The distinction between two types of upstream ion populations has been made on the basis of pronounced differences in their distribution functions. The 'reflected' ions represent a fast beam with temperatures typically 1 to 5 times 106 K and speeds up to five times the solar wind speed. An important feature of the reflected ion distributions in their strong temperature anisotropy, with T⊥ exceeding T∥ by a factor of two to three. In contrast, the 'diffuse' ions occupy a much larger region of phase space, both in energy and angle; their distribution function generally has the form roughly of a circular ridge in 2 dimensions and a spherical shell in 3 dimensions. Accordingly, their temperature is much larger (>107 K), and their bulk speed typically is smaller than the solar wind speed. Both ion populations have densities of the order of 0.1 cm-3. At times transitions between the two extremes, represented by the reflected and diffuse ion populations, are observed. These 'intermediate' distributions are cresent shaped, with the center of curvature near the solar wind velocity. This property suggests that the intermediate distributions result from pitch angle scattering of the reflected beams in the solar wind frame and supports the idea that the reflected ions are the origin of the diffuse ions. At times the diffuse ion distributions exhibit considerable structure and rapid temporal variations. Reflected and diffuse ions can also be distinguished by their occurrence as a function of the angle ϑ between the local shock normal and the interplanetary magnetic field. Whereas the diffuse ions occur predominantly for small ϑ, the reflected ions are observed most frequently for ϑ>45¿.