Velocity and abundance of silicon ions in the solar wind

Using data from the ISEE‐3 ion composition instrument (ICI), velocities and abundances of silicon ions in the solar wind have been determined. The period of investigation covers the maximum of solar cycle 21, beginning with launch of ISEE‐3 in August 1978 and ending at the removal of the spacecraft from the Lagrangian Point L1 in June 1982. The results generally confirm previous ICI observations of iron, the other heavy element with a low first ionization potential measured with the ICI. Silicon ions (and other ions in the same M/Q range) tend to stream at the bulk velocity of 4 He ++ in low‐speed solar wind. At high‐speed solar wind, Si lags by up to or about 20 km/s after 4 He ++ . By means of a minimum variance estimation technique, fluxes (and densities) of silicon in the solar wind have been obtained free of bias. An average Si/O flux ratio of 0.19±0.04 is derived. This value is larger by a factor of 3 or 4 than the Si/O abundance ratio at the solar surface.