Observations of structured optical emissions and particle precipitation equatorward of the traditional auroral oval

Abstract

High‐sensitivity all‐sky images from south‐central Alaska show the common occurrence of slow‐moving, faint optical emissions at 557.7 and 427.8 nm well equatorward of the discrete aurora. These emissions appear over a continuum of forms ranging from homogenous longitudinal bands to bands with irregular structure on their poleward edge to widely distributed arrays of vortex‐like curls and widely spaced spots on the order of 10 km in diameter. These forms appear to correspond to various stages in the temporal evolution of nearly corotating precipitation regions populated by particles injected from more distant areas of the magnetosphere and may exhibit morphological control by an instability operating on cold plasma near the plasmapause. Although these phenomena are most common in the evening hours and typically persist for hours at a time, one case demonstrates that the features can remain throughout the night on occasion. These faint optical features appear to be colocated with regions of enhanced background counts in DMSP particle measurements and to be associated with the ring current/outer radiation belt region equatorward of subauroral polarization streams (SAPS). The features can occur in conjunction with HF and MF radio frequency absorption, at levels which are often too low to show up on routine absorption instruments such as the riometer. This has practical implications on the operation and interpretation of ionospheric interaction experiments carried out in the subauroral region.