The solar cycle variation of the solar wind helium abundance

This paper is concerned with a critical survey of the experimental evidence for a variation of the relative abundance by number h=nα/np of helium in the solar wind. The abundance is found to vary by Δh=0.01 ±0.01 from 0.035 to 0.045 over solar cycle 20. The change in the average bulk speed during the solar activity cycle was insufficient to account for this increase in 〈h〉 with the solar cycle. The slope of the linear relation between h and the plasma bulk speed is also found to vary, being greatest around solar maximum. An attempt is made to explain the 30% variation in 〈h〉 as the result of the variation in the number of major solar flares over a solar cycle. These obvious transients are apparently not numerous enough to explain the observed variation, but the reasonable expectation remains that the transients observed recently by Skylab, which occur more frequently than major flares, could augment those associated with major flares. Since the solar wind flux is not observed to increase at solar maximum, the abundance of helium cannot be proportional to the proton flux leaving the sun unless the solar wind comes from a smaller area of the sun at maximum than at minimum.