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The Solar Wind and the Earth's Magnetosphere

Flowing from the Sun is the solar wind, whhch continuously carries magnetized plasma and energetic solar particles into the vicinity of the Earth. The Earth and its atmosphere are shielded from the direct impact of these particles and plasmas by the magnetosphere, a relatively self-contained region in space whose global topology is organized by the intrinsic magnetic field of the Earth. This field, which may be represented to a reasonable approximation by a dipole originating in the Earth's molten metal core, extends far into space and serves to deflect the onrushing solar wind. The stand-off distance (the magnetopause), commonly about 10 Earth radii (RE ) at the subsolar point, depends on the solar wind pressure and is highly variable. In the outer reaches of the Earth's near-space environment, tangential stresses applied by the solar wind set up a system of boundary region currents that effectively constrain the outer geomagnetic field to a comet-shaped form with a long tail extending downstream from the Sun (Figure 5.1). Thus, the Earth's magnetosphere extends from the upper atmosphere/ionosphere to altitudes of about 10 RE on the sunlit dayside and to more than 1000 R E on the nightside.

Mass, momentum, and energy are imparted to the magnetosphere with great variability by the continuously flowing solar wind. The primary form of plasma energy available at 1 astronomical unit (AU) is kinetic, as a result of the motion of the solar wind relative to the Earth. Solar wind plasma interacts with the projected cross-section of the entire magnetosphere (a disk of radius about 20 RE ), so that the total power intercepted due to the solar wind kinetic energy is about one thousandth of the radiant energy intercepted by the disk of the Earth. This energy transfer occurs with much greater variability than the radiant heating variations associated with the 0.1 percent solar cycle change in total solar irradiance. However, it is not the solar wind kinetic energy flux per se that seems to control geomagnetic activity, but rather the embedded solar wind magnetic field.

The major processes that extract, store, and dissipate energy from the solar wind flowing past the Earth, subsequently disturbing the geospace environment, involve the generation of plasma and energetic particles from stored magnetic fields. Three primary forms of energy dissipation detectable in the Earth's atmosphere are auroral particle precipitation,



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