[[spi:math]] (A2) marks the vicinity where energetic particle interactions with the atmosphere (such as airglow emissions) occur. Electric fields [[spi:math]] in the magnetosphere (with exception in the auroral region) are primarily perpendicular to the magnetic field [[spi:math]]. Under the influence of these two fields, a charged particle drifts with velocity [[spi:math]] perpendicular to both [[spi:math]] and[[spi:math]] [[spi:math]] (A3) In addition, the charged particle executes its usual gyration motion in the magnetic field with a gyration frequency [[spi:math]] = eB/mc and a gyroradius (Larmor radius) [[spi:math]]. Since the geomagnetic field is dipole-like and particle velocity generally has parallel ([[spi:math]]) and perpendicular components ([[spi:math]]), charged particles in the magnetosphere execute a gyrating-bouncing motion in much the same way as ions in a magnetic bottle. The location on a magnetic field line (customarily labelled by its equatorial geocentric distance R in units of RE, L [[spi:math]] R/RE) where the particle bounces back is known as the mirrorpoint. If the mirror-point of a charged particle happens to be below the altitude zce, the particle charge-exchanges and never returns to its former trapped motion in the magnetic field. The mirror-point of a particle depends on its initial velocity [[spi:math]] with respect to the magnetic field; in particular, the mirror-point is determined by the initial pitch-angle [[spi:math]] where [[spi:math]] is the component of [[spi:math]] parallel to the magnetic field. In other words, if an ion is injected at a geocentric distance r0 on a given dipole A-4
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