Observations and theory of the formation of stable auroral red arcs

A population of protons with energy of some tens of keV, called the ring current, is found near the equatorial region of the magnetosphere at several earth radii. During the main phase of geomagnetic storms the ring current shifts toward lower L values into the region of the plasmapause, which is characterized by steep gradients in the plasma density. This interaction together with an anisotropic pitch angle distribution leads to ring current instability and the growth of ion cyclotron wave turbulence. As wave energy is dissipated in the ambient electron gas by Landau damping, the plasmapause electron temperature is raised to a few electron volts, and a substantial temperature gradient is created with respect to the ionosphere. The energy transferred to the ionosphere by pitch angle scattering in the low collision frequency region and by heat conduction in the collision-dominated regime raises the ionospheric electron temperature to several thousand degrees. Therefore an appreciable number of electrons in the high-energy tail of the Maxwellian distribution, i.e., electrons with energy greater than 2 eV, exist in the F region of the ionosphere at about 400 km, where atomic oxygen is the dominant neutral gas constituent. Two eV is the threshold for excitation of oxygen atoms to the metastable ¹D level, and these O(¹D) atoms emit 6300-Å radiation, the signature of stable auroral red (SAR) arcs. Although the energy input rate required to produce electron temperatures sufficient to cause average SAR arcs is less than 0.1 erg cm⁻² s⁻¹, the energy radiated in the red line is only about 0.003 erg cm⁻² s⁻¹. Thus an SAR arc is an optical manifestation of a slow release of energy from the magnetosphere during a geomagnetic storm. Energetically it is small in comparison with high-latitude auroral processes.