Solar Constant and Total Solar Irradiance Variations

Abstract

Glossary Sun – A so-called main-sequence star -converting He to H -with an age of 4.6 billion years. It rotates with a period of 27.28 days (seen from Earth) around its axis which is inclined to the ecliptic by 7.09 • with an ascending node around 7–8 July. Photosphere – Visible surface of the sun with T eff ≈ 6050 K. It is defined as τ = 1 at 500 nm which means that the 'surface' at optical depth one is seen at the maximum of sensitivity of the human eye. Solar Cycle – It is the main source of periodic solar variation and is observed by counting the number of sunspots visible on the Sun. It has an average period of about 11 years and the last cycle with the number 23 started in August 1996 and ended November 2008. Sunspot Number – The sunspot number R is defined as R = k(10g + f) with the number of groups g, the number of spots f and a calibration factor k which accounts for instru-mental and seeing condition of the observation. R is also a rough estimate of the total magnetic field penetrating the visible hemisphere. Open magnetic field – This is the radial component of the magnetic field in the heliosphere. It is measured at the orbit of Earth, that is at 1 AU (astronomical unit = 1.49598 × 10 11 m) and varies with the solar cycle and on secular time scales. Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, CH–7260 Davos Dorf, Switzerland 2 Active region – Complex of sunspots and faculae clustered together, starting at the begin-ning of a solar cycle at latitudes of about ±40 • and migrating towards the equator during the cycle. Ephemeral region – Small bipolar region with sizes typically less than 20 000 km along their long axis, lifetimes of a few hours, and no sunspots. Their numbers vary in phase with the solar cycle. Unlike active regions ephemeral regions occur over the entire solar surface. Sunspot – Dark magnetic feature on the solar surface with diameters of 3000–70000 km. Spots larger than about 10000 km have besides the dark umbra (T eff ≈ 4240 K) a less dark penumbra (T eff ≈ 5680 K). Normally they exist in pairs with the leading (in the sense of rotation) spot having the opposite magnetic polarity of the cycle (after reversal of the polarity at solar maximum). Facula – Bright magnetic feature on the solar surface, mainly in active regions with similar sizes as sunspots. In contrast to sunspots with lifetimes of the order of a month, faculae can live for several rotations, even after the accompanying spot of the active complex has disappeared. Plage – The same as faculae but observed in the chromosphere in e.g. CaK-II at 393.36 nm. They are larger in size because of the fan out of the magnetic field with height in the solar atmosphere, and can be used as a proxy for faculae with appropriate scaling. Network – Besides faculae and plages there are small magnetic features summarized by the term network. They are mainly from decaying active region magnetic fields and are migrating poleward during a solar cycle. Solar constant – The amount of total solar energy of all wavelengths received by unit time and unit area at the mean sun-earth distance in the absence of the earth's atmosphere. The term 'Solar Constant' is somewhat misleading, because the irradiance is varying with the 11-year solar activity cycle and possibly over centennial time scales and hence 'Total Solar Irradiance' or TSI is used. An irradiance of 1365 Wm −2 correspond to an effective temperature of the sun of 5777 K.