WHAT ARE SUNSPOTS?

In 1610, shortly after viewing the sun with his new telescope, Galileo Galilei made the first European observations of Sunspots. Daily observations were started at the Zurich Observatory in 1749 and with the addition of other observatories continuous observations were obtained starting in 1849. As a climatologist, I always found it amazing that we have had regular sunspot data far longer than we have had reliable coverage of temperature or precipitation.

Sunspots appear as dark spots on the surface of the Sun. Temperatures in the dark centers of sunspots drop to about 3700 K (compared to 5700 K for the surrounding photosphere). They typically last for several days, although very large ones may live for several weeks. Sunspots are magnetic regions on the Sun with magnetic field strengths thousands of times stronger than the Earth's magnetic field.

Sunspots usually come in groups with two sets of spots. One set will have positive or north magnetic field while the other set will have negative or south magnetic field. The field is strongest in the darker parts of the sunspots - the umbra. The field is weaker and more horizontal in the lighter part - the penumbra. Faculae are bright areas that are usually most easily seen near the limb, or edge, of the solar disk. These are also magnetic areas but the magnetic field is concentrated in much smaller bundles than in sunspots. While the sunspots tend to make the Sun look darker, the faculae make it look brighter. During a sunspot cycle, the faculae actually win out over the sunspots and make the Sun appear slightly (about 0.1%) brighter at sunspot maximum that at sunspot minimum.

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 and ten times the number of groups. Monthly averages (updated monthly) of the sunspot numbers show that the number of sunspots visible on the sun waxes and wanes with an approximate 11-year cycle, The last five cycles are shown in the diagram below.

You can see from this diagram that the cycles are not equal in magnitude (110 to 200) or period (9.8 to 12+ years). If you superimpose the five cycles you can see this even more dramatically. The chart has the annual average sunspot number and starts with the year of the solar minimum (lowest sunspot average). You can see that cycles 19, 21, and 22 were higher amplitude and shorter periods (bottoming out in years 9-10 and then rebounding rapidly). Cycles 20 and 23 were less amplified and longer. Cycle 20 lasted about 11.6 years. It appears from the evidence we will present that cycle 23 has not yet bottomed out and thus is at least 12 years long.

Looking back at the full record of sunspot cycles, we can see this general behavior of short active cycles and longer, quiet ones. Successive 11 year cycles are different in their magnetic fields and the 22 year Hale cycle has in the past been related to some phenomena such as drought. Longer term cycles are apparent when you carefully examine the data. Very obvious from the long term plot of the 11 year cycles is the approximate 100 (106) year cycle. There is also a 213 year cycle. The last 213 minimum was in the early 1800s. The turn of each of the last 3 centuries has started with quiet long cycles with mid-century shorter, higher amplitude cycles. The quietest period was in the early 1800s (the Dalton Minimum). The 100 and 200 year minima are due the next decade suggesting a quieter sun ahead.

SOLAR CYCLES AND TEMPERATURES

When the sun is more active, it is brighter than when it is quiet. This difference in brightness or irradiance over the 11 year cycle is about 0.1%. The irradiance change though since the little ice age may be as high as 0.4 or 0.5% and has been used to explain the ice age and recovery since. Even on the shorter term, there are other solar factors at play besides simple brightness or irradiance changes.

While irradiance changes just 0.1% over the 11 year cycle, ultraviolet radiation changes 6 to 8% and shorter wavelengths even more. Ultraviolet radiation destroys ozone, an exothermic process which produces warming in the higher atmosphere in low to mid latitudes that works its way down to the middle troposphere in time. Labitzke (2001) has shown statistically significant differences of temperatures in the lower stratosphere into the middle troposphere with the 11 year solar cycle (warmest at max) which she attributes to solar flux induced ultraviolet variance. Shindell et al NASS GISS (1999) showed results from a global climate model including ozone and UV found UV induced stratospheric ozone changes and generated heat that penetrates into the troposphere, in effect confirming Labitzke’s findings.

Also an active sun causes a diffusion of cosmic rays, which have a low cloud enhancement effect through ion mediated nucleation. Low clouds reflect radiation and have a net cooling effect. The absence of them during active solar periods means more radiation reaching the surface. Enhanced low cloudiness during solar minima would augment cooling.

These factors magnify the solar irradiance factors acting to enhance warming during active solar periods and diminish it during quiet periods. Scafetta and West (2007) have indicated that Total Solar irradiance as a proxy for the total solar effect could be responsible for over 50% of the changes since 1900.

MAUNDER MINIMUM

Early records of sunspots indicate that the Sun went through a period of inactivity in the late 17th century. Very few sunspots were seen on the Sun from about 1645 to 1715, a period known as the Maunder Minimum.