The superposition of rotating and stationary magnetic sources: Implications for the auroral region

Magnetohydrodynamic theory has been widely and successfully applied to explain many aspects of magnetospheric behavior. In particular, it has been used to predict where and when field-aligned auroral currents of electrons and ions will flow between the magnetosphere and ionosphere. On the other hand, in the auroral region, precipitating particles with energies of many keV are observed and are widely presumed to have been accelerated by field-aligned electric fields, which have been observed directly. These observations imply that additional plasma and/or external processes are present. In this report, an expression for the field-aligned electric field due to the superposition of the Earth's rotating magnetic dipole and a stationary magnetic source is derived and evaluated at various locations throughout the magnetosphere. The results show a striking agreement with both the location and the energies of precipitating ions observed over a two-year period by the Defense Meteorological Satellite Program and also agrees with the size of the auroral oval as measured by ultraviolet data from the Viking satellite. The degree and characteristics of this agreement suggest that the parallel electric field associated with the rotating magnetic dipole is indeed present, is not canceled by polarization effects, and is a prime cause of energetic-ion precipitation.