An auroral theory is developed from the point of view of particle orbits in an inhomogeneous plasma confined by a magnetic field. Specifically, a mechanism is proposed for ejection into the atmosphere of geomagnetically trapped protons and electrons. It is assumed that the energetic particles are distributed in longitude irregularly. The tendency for positive and negative particles to drift in opposite directions will then lead to momentary electrostatic fields, arising from excess charges of one sign aligned along a magnetic line of force. As particles drift into this potential, they lose transverse kinetic energy, and a portion of the particles immediately spiral out the ends of the flux tube into the atmosphere As the potential grows, the drift of particles into this "discharge tube" is inhibited, but more of those entering the potential with high velocity are ejected, regaining their lost transverse kinetic energy in accelerated motion along the magnetic field. The potential may rise sufficiently to discharge particles with energies of several kev within an interval of less than a second, which is rapid enough to render neutralization by ionospheric ions and electrons unimportant. If the density fluctuations of auroral plasma exceed a certain critical value, the electrostatic field will cause them to grow rapidly. This instability is identified with auroral-ray structure. A density fluctuation may maintain its identity, even though individual particles are constantly moving thorugh it. This characteristic may be associated with the fading and reappearance of rayed structures. The basic mechanism of electrostatic fields arising from the particle drifts will also produce local accelerations of particles, by tending to establish an equipartition of energy between protons and electrons. This is presumably the mechanism for the local acceleration of auroral electrons, although it will also modify, but less severely, the energy spectrum of trapped protons. Various other consequences of these macroscopic, but short4ived, electric fields are examined, with a view toward understanding auroral morphology. It is proposed that an B X B drift accounts for the statistical preference for auroral patterns to move toward the sunlit hemisphere and for the departures of auroral forms from alignment along circles of geomagnetic latitude, even in the polar cap. The B field, when transferred to the atmosphere by bombardment and by ordinary conduction, will produce a Hall current parallel to auroral motions; this current is identified with the auroral electrojet and associated magnetic disturbances. It is suggested that these considerations also have a bearing on the north-south thickness of auroral forms, on auroral breakup, and on daily variations.