Another variation in solar output is generated by the 27-day solar rotation cycle. When the Sun is active, certain regions of the Sun emit more radiation. However, Earth only encounters these changes when these regions are facing Earth. At its equator, the Sun rotates with a period of about 27 days. As active areas of the Sun rotate from the front to the back side of the solar disk, the intensity of the radiation reaching Earth varies accordingly. The 27-day cycle in the solar irradiance is clearly visible in Figure 10. The effect of the solar rotation cycle is largest during periods of high solar activity, when the solar cycle is at a maximum. During periods of solar minimum, the number of disturbances on the Sun is low. If we look at the

Figure 10. ????

Lyman-Alpha band, we can note a variation in solar output of about 20% during 27-day rotation cycle in early 1992, decreasing to under 5% by 1995. The cycle produces changes from about 4% to 1% over the time period shown at the 0.2 mm wavelength. It is interesting to note that in this wavelength range, the effect of the 27-day cycle is roughly the same size as the 11-year cycle during the period of solar maximum (early in the period). As we see with the 11-year solar cycle, the amplitude of the variation continues to decrease with increasing wavelength. Modeling and observational studies of the ozone response to the 27-day solar cycle indicate variations of about 2% during periods of solar maximum (27-day solar variations of about 5%).