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UPPER-ATMOSPHERE ELECTRIC-FIELD SOURCES 196 poses, representing the level where the density of electron-ion pairs falls to roughly 1010 mâ3 and below which electric currents become relatively small. The ionization is formed largely by the effect on the upper atmosphere of solar extreme-ultraviolet and x-ray radiation at wavelengths shorter than 102.6 nm, but energetic particles impacting the upper atmosphere from the magnetosphere also create important enhancements. The ionospheric plasma has a temperature on the order of 1000 K, which is much cooler than the energetic plasma farther out in the magnetosphere. Collisions between charged particles and neutral atmospheric molecules become important below 200 km altitude and strongly affect the electrodynamic characteristics of the ionosphere. Figure 14.1 Configuration of the magnetosphere. The magnetic field is shown by continuous lines, and locations of important magnetospheric features are pointed out. At high latitudes, where magnetic-field lines connect the ionosphere with the outer magnetosphere, the ionospheric features are quite complex. Ionospheric phenomena become better organized in a coordinate system based on the geomagnetic field than in geographic coordinates, with the difference arising mainly from the 11Â° tilt of the dipolar field from the Earth's axis. Different magnetic coordinate systems exist, but for descriptive purposes the differences are not crucial, and the simple terms "magnetic latitude" and "magnetic local time" will suffice here. At high magnetic latitudes, aurora are produced as energetic charged particles, mainly electrons, precipitate into the upper atmosphere from the outer magnetosphere, creating both visible emissions and ionization enhancements. The aurora form a belt around the magnetic pole, called the auroral oval. The oval is in fact roughly circular, of variable size, and has a wider latitudinal extent on the nightside than on the dayside of the Earth. The entire oval is shifted toward the nightside, so that aurora appear at lower magnetic latitudes at night (roughly 67Â°) than during the day (roughly 78Â°). The nighttime particle precipitation also tends to be more intense and widespread than on the dayside. Contained within the auroral oval is the polar cap, where auroras are less frequent but where on occasion very energetic protons from solar flares enter and penetrate relatively deep into the upper atmosphere. A more detailed description of the Earth's space environment can be found in several of the references listed at the end of this paper, especially in the book of Akasofu and Chapman (1972). ELECTRODYNAMIC PROCESSES IN SPACE The charged particles of a plasma react strongly to electric and magnetic fields. There is a strong tendency for the particles to short out any electric fields, so that it is often a good approximation to treat the electric field in the frame of reference of the plasma as vanishing: This is often called the magnetohydrodynamic (or MHD) approximation (e.g., Roederer, 1979). The frame-of- reference choice is important if the plasma is moving and if a magnetic field is present, because the electric field observed in a different reference frame is not the same. If we let E be the electric field in an Earthfixed reference frame, V be the velocity of the plasma with respect to the Earth, and B be the magnetic-field vector, then a (nonrelativistic) Lorentz transformation yields where the vector product V Î§ B results in a vector directed perpendicular to both V and B. The electric field is then simply related to the plasma velocity and magnetic field by the approximate relation Alternatively, the velocity component perpendicular to B can be related to E and B as Equations (14.3) and (14.4) express the same fact from two different points of view: the electric field and plasma velocity are closely interrelated and help to determine each other. In some cases, as in the solar wind where plasma momentum is high, the electric field quickly adjusts toward the value given by Eq. (14.3). In other cases, as in the upper ionosphere where electric