![]() Quantification of OSF is also a key means of verifying global coronal models. Understanding this open solar flux (OSF) is central to space weather, as the OSF forms the heliosphere, magnetically connects the Sun to the planets, and dominates the motion of energetic particles. Global Horizontal Irradiance (GHI) is the total irradiance from the sun on a horizontal surface on Earth.A fraction of the magnetic flux threading the solar photosphere extends to sufficient heliocentric distances that it is dragged out by the solar wind.There would be almost no DHI in the absence of atmosphere. It is measured on a horizontal surface with radiation coming from all points in the sky excluding circumsolar radiation (radiation coming from the sun disk). Diffuse Horizontal Irradiance (DHI), or Diffuse Sky Radiation is the radiation at the Earth's surface from light scattered by the atmosphere.The irradiance above the atmosphere also varies with time of year (because the distance to the sun varies), although this effect is generally less significant compared to the effect of losses on DNI. ![]() Losses depend on time of day (length of light's path through the atmosphere depending on the solar elevation angle), cloud cover, moisture content and other contents. Direct irradiance is equal to the extraterrestrial irradiance above the atmosphere minus the atmospheric losses due to absorption and scattering. It excludes diffuse solar radiation (radiation that is scattered or reflected by atmospheric components). Direct Normal Irradiance (DNI), or beam radiation, is measured at the surface of the Earth at a given location with a surface element perpendicular to the Sun.The solar constant is a conventional measure of mean TSI at a distance of one astronomical unit (AU). It is measured perpendicular to the incoming sunlight. Total Solar Irradiance (TSI) is a measure of the solar power over all wavelengths per unit area incident on the Earth's upper atmosphere.There are several measured types of solar irradiance. The study and measurement of solar irradiance have several important applications, including the prediction of energy generation from solar power plants, the heating and cooling loads of buildings, and climate modeling and weather forecasting. Solar irradiance affects plant metabolism and animal behavior. Irradiance on the Earth's surface additionally depends on the tilt of the measuring surface, the height of the Sun above the horizon, and atmospheric conditions. Irradiance in space is a function of distance from the Sun, the solar cycle, and cross-cycle changes. Irradiance may be measured in space or at the Earth's surface after atmospheric absorption and scattering. This integrated solar irradiance is called solar irradiation, solar exposure, solar insolation, or insolation. Solar irradiance is often integrated over a given time period in order to report the radiant energy emitted into the surrounding environment ( joule per square metre, J/m 2) during that time period. Solar irradiance is measured in watts per square metre (W/m 2) in SI units. Solar irradiance is the power per unit area ( surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Note that the two images use the same color scale. The top image is the annual mean solar irradiation (or insolation) at the top of Earth's atmosphere (TOA) the bottom image shows the annual insolation reaching the Earth's surface after passing through the atmosphere. The shield effect of Earth's atmosphere on solar irradiation.
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