On the Derivation of Empirical Limits on the Helium Abundance in Coronal Holes below 1.5 R _s

Abstract

We present a simple technique describing how limits on the helium abundance, a, the ratio of helium to proton number density, can be inferred from measurements of the electron density, temperature and their gradients below 1.5 R,. As an illustration, we apply this technique to emission line intensities in the extreme ultraviolet, measured in polar coronal holes. The example indicates that or can be significantly large in the inner corona. This technique could be applicable to the more extensive data to be obtained from coordinated ground and space-based observations during the Ulysses south polar passage and the Spartan flight, and subsequently during the SOHO mission. Limits on the helium abundance in the solar wind can thus be derived from its source region and compared to interplanetary values.