Sunspots :


Sunspots appear as darker features on the photosphere, and are regions of slightly lower temperature (typically 4000 K/6680° F) that result where the emergence of strong magnetic fields from the solar interior disrupts the normal pattern of convection. A typical sunspot has a magnetic-field strength of 0.25 tesla, compared with the Earth’s magnetic-field strength of less than 0.0001 tesla. Sunspots range in size from pores 1,000 km (625 mi) in diameter, to extensive, complex groups that may cover up to 0.5 per cent of the visible solar hemisphere. Sunspot numbers vary over long time-scales, reaching a maximum roughly every 11 years. The underlying magnetic cycle which is believed to cause sunspot activity takes 22 years to return to its starting configuration. Sunspots appear to be a consequence of the interaction between deep-seated magnetic activity in the Sun, and the differential rotation of the outer, convective layers: at its equator, the Sun rotates on its axis once every 25.6 days, but at the poles, the rotation period is in excess of 30 days. As a result of the differential rotation, the solar magnetic field becomes wrapped around itself, so that loops are forced up and out through the photosphere: sunspots form at the sites of emergence.