Analysis of incoherent scatter radar measurements at Arecibo routinely take into account the presence of light ions above an altitude of 400 km. Combined with field-aligned ion velocity observations it is possible to determine the diurnal variations of the proton flux. We find that the near-zero daytime proton outflow in the topside ionosphere during equinox is not generally balanced by a larger nighttime inflow, as appears to be the case for other ions such as O+. The proton flux and concentration does respond, however, to external influences such as disturbed magnetic conditions. One such isolated magnetic storm occurred during the day during the Equinox Transition Study, a World Day investigation in September 1984. We show the response to the storm of several measured topside parameters, in particular the proton concentration and its flux, where significant departures from their nominally zero conditions occurred. ¿ American Geophysical Union 1989