Atmospheric response to heating within a stable auroral red arc

Abstract

We assume the electron conduction hypothesis for the stable auroral red arc. That is, energy conducted in the electron gas from the magnetosphere atong the geomagnetic field lines into the ionosphere heats the ambient F-region electrons. These, in turn, excite the ( 1D) term of atomic oxygen by electron impact giving rise to the λ6300 emission characteristic of the arc. A two-dimensional model of a red arc is constructed using solutions of the one-dimensional electron heat-conduction equation so that the calculated two-dimensional λ6300 volume emission-rate contours model the contours determined from photometric observations. The resulting electron temperature, λ6300 volume emission-rate, and neutral heating-rate contours are obtained for the arc model. More than 99 per cent of the energy conducted from the magnetosphere is transferred to the neutral gas in the ionosphere through elastic and inelastic collisions of the electrons and ions with the neutrals. A two-dimensional steady-state dynamic model of the neutral thermosphere incorporating thermal conduction, viscosity, and ion drag is used to calculate the temperature perturbation and circulation pattern within the arc region. The pressure forces originating from the red-arc heating drive a thermal cell with upward motion in the red-arc region and a stow subsidence over a much greater area outside the arc. The adiabatic warming and cooling by these motions significantly reduce the neutral temperature increase within the red arc compared to the increase obtained in the absence of motions, but the warming and cooling enlarge the horizontal scale of the region of temperature increase.