ABSTRACT

The most prominent features on the solar surface are the sunspots. The amount of solar energy received at a given location on Earth obviously depends on the Earth’s distance and orientation with respect to the Sun. The variability of the solar radiative output strongly depends on the wavelength. Solar ionizing radiation can damage electronic systems, such as satellites and spacecraft, navigation and communication systems, electric power transmission. The model is remarkably successful in reproducing the observed irradiance variability, assuring us that irradiance changes on the relevant timescales are indeed dominated by the solar surface magnetism. The longest direct solar observation is the counting of sunspots and their groups. A reconstruction of the solar irradiance from the sunspot number is less straightforward than from direct observations of the solar surface magnetic field. Reconstructions of past solar activity from the isotope data suggest that solar activity varies on centennial and millennial timescales, too.