Observations of particles precipitating into detached arcs and patches equatorward of the auroral oval

Abstract

Particle observations made by the energetic particle detector on board Isis 2 are examined at times when the spacecraft intersects field lines mapping into regions of optical emission equatorward of the auroral oval. These emissions appear as arcs and patches and have been called detached arcs [Anger et al., 1978] and patches [Moshupi et al., 1977]. The forms are found at the feet of field lines having a trapped population of electrons 1 ≤Ee ≤10 keV as well as the usual higher‐energy radiation belt particles. Precipitation of these trapped particles at all energies up to 210 keV is observed above the optical forms. In two cases, detailed comparison confirmed that the precipitating electrons carries sufficient energy flux to account for the observed emission intensities. It is found that each of the detached arcs and all but one of the patches occur on field lines which have one end ‘sunlit’ and presumably carry photoelectrons. The geographic and local time distribution of field lines which are dark at one end and sunlit at the other is compatible with the observed distribution of detached arcs, although the role of photoelectrons in causing precipitation is not clear. The particle observations require that precipitation be caused by pitch angle scattering with scattering angle slightly larger than the loss cone. An empirical model is suggested in which a trapped, residual plasma sheet population drifting about the earth is precipitated when it enters scattering regions fixed in L‐MLT outside the plasmapause. The spatial confinement of the precipitation and its similarity to observed detached plasma regions suggest that the scattering regions have enhanced cold plasma densities. Electron cyclotron resonance is examined as the cause of the pitch angle scattering and found to be consistent with the observations.