The relation of quiescent auroral structure to ionospheric electric and magnetic fields

Observational and experimental results concerning auroral structure, auroral zone magnetic bays, and auroral electrons are briefly reviewed and a brief outline of some of the results of the theory of adiabatia charged particle motion are given. These are used to show that the auroral zones are apparently connected by closed magnetic field lines, and that there exist in the auroral zone ionosphere electric fields which are essentially transverse to the magnetic field and which typically have magnitudes on the order of 50 millivolts/meter. These electric fields provide the driving force for localized ionospheric currents, the auroral electrojets, which flow within the regions of enhanced ionization provided by the visible auroral forms. It is shown that auroral arcs and bands are apparently ordinarily so oriented as to have their horizontal axes perpendicular to these electric fields. It is further shown that if the auroral zone magnetic field lines are equipotentials, adiabatic acceleration and precipitation of electrons will occur. In the event that the motion of the electrons is not entirely adiabatic, but involves slow diffusion in pitch angle, precipitation will still occur. Either of these processes will lead to the precipitation of electrons in extended regions of glow, and to the development of a pitch angle distribution which is highly peaked outside of the loss cone. With such a pitch angle distribution, the distortion of ionospheric electric and magnetic fields associated with localized auroral electrojets leads to a modulation of auroral precipitation, giving parallel bright and dark bands of auroral emission, which are aligned nearly perpendicular to the ionospheric electric field