We present a three-dimensional, collision by collision model of the transport and energy degradation of energetic protons incident on the atmosphere. A test of the model was carried out using NOAA 12 satellite measurements of incoming energetic protons as input and simultaneous spectrometric measurements of the resulting dayside auroral hydrogen emission. There is a good match between the calculated and the observed emission profiles. The emission peak of the narrow and symmetric emission profile is positioned 4 ¿ to the blue of the unshifted line center, which is characteristic of the monoenergetic proton precipitation associated with the magnetospheric cusp/cleft, and the velocity filter resulting from dayside magnetic field line merging and convection. The intensities of the emission was reproduced using a proton number flux of approximately one-half of that which was obtained by NOAA. The portion of the emission profile to the red of the unshifted line was well within the instrumental broadening. From this we conclude that the contribution of upward moving hydrogen atoms to the observed dayside H emission must be of the order of 10 percent. ¿ 1998 American Geophysical Union