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The relationship between riometer-measured absorption of 30 MHz cosmic radio waves at Thule, Greenland, and the extramagnetospheric 1-100 MeV solar proton event fluxes observed by satellite OV5-6 during 1969–1972 is studied, with night and day conditions at Thule being separated. In the 70–90 km region in which most nighttime absorption occurs, ionization conditions are, in fact, relatively simple. This leads to a nighttime relationship between absorption and the square root of the integral proton flux, which is similar to that for daytime contlitions. Based on previously measured effective recombination coefficients, theoretically optimum energy thresholds are determined for use in this approximate relationship. The values are 2.2 MeV and 5.2 MeV for night and day conditions, respectively. Both the approximate absorption, and that calculated by a reformulation of the standard approach, compare satisfactorily with extensive riometer measurements at Thule. The approximate relationships are employed in a real-time system that uses satellite proton data for prediction of radio wave (riometer) absorption.