Solar Radiation As the Earth spins on its axis and orbits the Sun, it is bathed in solar radiation. The Sun is a stable star: it is burning steadily, giving out energy (heat and light). Satellite measurements have shown that the short-term variations of the energy output (the "solar constant") are at the most of the order of a percent. Radiation and particles The radiation level, as measured at the Earth's surface, is just at the appropriate level to maintain the Earth's favourable climate. But without protection, the radiation level would be dangerous: direct exposure to the ultra-violet (UV) component of solar radiation is harmful to most species including Homo sapiens, since UV radiation rapidly and efficiently breaks down bonds between carbon atoms, destroying organic compounds. The Sun is also the source of a second potentially destructive phenomenon: the solar wind is a stream of charged particles that flow outwards from the Sun. The particles can interact with species in an atmosphere, stripping it away by a process known as sputtering. Lines of defense The Earth has two lines of defence that protect its surface from the effects of excess UV radiation and the solar wind: its atmosphere and its magnetic field. The atmosphere absorbs and attenuates the UV radiation, decreasing the level of harmful radiation that reaches the planet's surface. The insulating effects of the atmosphere, as well as keeping radiation out, also keeps heat in, maintaining a regular temperature at the surface. Without the atmosphere, surface temperatures would drop to below 0°C. The Earth's magnetic field, generated by convection currents in the Earth's metallic core, deflects the solar wind. Without its atmosphere and its magnetic field, the surface of the Earth would be uninhabitable by carbon-based species like us.