The Sun is the primary source of the energy that sustains life on Earth. Solar energy is the principal driver of all Earth's atmospheric phenomena, from weather patterns in the lower layers, through auroras in the upper layers, to the "space weather" environment of energetic particles at the altitudes of orbiting satellites. The energy originates from the nuclear fusion of hydrogen into helium in the Sun's core. Over millions of years, a combination of radiative and convective processes transports the energy thus generated outwards to the visible surface, where it is ultimately radiated into space.

Convective motions and differential rotation beneath the surface generate intense magnetic fields, which erupt through the surface, often to produce regions of strong magnetic field. The field acts as an energy conduit between the turbulent convective motions beneath the Sun's surface and the outer, observable, layers of the Sun's atmosphere. Phenomena associated with magnetically-active regions include the formation of sunspots, enhanced heating of the solar atmosphere, solar flares, and the ejection of billions of tons of material in so-called coronal mass ejections. The solar magnetic field varies cyclically over about a decade and can also change unpredictably within a given cycle. These variations lead to changes of a few tenths of a percent in the total solar radiative output, much more dramatic changes in specific wavelength bands (notably radio, ultraviolet, and X-rays), large variations in the strength and speed of the solar wind, and a host of energetic processes such as the acceleration of electrons and protons to ultrarelativistic energies. Not only are these processes of fundamental scientific interest, but many also have a direct effect on bodies within the solar system&--in particular on the Earth. The nature of the near-Earth environment, and even events on Earth's surface, can be substantially influenced by changes in the interior and atmosphere of the Sun.

While these facts are quite well established empirically, very little is understood about the underlying physical causes. For example, we do not know how the solar corona is heated to a few million degrees (several hundred times hotter than the Sun's surface); the mechanisms of particle acceleration in flares are a mystery; the driving force behind the solar wind has not been determined; and the triggering mechanisms for solar mass ejections remain elusive. Even the fundamental mechanism for generating the all-important magnetic field itself has not been conclusively established.

A bold series of new missions designed to address these questions will yield new insights into the nature of our closest star, major advances in many areas of science and technology, and, most importantly, a deeper understanding of the Sun-Earth connection and its importance for our civilization.