Linear relationships between daytime 30 MHz polar-cap absorption and the square root of integral and differential proton fluxes have been studied experimentally, using satellite and riometer measurements, and theoretically, by means of power-law model proton spectrums together with D-region ionization models developed previously by the author. Relationships of riometer absorption, A, to the 2π omnidirectional integral proton fluxes greater than the threshold energy E0 were found to be least sensitive to varying model proton spectrums for E0 = 7 Mev. In this case the following formula was computed: A = 0.083[J(>7 Mev)]½ db, where the flux units are cm−2sec−1. Using data collected from the 2π omnidirectional integral proton detectors on the polar-orbiting satellite 1963 38C with E0 = 2.2, 8.4, and 25 Mev on 93 passes and polar cap 30 MHz riometer data during three markedly different events, the best linear correlation was found to be A = 0.084[J(>8.4 Mev)]½ db with a root-mean-square deviation of 0.38 db. Daytime 30 MHz absorption during PCA's is affected most by 15 Mev protons, which produce their greatest ionization at a 65 km altitude in the D region.