Helium and Hydrogen Abundances in the Heliosphere and the Local Interstellar Medium

The sun is generally thought to have the same helium abundance as that in the solar nebula from which it was formed 4.6 billion years ago. In contrast, the interstellar medium reflects current galactic conditions. The departure of current abundances from the primordial and protosolar values may help trace the processes that drive the nucleosynthesis evolution of the galaxy. Here, we use the solar He 58.4 nm line measured by the Solar Heliospheric Observatory (SOHO) to derive the He abundance inside the focusing cone. We also use Hubble Space Telescope, SOHO, and Voyager observations to constrain the properties of the interplanetary hydrogen flow, particularly at the heliospheric interface. Finally, we compare H and He abundances derived here with primordial and protosolar values, stressing the unique opportunity offered by inner heliosphere observations and future Voyager in situ local interstellar medium measurements to derive the He abundance in the very interstellar cloud in which we reside.