We have performed a statistical analysis of particles >30 keV/charge upstream of the earth's bow shock. The rate of occurrence of upstream particle events has been determined relative to the angle between the magnetic field and the shock normal at the shock intersection point as well as relative to the angle between the magnetic field and the radial direction (i.e., the sun-earth line). the probability for upstream particle events is highest for small angles between the magnetic field and the radial direction, indicating that the occurrence is determined by the bow shock connection time of a field line convected with the solar wind. We have therefore determined for a model bow shock the occurrence rate of upstream particle events relative to the bow shock connection time. Assuming that the particles are accelerated by multiple reflections between upstream and downstream scattering center, we can use the observed time constants in order to derive propagation parameters. We find a diffusion coefficient depending linearly on energy per charge, the value of the mean free path of a 30-keV proton to be ~4 RE, and the region of strong scattering to be limited to ~30 RE in front of the bow shock.