The solar wind helium abundance: Variation with wind speed and the solar cycle

We investigate the helium abundance in the solar wind and variations thereof on a time scale of years. Data from the WIND/SWE experiment gathered between the end of 1994 and early 2000 are analyzed. In agreement with similar work for previous solar cycles, we find a clear dependency of the He/H ratio in the solar wind on the solar cycle. In the slow solar wind, the average He/H rises from a minimum of less than two percent around solar minimum to about 4.5% in early 2000. The solar cycle dependency is stronger the lower the speed that is used to sort the data. We observe the strongest dependency of the He/H ratio on the solar wind speed around solar minimum and it weakens as the solar activity increases. We speculate that the expansion factor of the magnetic field close to the Sun changes over the solar cycle and thereby changes the efficiency of the Coulomb drag. Inefficient Coulomb drag leads to a low helium abundance in the solar wind.