field line L [[spi:math]] R/RE, then we can determine a critical initial pitch-angle [[spi:math]] such that all particles with initial pitch-angles [[spi:math]] will have mirror-points below a given geocentric distance [[spi:math]] on the same field line (A4) If [[spi:math]] is identified with the precipitation altitude zce , then [[spi:math]] is called the loss cone angle because all particles with pitch angle less than [[spi:math]] are "lost". According to (A4), if an ion is injected at LEO ([[spi:math]]) with [[spi:math]], it will almost certainly be lost, because [[spi:math]] 90° at LEO. Conversely, at GEO (r0 » [[spi:math]]) the loss-cone tends towards 0°; hence, only those ions injected with [[spi:math]] almost entirely parallel to the magnetic field are lost. Such considerations are very important in assessing a number of effects such as airglow intensity and plasma instabilities generated by SPS spacecraft exhaust emissions. The lower boundary of the magnetosphere is, strictly speaking, the spherical ionospheric layer known as the E-layer in which particle motion is no longer controlled by the magnetic field. However, for purposes of SPS assessment, ionospheric effects are dealt with elsewhere, therefore, we shall regard LEO as the lower boundary altitude of the magnetospheric effects, although because of magnetospheric-atmospheric interactions such a common lower boundary does not exist for all magnetospheric processes. A-5 [[spi:math]]
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