Time-dependent behaviour of a stable auroral red arc excited by an electric field

Abstract

We examine the electric field hypothesis as a possible explanation of a stable auroral red arc. An electric field perpendicular to the geomagnetic field in the ionosphere heats the ambient F-region electrons and ions. Given large enough electric fields, the electrons can be heated sufficiently to excite the OI ( 1 D) term of atomic oxygen by electron impact, giving rise to the λ6300 emission characteristic of the red arc. The electron and ion heating rates are determined by the relative drift between the plasma and neutral gas. The electric field, acting through ion drag, accelerates the neutral gas in a time scale small compared to the observed lifetime of the arc. If the electron density within the red arc is high, the relative drift between the plasma and neutral gas is significantly reduced and the red arc excitation is effectively shut off because of the consequent lower electron and ion heating rates. For low electron densities, the electric field strength necessary to excite the red arc is far greater than any fields which have been observed. For all electron densities between these two extremes, the calculated features of a red arc excited by a perpendicular electric field cannot be made consistent with both satellite observations of electron temperature and ground-based observations of λ6300 emission. We conclude that it is not possible for a perpendicular electric field to excite a red arc such as observed, and thus support the contention that conduction of heat in the electron gas from the magnetosphere alone is sufficient to excite and maintain the stable auroral red arc.