During the solar particle event of Arpil 12-14, 1973, a flux depression of solar protons in the dawn side pseudotrapping region relative to the flux in the dusk side pseudotrapping region was frequently observed with the low-altitude polar-orbiting satellite Esro 4. This flux decrease is interpreted as being due to weak pitch angle scattering of the solar protons during their drift from dusk to dawn. Treating the pitch angle diffustion as a simple initial value problem, We derive quantitative values of the equatorial bounce-averaged pitch angle diffusion coefficients at small pitch angles for different energies. These values represent lower limits for magnetospheric conditions at the time for the Esro 4 observations. Through the assumption that the pitch angle diffusion is a gyroresonant interaction process of the particles with either Alfv¿n or ion cyclotron waves, the necessary power of the waves at the resonant interaction frequency is calculated.