This is a study of the ionization input of interplanetary (including solar flare) energetic protons and alpha particles into the south polar ionosphere over the interval 1982--1985. Energetic particle data are collected by the charged particle measurement experiment (CPME) of the Applied Physics Laboratory/Johns Hopkins University, onboard the IMP 8 satellite orbiting the Earth at 30--35 Earth radii. It is well known that interplanetary ions have full and prompt access to the polar ionosphere. The incremental ionization produced at 20--120 km altitudes causes enhanced radio wave absorption which is observed by riometers operated by the University of Maryland, at South Pole, Antarctica. We compute the expected absorption from the vertical structure of the ionization deposited by these energetic particles and compare the computed values with the observations. The contribution of the alpha particles is found to be quite small as a percentage of the absorption except at the peak of the day 35, 1983, event, when their contribution to the absorption is about 0.6 dB out of a total of 3.4 dB. The dominant contribution to absorption at 30 MHz usually arises from protons below 10 MeV, specifically in the 2- to 4-MeV interval. We have propagated the observed fluxes and energy spectra of protons and alpha particles through a seasonally adjusted slab model of the polar atmosphere. The atmospheric ionization resulting from the slowing and stopping of protons and alpha particles is used to estimate an equilibrium vertical ionization profile which is then convolved with an absorption efficiency profile to yield a calculated absorption. There is good agreement between the computed and observed absorption when the daily averaged absorption is above 0.1 dB; this shows that the interplanetary ions are the dominant contributors on those days. This set of observations constitutes the most complete and definitive illustration to date of the control by interplanetary particles of the shape and magnitude of the ionization profile in the lower ionosphere. ¿ American Geophysical Union 1989