The electromagnetic radiation and particles (electrons, protons, alpha particles, and rarer heavy atomic nuclei) emitted by the Sun. The electromagnetic radiation covers a wavelength range from x-rays to radio waves, that is, from about 0.01 nanometer to 30 km. The annual mean irradiance at Earth, integrated over the whole spectrum, amounts to 1365 W · m⁻², and 99% of its energy is carried by radiation with wavelengths between 278 and 4600 nm, with the maximum at 472 nm. The division into two energetically equal parts is at a wavelength of 731 nm. Although the total solar irradiance is often called the solar constant, it varies with time, for example, with the solar activity cycle with a period of about 11 years. These variations have been monitored since 1978 by radiometers on space platforms and amount to about 0.1%, with the radiation higher during the maximum of solar activity. There are also shorter-term variations such as decreases due to the passage of dark sunspots or increases due to faculae.