Auroral polar cap boundary ion conic outflow observed on FAST

We observe large ion outflow fluxes (> 10(8) cm(-2)s-(1) mapped to 100 km) flowing from the auroral polar cap boundary on many FAST auroral passes near midnight. The outflow is in the form of ion conics composed mostly of light ions with energies of hundreds of eV. A statistical study off 606 FAST orbits during January and February 1997 was done to determine the MLT distribution and ion outflow properties relative to substorms. We find that the ion conic events occur most frequently near midnight, and therefore their direct magnetic connection to the nightside central plasma sheet makes this ion outflow a candidate for the ionospheric source of the nightside plasma sheet. To study the relationship between the ion conic outflow events and auroral substorm conditions, we use global auroral images acquired from the ultraviolet imager (UVI) on Polar. We find that the outflow events are well correlated with substorm expansion phase, but ion conic outflow events also occur during generally active aurora, when onset and expansion phases were not clearly identifiable. By combining FAST data to determine ion outflow fluxes and ILAT extent and Polar UVI images to determine MLT extent, we estimate the total outflow due to these ion conic events to be 10(22), 10(24) ions/s. By comparing with earlier ion outflow studies, we conclude that these polar cap boundary ion conic events are not the dominant or only ionospheric source of ion outflow over the entire auroral oval but are the dominant nightside auroral ion outflow.