Introduction to several papers on special section of 'The Time varying Sun'

The Sun is at the origin of life on the Earth, and it influences all the Earth's environment, human life, climate, geomagnetism as well as many new technologies systems as for example navigation system. This is why the time varying sun deserves to a trans-disciplinary approach of many topics. In the following 8 papers, we will consider the sun itself its relation with the earth (geomagnetism and climate). (...)