Ion mass spectrometer measurements of He++ and H+ ion spectra on a satellite pass through the low-altitude (800 km) day side cusp during the magnetically active (Kp=60) period of March 7, 1972, are reported. These measurements in the energy-per-unit-charge range between 0.7 and 12 keV show a systematic velocity dependence in the latitudinal distributions. This dependence is explained by a dawn-dusk convection electric field of 30--60 mV/m operating in the cusp. The average velocity spectrum of the He++ is found to be significantly harder than the accompanying H+ spectrum, suggesting the possibility of an electrostatic deceleration mechanism operating between the solar wind source region and the low-altitude cusp.