The auroral ion measurements (0.4--30 keV) made from the satellite Aureol 1 have revealed two types of precipitation with different energy characteristics in the auroral and subauroral nighttime zone: (1) In the entire nighttime auroral zone a proton precipitation with an average energy ranging from ?5 keV to at least 25 keV, which coincides with the region of auroral Balmer series lines emission as observed from the ground (Crasnier et al., 1974). The large scale characteristics of this precipitation reflect the adiabatic acceleration of the protons in the plasma sheet and their drift motions in the outer magnetosphere (Galperin et al., 1976, 1978) (2) A subauroral precipitation of ions with energy less than 12 to 15 keV limited to the morning sector between approximately 0000 and 0500--0600 MLT. This newly observed structure is sporadic and takes place in the hours following the injections of plasma sheet particles into the inner magnetosphere at the onset of magnetospheric substorms. The ionospheric region affected by this precipitation corresponds, in the euqatorial plane, to the zone in which Alfv¿n layers result from the ion drift pattern (from the added effects of convection and corotation electric fields and from magnetic field gradient). It is shown that the electric and magnetic field models derived by McIlwain (1972) from the energy dispersion of the particles injected at 6.6 RE at the substorm onset accounts for the main spatial and energy characteristics of these low-energy ion subauroral precipitation. This result is obtained by extending the logic used to study the proton 'noses' (Smith and Hoffman, 1974: Konradi et al., 1976) from the evening side to the morning side of the inner magnetosphere, and this makes it possible to obtain, from the injection boundary concept, a coherent approach to the substorm injection features at different local time sectors.