On the observations of flat energy spectra upstream of fast interplanetary shocks

Several interplanetary shock crossings by spacecraft at 1 astronomical unit have revealed enhanced energetic-ion fluxes that extend far upstream of the shock. In some shock events, ion fluxes with energies between 100 keV and 2 MeV acquire similar values, which we refer to as ``overlapped'' fluxes, corresponding to flat energy spectra in that energy range. On the other hand, close upstream, the fluxes values are observed to depend on energy. In this work, we analyze three interplanetary shock-related energetic particle events observed by the Advanced Composition Explorer spacecraft where flat ion energy spectra were observed upstream of the shock. We interpret these observations via a velocity filter mechanism for particles in a given energy range. In particular, ions with velocity parallel to the local magnetic field larger than the speed of the upstream plasma, in the reference frame of the shock, can easily propagate back upstream, while lower energy ions tend to be confined to the shock front, thus reducing their fluxes far upstream and giving rise to flat energy spectra.