Sunspots are regions on the solar surface that appear dark because they are cooler than the surrounding photosphere, typically by about 1500 K (thus, they are still at a temperature of about 4500 K, but this is cool compared to the rest of the photosphere). They are only dark in a relative sense; a sunspot removed from the bright background of the Sun would glow quite brightly.
Basic Features of Sunspots
The largest sunspots observed have had diameters of about 50,000 km, which makes them large enough to be seen with the naked eye. Sunspots often come in groups with as many as 100 in a group, though sunspot groups with more than about 10 are relatively rare. There are well established methods for measuring the number of sunspots. Sunspots develop and persist for periods ranging from hours to months, and are carried around the surface of the Sun by its rotation (a fact known to Galileo). A typical sunspot consists of a dark central region called the umbra and somewhat lighter surrounding region called the penumbra.
Solar Rotational Period
Historically, the first measurements of the period for solar rotation were made by tracking sunspots as they appeared to move around the Sun. Galileo used this method to deduce that the Sun had a rotational period of about a month. Because the Sun is not a solid body, it does not have a well defined rotational period. Modern measurements indicate that the rotational period of the Sun is about 25 days near its equator, 28 days at 40 degrees latitude, and 36 days near the poles. The rotation is direct, that is, in the same sense of the motion of the planets around the Sun. Sunspots have been monitored since the time of Galileo. One striking feature that emerges from the long-term data is that the number of sunspots observed in a given year varies in a dramatic and highly predictable way.
11-Year Sunspot Cycle
If one plots the total number of sunspots observed in a year as a function of the year, there is a striking variation in the number of sunspots that is cyclic, with a period of approximately 11 years. This 11 year periodicity is called the sunspot cycle. The last solar maximums (period of maximum sunspot activity) were in the years 2000 and 2012. The last solar minimum was 2009.
The Active and Quiet Sun
Sunspot maxima correspond generally to periods of high solar activity. This activity includes increased solar wind and phenomena like aurorae and magnetic storms that are correlated with the solar wind, increased flares, and increased non-thermal radio and X-ray emission. Conversely, near sunspot minima the Sun is much quieter with respect to these phenomena. In addition, as we have seen there are significant differences in the nature of the corona during periods of active and quiet Suns.
Hurricanes and Sunspot Activity
A recent study suggests that hurricane intensity may be linked to the number of sunspots on the Sun. A decrease in the number of sunspots may be related to an increase in hurricane intensity. After examining the past 100 year hurricane records of the United States and Caribbean, James Elsner and Thomas Jagger of Florida State University in Tallahassee conclude that their intensity may be linked to 10 to 12 year solar magnetic activity cycles. Data from the National Hurricane Center, Miami, Florida, was used in the study. Sunspots are areas on the Sun with increased magnetic activity. The number of sunspots vary during the solar cycles. Increased solar activity will allow more ultraviolet rays to reach Earth, resulting in warming of the relatively colder upper atmosphere. Decreased solar activity reverses this phenomenon. It is believed that the greater the temperature difference between upper and lower atmospheric regions, the higher the hurricane intensity. Establishing a link between sunspots and hurricane intensity can provide a valuable tool for predicting storms. Other scientists however question the statistical basis of the study and the physical processes attributed to changes in hurricane activity.