Ion concentration measurements during the initial elliptical phase of the Atmosphere Explorer C mission reveal an anomalous behavior as reflected in the height and longitudinal distributions of the [H+>/[O+> ion concentration ratio above 400 km. We emphasize a description and properties of this spatial variability with data acquired during August 1974 when the satellite sampled this height region during late afternoon at southern winter and northern summer mid-latitudes. A well-defined relative enhancement in the ion ratio near 500 km occurs in the winter hemisphere over a broad longitude interval about -90¿; the general pattern persists throughout the month, but the magnitude of the relative variation decreases as the satellite orbit precessed southward. The observations at northern summer mid-latitudes indicate a smaller relative depression of the ion ratio within the same general longitude sector. Available measurements and theoretical arguments preclude the presence of similar changes of the observed magnitude in the relative abundance of neutral hydrogen and oxygen at these altitudes. We must thus infer significant deviations of the ion ratio and the H+ concentration from charge exchange equilibrium to altitudes as low as 400 km in the longitude region of abnormal behavior. Height distributions of the observed H+ concentration are consistent with flows of this ion downward into the winter and outward from the summer afternoon F regions, but flows that occur preferentially at longitudes in the vicinity of -90¿ for the geophysical conditions being sampled. The corresponding horizontal patterns of this ion flow are in general accord with those indicated by Explorer 32 ion composition data at higher altitudes, with neutral winds as the probable driving source. Estimates further suggest that the wintertime H+ fluxes might be substantial, with unusually large vertical flow velocities possible within a localized geographic region. While quantitative aspects of this interpretation remain uncertain, the observed behavior in the [H+>/[O+> ratio provides strong evidence as to the presence and effects of ion flow in the topside ionosphere. ¿ American Geophysical Union 1992