A measure of the flux of neutrinos from the Sun reaching the Earth. The neutrinos are produced in nuclear reactions at the Sun’s centre. It is estimated that 65 billion (6.5 × 10¹⁰) solar neutrinos pass through each square centimetre of the Earth per second. The first solar neutrino detector, which began operation in 1968, consisted of a tank of fluid containing chlorine, situated deep underground in the Homestake gold mine, South Dakota. A tiny proportion of the chlorine atoms interact with solar neutrinos to produce argon atoms. The number of neutrino interactions is measured by the solar neutrino unit (SNU), 1 SNU equalling 1 interaction per second per 10³⁶ chlorine atoms. The neutrino rate predicted by solar models is about 8 SNU, but measurements from Homestake and from the gallium detectors SAGE (in Russia) and Gallex (near Rome, Italy), indicate a neutrino rate about one-third that expected. However, these instruments detect only so-called electron neutrinos, one of three types of neutrino. More recent results from the Sudbury Neutrino Observatory in Ontario, Canada, which contains heavy water and is sensitive to all three types of neutrino, demonstrate that the right number of neutrinos are produced in the Sun but that many of the electron neutrinos change into muon neutrinos and tau neutrinos on their way to Earth.