The Sun And Earth
The radiations from the Sun influence the Earth and planets in many ways apart from the obvious effects of heating and illuminating their surfaces. The study of the interactions between the Sun and Earth is a branch of astronomy termed solar-terrestrial relations. The short-wavelength photons from the Sun have sufficient energy to ionize any constituents in the outer atmospheres of planets. The resulting IONOSPHERES, as the ionized outer layers are termed, affect the propagation of radio waves; in particular they make long-distance radio communication possible on Earth be¬cause the ionosphere forms a reflecting screen. Since the extreme ultraviolet and X-ray emissions from the Sun are highly variable, the height of the ionosphere and concentration of the ions are also variable.

Charged particles may be trapped in planetary magnetic fields, and so give rise to radiation belts encircling the planet. The total extent of the planetary magnetic field is restricted to a region termed the magnetosphere by the pressure of the solar wind stream¬ing past the planet. When a solar storm occurs, shock waves and clouds of plasma propagate out and can interact with a magneto-sphere to give rise to magnetic storms and aurorae.

Many of these phenomena were unknown, or their existence only indirectly inferred, before the era of space vehicles. The direct probing of the interplanetary space in the vicinity of Earth has now allowed detailed pictures to be constructed of conditions and processes, largely unattainable on Earth itself, which take place as the emissions from the Sun interact with the planets.

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