We study temporal structures, energy spectra, and spatial gradients of 25--70 keV protons during four intense upstream ion events observed on December 3, 1977, by the medium-energy particle telescope (KED) on ISEE 2. The strong role of the bow shock connection time in controlling the absolute intensity and spectral shape of the upstream ions is confirmed. The path along which the convected magnetic field is carried on the bow shock surface has no observable influence. During the plateau phases, we determine a field-aligned gradient pointing toward the bow shock with an e-folding distance L=6.5¿1.5 RE for ≈30 keV protons. The combination with anisotropy data leads to a direct determination of the mean free path &lgr;∥ =2.6¿0.6 RE. A gradient perpendicular to the magnetic field points toward the nose of the bow shock with a north-south component of about 6%/RE. Its conversion to a spatial scale allows to estimate the perpendicular diffusion coefficient. We conclude that lateral diffusion is not the main escape mechanism which determines the exponential energy spectrum. The control of the acceleration efficiency by local characteristics of the bow shock is suggested by various observations: (1) fluctuations on a temporal scale of about 20 min and with a peak-to-peak amplitude of about 50% superimposed on the plateau phases; (2) structured onsets of events during smoothly improving connection conditions; (3) strong intensity modulation during marginal acceleration conditions when the connection time is of the order 10 to 12 min; (4) convection of differently populated field lines across the observer. We tentatively relate the variable acceleration efficiency to changes in the normal angle (the angle between the shock normal and the local magnetic field direction). The hitherto unexplained existence of the limits in flux during the plateau phases is stressed.