Spacelab 2 measurement of the solar coronal helium abundance

Abstract The abundance of helium relative to hydrogen has been measured with the “Coronal Helium Abundance Spacelab Experiment” (CHASE) from the space shuttle Challenger in 1985. Previous solar measurements have proved difficult due to the temperature-sensitivity of the electron excitation rates for the observed lines. In this approach scattered Lyman Alpha (Lyα) radiation of belium and hydrogen formed in the corona were measured with a grazing-incidence spectrometer and compared with the intensity of the illuminating flux from the solar chromosphere. The abundance ratio by number of atoms was found to be 0.070 with an uncertainty of 0.011. Scattered light in the telescope is the main source of error.