A regular variation in the number of sunspots visible on the solar disk is fairly easy to recognize. In fact, this is one of the few instances where we have a set of reliable data that date back over several centuries. As you can see in Figure 4.08, there is a regular cycle, whose period is around 11 years, in the number of observed sunspots. This regular cycle is called, simply, the solar cycle.
Sunspots are disturbances on the surface of the Sun where less radiation is being emitted at visible wavelengths; thus the spots appear dark compared to the solar disk. However, when sunspots are present, there is an increase in radiation emitted at other wavelengths, particularly in the UV spectral region. Solar flares, giant explosions from the solar surface, also increase the high energy emissions from the sun. They are more prevalent during the active portion of the 11 year solar cycle.
Because the electromagnetic radiation and particle fluxes associated with solar storm activity affect the upper part of Earth's atmosphere, including the ozone layer, we find it useful to have a quantitative measurement of solar activity that is somewhat more quantitative than a simple count of visible sunspots. Since 1947, such a measurement has been made by the National Research Council of Canada, which is a measure of the radio flux coming from the solar disk at 2800 MHz. This frequency corresponds to a wavelength of 10.7 cm.
Long-term measurements of 10.7 cm wavelength (radio wave region) solar radiation also show the cycle in solar output. Radiation at this particular wavelength is highly correlated with the sunspot cycle, which may be seen in Figure 4.09 that shows the record of intensity of radio emissions from the Sun at a wavelength of 10.7 cm (2800 Mhz) since measurements were initiated in1947. The 10.7 record is often used as a proxy for the solar cycle in long-term ozone time series analysis because of the close relationship between these two solar properties and its comparatively long data record.
Measurements of the variation of the Sun's output at other wavelengths have been made from satellite instruments as well. The solar irradiance from the period of solar maximum in late 1991 through the next solar minimum in 1996 in the Lyman-Alpha emission band and in the 205-210 nm ultraviolet region has been measured by UARS SOLSTICE. At wavelengths longer than about 260 nm, the solar cycle becomes negligible. Stratospheric ozone amounts and temperatures are most sensitive to the solar cycle at wavelengths near 200 nm. As we will see in later sections, the interaction between solar radiation, ozone, and temperature is very complicated. It is therefore difficult to separate out the changes due specifically to the solar cycle, but measurements and models indicate that the solar cycle is responsible for a maximum temperature variation in the stratosphere of about 2-3 K at the stratopause, and an ozone variation on the order of 5% in ozone in the upper stratosphere, at about 43 km (~2 mb) [Brassuer and Solomon, p. 384 Chandra and McPeters].