Solar wind driving of the auroral outflow during the 23–26 September 1998 storm

Abstract. Data from the FAST spacecraft are used to study the temporal progression of the energy inputs to the dayside cusp and the nightside aurora, including Poynting flux, electron number flux and amplitude of extremely low frequency (ELF) waves, during a storm driven by CME (Coronal Mass Ejections), and the resulting H+ and O+ outflows. The results show that (1) On the dayside, Poynting flux, ELF waves activity and soft electron precipitation are all enhanced during the initial and main phases of the storm, and decrease during the recovery phases. On the nightside, the Poynting flux increases during the initial and main phase, but the enhancements are smaller than on the dayside. The variations in the ELF wave activity and electron precipitation are similar before and during the storm. (2) The energy inputs are strongly correlated with the solar wind – magnetosphere coupling functions, dΦMP/dt and p1/2dΦMP/dt, especially in the dayside cusp region where the energy inputs and the ion outflows are localized. (3) The O+ and H+ ion outflow flux, fO+ and fH+, and the flux ratio fO+/fH+ all increase during the storm. Both the fluxes and the flux ratio reach their peaks on the initial phase and are enhanced during the main phase. Nightside auroral H+ and O+ outflows have lower outflow number fluxes than that in the dayside cusp region. These observations show how the solar wind changes characteristics of CME storms and results in strong sustained ion outflow during the initial and main phases.