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J. L. Lean (Naval Research Laboratory)
Photons with wavelengths spanning the entire electromagnetic spectrum, ionized particles with MeV energies and plasma at speeds near 450 km/s impinge upon the Earth as it orbits the Sun. Driven by the Sun's internal dynamo, which generates magnetic fields that alter the solar atmosphere, the constantly changing fluxes of photons, particles and plasma define a dynamic external environment to which the Earth continually responds and adjusts. That part of the Sun’s energy most crucial for life on Earth -- electromagnetic radiation in the visible and near-IR spectrum -- is the least variable. This energy heats the Earth's surface directly. Atmospheric layers extending many 100 km above the surface determine the deposition of solar photon energy at shorter wavelengths. Radiant UV energy produces the ozone layer in the stratosphere, while EUV energy heats the thermosphere to thousands of K and ionizes gases to produce the ionosphere. As it flows past, the solar wind shapes and distorts the Earth's magnetic field. Solar and galactic particles are constrained to enter the terrestrial system mainly at high latitudes, along field lines that extend into space. The evolution of solar activity during the 11-year cycle and episodic eruptions produce energy output changes that have a wide array of terrestrial impacts on a wide range of time scales. The Earth's response to this solar forcing is muted and slow near the surface, where solar cycle temperature changes may be of order 0.1 C, and are controversial. Terrestrial responses to solar variability grow in amplitude and occur more rapidly at increasing heights above the surface. Solar cycle temperatures changes of order 600 K occur in the upper atmosphere. Today, the impacts on the Earth of the Sun's energy output variations have implications for policy making on global change, the economics of space-based technologies, and national defense.
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Bulletin of the American Astronomical Society, 36 #2
© YEAR. The American Astronomical Soceity.