24 began in January 2008 However, the reverse polarity sunspot that signaled the start of cycle 24 on January 4th dissolved within two days after that.
In fact, while 2008 was supposed to signal the beginning of cycle 24 and an increase in sunspot numbers, the lack of sunspots in 2008 made it a century-level year in terms of solar quiet. Remarkably, sunspot counts for 2009 have dropped even lower. As of March 31, 2009, there were no sunspots on 78 of 2009’s 90 days to that date (87%). The 100-year record for a full year is 311 spotless days (85%) in 1913.
Why do Sunspots Matter?
Low sunspot numbers have been historically linked to global cooling several times over the past 400 years or so. The Maunder Minimum is the name given to the period roughly spanning 1645 to 1715 when sunspots became exceedingly rare, as noted by solar observers of the time. The period was named after the solar astronomer Edward W. Maunder (1851–1928).
During one 30-year period within the Maunder Minimum, astronomers observed only about 50 sunspots, as opposed to a more typical 40,000–50,000 spots in modern times. More importantly, the Maunder Minimum coincided with the coldest part of the Little Ice Age, during which Europe and North America, and perhaps much of the rest of the world, were subjected to bitterly cold winters.
The Dalton Minimum was a period of low sunspot activity. Named for the English meteorologist John Dalton, this minimum lasted from about 1790 to 1830. Like the Maunder Minimum, the Dalton Minimum coincided with a period of lower-than-average global temperatures. For example, some weather reporting stations experienced a 2.0 °C decline over 20 years. The Year Without a Summer, in 1816, also occurred during the Dalton Minimum.
Whether there is a definite causal connection between low sunspot activity and colder winters and cooler summers is the subject of ongoing debate. However, even the possibility of such a connection given the overdue beginning of cycle 24 should give one pause. All humans can do is wait and watch.
 |
