Asymmetric sunlight effect on dayside/nightside auroral precipitation

Abstract

There are considerable evidence supporting the novel finding from intensive particle observations that electron acceleration events responsible for producing discrete arcs are more common in darkness than in sunlight [Newell et al., 1996a]. This sunlight effect on the production of auroral arcs is most dramatic in the pre-midnight sector, and it is still unclear whether the dayside auroras have the same response to the sunlight as the nightside auroras. In this paper we investigate this issue by using auroral images acquired from the ultraviolet imager (UVI) on board the Polar satellite. By analyzing auroral emission in the long Lyman-Birge-Hopfield band (160 – 180 nm), which is approximately proportional to the total energy flux of precipitating electrons over a wide range of auroral electron energies, one can estimate the total energy deposition rate from auroral precipitation. To emphasize the sunlight effect on auroras seasonal averages of auroral luminosities are derived for the winter of 1996 and the summer of 1997 (4 weeks before and after the solstices). It is shown again that auroral intensity in the premidnight sector is suppressed in sunlight. On the contrary, dayside auroral intensity is generally enhanced in sunlight, indicating a different source mechanism for the dayside aurora from the nightside auroras. The suppression of nightside auroras in sunlight is usually attributed to a feedback mechanism, associated with the ionospheric conductivity, when the nightside magnetosphere acts as a current generator. Since precipitating electrons that produce dayside auroras are associated with upward field-aligned currents, the enhancement of dayside auroras in sunlight can be interpreted as a simple circuit system in which the ionosphere is a load and the dayside magnetosphere is a voltage generator.