Helium in the solar wind

Abstract

Data obtained from electrostatic analyzers on the Vela 3A and 3B satellites from July 1965 to July 1967 have been analyzed to obtain relative helium abundances and plasma properties. Ninety percent of the 10,314 spectra used from this period gave helium to hydrogen density ratios from 0.01 to 0.08. The long term average was 0.037, which is lower than the recently reported solar-surface ratio of approximately 0.063. For the time period covered in this analysis, only 10% of the spectra yielded helium to hydrogen density ratios greater than 0.063. Large variations in the helium to hydrogen density ratio were observed even when averaged over a solar rotation. Sudden increases in the solar wind helium content followed simultaneous occurrences of Forbush decreases and sudden-commencement geomagnetic storms. The magnitude of the helium to hydrogen density ratio, however, was not correlated to the magnitude of the geomagnetic storm, as measured by the Kp index. The ratio of the helium to hydrogen velocities was strongly peaked around 1.0, as might be expected. The distribution of the helium to hydrogen temperature ratios had a full width at half maximum of approximately 2.5 and a mean of approximately 4.0.