Sunspots and climate Sunspots and climate

Sunspots have a diameter of about 37,000 km and appear as dark spots within the photosphere, the outermost layer of the Sun. The photosphere is about 400 km deep, and provides most of our solar radiation. The layer is about 6,000 degrees Kelvin at the inner boundary and 4,200 K on the outside. The temperature within sunspots is about 4,600 K. The number of sunspots peaks every 11.1 years. There is a strong radial magnetic field within a sunspot, as implied in the picture, and the direction of the field reverses in alternate years within the leading sunspots of a group. So the true sunspot cycle is 22.2 years. There is also a superimposed fluctuation with a period of 25 months, i.e. a quasi-biennial oscillation. Sunspots were observed in the Far East for over 2000 years, but examined more intensely in Europe after the invention of telescopes in the 17th century. In 1647 Johannes Hevelius (1611-87) in Danzig made drawings of the movements of sunspots eastwards and gradually towards the solar equator. In 1801 William Herschel (1738-1822) attempted to correlate the annual number of sunspots to the price of grain in London. The 11-year cycle of the number of sunspots was first demonstrated by Heinrich Schwabe (1789-1875) in 1843. There have been several periods during which sunspots were rare or absent, most notably the Maunder minimum (1645-1715), and less markedly the Dalton minimum (1795-1820) (Fig 2.8 in the book). During the Maunder minimum the proportional concentration of radio-carbon (14C) in the Earth?s atmosphere was slightly highe