Upstream ion events with hard energy spectra: Lessons for their origin from a comparative statistical study (ACE/Geotail)

In this paper we present statistical results from a comparison of ion events observed almost simultaneously by the Geotail spacecraft near the Earth's bow shock and by ACE moving around the libration point L1 (∼220km). The main result of this study is that important features of the ACE ion events, as for instance, the ion flux, the ion energy spectral slope, and the particle composition, change drastically through propagation from the magnetosphere to the L1 point. Among other results we found that the ACE events show (1) a strong spectral hardening compared to the spectral index γGeotail value observed just outside the magnetosphere. It is a decreased value by an average factor≈3, and (2) a percentage as low as ∼22% of the Geotail electron events which is accompanied by the presence of electrons at the position of ACE. We infer that a short duration ion event with a hard “solar” type energy spectrum, which is non-accompanied by energetic electrons, can originate from the Earth's magnetosphere, and that therefore, these results should be taken into account in space weather prediction research. More detailed information on the varying features of traveling ions and electrons from the bow shock to far distances are important with respect to the problem of their origin and are also presented and discussed in the paper. •The spectrum of upstream ion events hardens at large distances from the bow shock.•Far upstream ion events show a hard “solar” type energy spectrum.•Magnetospheric electrons show strong flux decrease far upstream from the bow shock.