Observations from the retarding ion mass spectrometer (RIMS) on the Dynamics Explorer 1 (DE-1) satellite arc used to examine He+ and its relation to H+. These observed values are also compared to theoretical values from the field line interhemispheric plasma (FLIP) model. It is found that the observed density ratio of He+/H+ may vary from less than 0.01 to over 1.0 in the plasmasphere. The average value of 0.2 is greater than has been previously found by either observation or theoretical calculation. In order to produce good agreement between the model and measured ion temperature, it is necessary to assume that approximately 55% of the energy of the ionospheric photoelectron escape flux is deposited in the plasmasphere. This proportion is much higher than found in previous theoretical estimates. When the plasmaspheric heating rate is adjusted to bring the model temperatures into agreement with the measured temperatures, generally good agreement is obtained between the modeled and observed ion density profiles, except at the highest altitudes. Typically, there is a region in the plasmasphere above ~4000 km and L=2.0 where the He+/H+ density ratio has been observed to maintain a nearly constant value of approximately 0.2. This characteristic is not reproduced by the model. It is also found that the H+ density varies primarily with the magnetic activity while the He+ density varies more with the solar activity. These effects combine in such a way that the ratio of these densities is found to be dependent on both indexes, with the solar activity having the greatest effect. This appears to account for the observation of large He+/H+ density ratios since the DE-1/RIMS observations took place during a period of unusually high solar activity. The model also indicates that there is a factor of 3 diurnal variation in the high-altitude He+ density, but little diurnal variations in the H+ density. The measured densities, which were limited to short local time periods in the morning and evening sectors, suggest more diurnal variation in the evening than the model. ¿ American Geophysical Union 1989