Cusp field-aligned currents and ion outflows

On September 24 and 25, 1998, the Polar spacecraft observed intense outflow s of terrestrial ions in association with the passage of an interplanetary shock and coronal mass ejection. The orbit of the Fast Auroral Snapshot (FA ST) Explorer was in the noon-midnight meridian during this ion outflow even t, and FAST passed through the dayside cusp region at similar to 4000 km al titude every 2.2 hours. FAST was therefore able to monitor the ion outflows subsequently observed by Polar. We show that while the outflows were more intense after the shock passage, the overall particle and field signatures within the cusp region were qualitatively similar both before and after the shock passage. FAST observations show that the cusp particle precipitation marks the lower-latitude leg of a pair of field-aligned currents and furth er, that both field-aligned current sheets appear to be on open field lines . Moreover, the polarity of the cusp currents is controlled by the polarity of the interplanetary magnetic field (IMF) y-component, such that the magn etic field perturbation associated with the pair of cusp currents is in the same direction as the IMF B-y. This is a consequence of the reconnection o f cusp-region field lines at the magnetopause, with the flux transport resu lting in electromagnetic energy being transmitted along field lines to the ionosphere as Poynting flux. We show that this Poynting flux can be as high as 120 mW m(-2) (120 ergs cm(-2) s(-1)) at FAST altitudes (similar to 500 mW m(-2) at ionospheric altitudes), presumably because of the strong IMF B- y (similar to 40 nT), and is the dominant energy input to the cusp-region i onosphere. Furthermore, we find that the peak ion outflow flux is correlate d with the peak downward Poynting flux, although only a few passes through the cusp centered around the time of the shock passage were used to determi ne this correlation. The energy carried by Poynting flux is dissipated as h eat within the ionosphere, through Joule dissipation. The heating will tend to increase the ionospheric scale height, allowing greater access of ionos pheric ions to the altitudes where transverse ion heating via ELF waves can occur. Thus electromagnetic energy supplied by the transport of reconnecte d magnetic flux is the essential first step in a multistep process that enh ances the outflow of ionospheric plasma in the dayside cusp.