The footprint of FAST passes the polar cap boundary at 0257:30 UT (67) in Figure 11, although some low-energy (1 keV) precipitating electrons are seen shortly before at 0257:10 UT (69). The polar cap boundary as seen by FAST agrees to within 1.5 of the poleward electrojet boundary inferred from the magnetometers at this time (Figure 7). The agreement is reasonable considering the spacing of the magnetometers.
Figure 11d suggests that the intensity of the emissions peaks near Arviat, 3 poleward of the most northern auroral arc intersected by FAST. The low-energy (1 keV) precipitating electrons located farther poleward of this arc are probably not energetic enough to be the source of auroral roar emissions; however, they may be effective in screening emissions. An inverted-V structure shown in Figure 11a is located at the latitude of Gillam as well as upgoing loss cone (180) and trapped (40-130) electron populations. In five other FAST overpasses (not shown), downgoing beams, upgoing loss cones, and trapped electron populations are present implying that energetic electrons capable of generating auroral roar are present near or equatorward of the emissions.