Asymmetric sunlight effect on dayside/nightside auroral precipitation

There are considerable evidence supporting the novel finding from intensive particle observations that electron acceleration events responsible for pr oducing discrete arcs are more common in darkness than in sunlight [Newell ct at, 1996a]. This sunlight effect on the production of auroral arcs is mo st 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 au roras. In this paper we investigate this issue by using auroral images acqu ired from the ultraviolet imager (WI) on board the Polar satellite. By anal yzing auroral emission in the long Lyman-Birge-Hopfield band (160 - 180 nm) , which is approximately proportional to the total energy flux of precipita ting electrons over a wide range of auroral electron energies, one can esti mate the total energy deposition rate from auroral precipitation. To emphas ize the sunlight effect on auroras seasonal averages of auroral luminositie s 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 auro ral intensity is generally enhanced in sunlight, indicating a different sou rce mechanism for the dayside aurora from the nightside auroras. The suppre ssion 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 t hat produce dayside auroras are associated with upward field-aligned curren ts, 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 mag netosphere is a voltage generator. (C) 2001 Elsevier Science Ltd. All right s reserved.