Solar activity: sunspots and F10.7 flux

The solar cycle in two curves

The sunspot number is the oldest continuous measure of solar activity: daily observations go back to 1818 and reconstructions to 1610. Sunspots are cooler regions where the magnetic field breaks through the surface; their number rises and falls on a cycle of about eleven years. SILSO, at the Royal Observatory of Belgium, keeps the world reference.

The F10.7 flux is the intensity of the radio waves the Sun emits at a wavelength of 10.7 cm, measured daily in Canada since 1947. It tracks the sunspot number closely but depends neither on the sky nor on the observer: about 70 units at solar minimum, above 150 at maximum. It is the index atmospheric models use to estimate the heating of the upper atmosphere.

The more active the Sun, the denser the ionosphere, and the more the Schumann resonance, the solar wind and the flares respond: this page gives the slow context behind the site's other curves.