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UPPER-ATMOSPHERE ELECTRIC-FIELD SOURCES 195 14 Upper-Atmosphere Electric-Field Sources Arthur D. Richmond National Center for Atmospheric Research The Earth's space environment is filled with electrons and positive ions, comprising a plasma of very low density. These charged particles collide only infrequently and are strongly influenced by magnetic and electric fields. In turn, the charged particles affect the distributions of the magnetic and electric fields in space. The space plasma environment, therefore, is dominated by electrodynamic processes. The regions of space involved in creating upper atmosphere electric fields are illustrated in Figure 14.1 and are described below. The solar wind is a plasma with electron and ion number densities of order 5 Ã 106 mâ3 flowing continually outward from the Sun at a speed of 300-1000 km/sec. Imbedded within it is the interplanetary magnetic field (IMF), which is maintained by electric currents flowing throughout the solar-wind plasma. The IMF strength at the orbit of the Earth is roughly a factor of 10â4 smaller than the strength of the surface geomagnetic field. Most of the time, an interplanetary field line near the Earth can be traced back to the surface of the Sun, where magnetic fields are ubiquitous. As the Sun rotates (once every 27 days) different magnetic regions influence the IMF near the Earth. The combination of solar-rotation and outward solar-wind flow produce a roughly spiral IMF pattern. In addition, the solar- wind velocity can change dramatically and produce both large-scale and small-scale distortions of the IMF so that the field direction and strength vary greatly. These changes have been found to influence the electrical state of the magnetosphere and ionosphere. The magnetosphere is the region of space where the geomagnetic field has a dominant influence on plasma properties. As the charged particles of the solar wind are deflected by the geomagnetic field, an electric current layer is formed at the boundary between the solar wind and the magnetosphere, called the magnetopause. This current layer distorts the geomagnetic field from the dipole-like configuration that it would otherwise have and helps to create a long magnetized tail trailing the Earth. Although the full extent of this tail has not yet been determined, it is known to be more than 500 Earth radii. The magnetosphere contains the radiation belt, composed of energetic charged particles trapped in the magnetic field. The number density of electron-ion pairs in the magnetosphere is highly variable, ranging in order of magnitude from a low of 106 mâ3 in parts of the tail up to 1012 mâ3 in the densest portions of the dayside ionosphere. The ionosphere is the ionized component of the Earth's upper atmosphere. It is not distinct from the magnetosphere, but rather forms the base of the magnetosphere in terms of electrodynamic processes. The lower boundary of the ionosphere is not well defined but can be taken as about 90 km altitude for the present pur