With observations from the retarding ion mass spectrometer on the Dynamics Explorer 1 from 1981 through 1984, we examine the He+ to H+ density ratios as a function of altitude, latitude, season, local time, geomagnetic and solar activity. We find that the ratios are primarily a function of geocentric distance and the solar EUV input. The ratio of the densities, when plotted as a function of geocentric distance, decrease by an order of magnitude from 1 to 4.5 RE. After the He+ to H+ density ratios are adjusted for the dependence on radial distance, they decrease nonlinearly by a factor of 5 as the solar EUV proxy varies from about 250 to about 70. When the mean variations with both these parameters are removed, the ratios appear to have no dependence on geomagnetic activity, and weak dependence on local time or season, geomagnetic latitude, and L shell.
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