Observations of loss cone-shaped back streaming energetic protons upstream of the Earth's bow shock

Energetic ions are at times observed in the upstream region of the Earth's bow shock, and their origin is considered to be in the interaction with the shock front. While the energy of the solar wind ions is a few keV at most, the energy of the back streaming ions ranges from ̃5 keV to several MeV In the present study we investigate back streaming energetic ions in the upstream of the Earth's bow shock observed by Geotail during two coronal mass ejection events. The observed local magnetic field rotated significantly during the events. Using the bow shock model and the observed magnetic field data, we found that the energetic ions appeared only when the upstream magnetic field was connected to the bow shock. The energetic ions showed two distinct distribution function characteristics, namely, the field-aligned beam (FAB) and the loss cone-shaped distribution. While the former is occasionally detected, the latter having higher energies (30 keV to several hundred keV, compared to <18 keV for FAB) has not been reported before. Using a bow shock model, we can also estimate the shock angle at the point on the shock surface that the upstream field line is connected to and find that the distribution function shape transits from the FAB to the loss cone-shaped distribution as the shock angle becomes larger (transition at Θ_Bn., = 70^°- 80^°). We discuss the possible mechanisms responsible for the production of the newly found member of the energetic upstream ion family.