Helium abundance variations

Using observations of the solar wind by Explorers 34 and 43, we confirm the existence of a linear relationship between the relative abundance of helium nα/np and the bulk speed U of the wind, recently described by Hirshberg, Asbridge, and Robbins. Use of this relationship nα/np = a(t) + bU allows the dependence of the relative abundance of helium on solar activity to be separated from its dependence on bulk speed. The proportion of helium in the solar wind at a given bulk speed is found to increase with increasing sunspot number, averaged over the period of observation. We show that the relationship between average relative abundance and temperature, deduced by using the bulk speed-temperature relation, is consistent with the data. The relative abundance of helium is not found to increase with increasing solar-wind flux. This confirms the result of Hirshberg et al.