Schematic illustration showing the variation in sunspot numbers as a function of the solar cycle. The Sun's magnetic activity varies with a period of 11 years. At the beginning of the cycle, sunspots - regions of relatively cool temperature - are found in small number at mid-latitudes. As the cycle progresses, the spots appear at progressively lower latitudes, and in greater numbers, eventually clustering around the equator at the end of the 11-year period. The phenomenon is though to occur because, as the Sun rotates differentially (faster at the equator than at the poles), its magnetic field tends to wind up over the years, enhancing sunspot numbers and dragger them to lower latitudes.