Bow Shock is not the Main Source of Upstream Energetic (> 40 keV) Ions

The origins of upstream energetic (40 keV - 1.5 MeV) ions have been investigated. Simultaneous multiple spacecraft observations show that the energetic ion flux upstream of the magnetopause is always lower than that in the high-altitude dayside cusp. During an upstream ion event when the INTERBALL-1 spacecraft was upstream near the bow shock, GEOTAIL was in the post dusk magnetosheath, and POLAR was near the cusp on open magnetospheric field lines, the measurements reveal that (1) the ion energy spectra, measured by these three spacecraft had a similar spectral shape, (2) no comparable energetic ion phase space density (PSD) was observed at the forward libration point, (3) ion PSDs measured near the cusp were about one order of magnitude higher than that measured near the quasi-parallel bow shock, (4) the time profiles of ion PSDs measured by POLAR and INTERBALL-1 seem to track fairly closely, (5) the ion PSD peaks were detected first by POLAR then by INTERBALL-1, and (6) the energetic ion PSD measured near the bow shock was independent of whether the bow shock was quasi-parallel or quasi-perpendicular. These observational facts demonstrate that the bow shock is not the main source of the energetic (> 40 keV) ions in this upstream event. These upstream energetic ions most likely come from the high-altitude dayside cusp along open field lines.