The Fe/O elemental abundance ratio in the solar wind as observed with SOHO CELIAS CTOF

Using data of the Charge Time‐of‐Flight (CTOF) mass spectrometer of the Charge, Element, and Isotope Analysis System (CELIAS) on board the Solar and Heliospheric Observatory (SOHO) from ∼80 days of observation around solar minimum we derive a value for the Fe/O abundance ratio for the inecliptic solar wind of 0.11 ± 0.03. Since Fe has a low first ionization potential (FIP) and O is a high‐FIP element, their relative abundance is diagnostic for the FIP fractionation process. The unprecedented time resolution of the CELIAS CTOF sensor allows a fine‐scaled study of the Fe/O ratio as a function of the solar wind bulk speed. On average, the Fe/O abundance ratio shows a continuous decrease by a factor of 2 with increasing solar wind speed between 350 and 500 km/s. This corresponds to the well‐known FIP effect dependence. Our value at ∼500 km/s agrees with the previously observed Fe/O ratio in the fast solar wind emerging from polar coronal holes whereas the value for speeds below 350 km/s is consistent with a remote abundance determination in the leg of a coronal streamer. The variability of the Fe/O abundance ratio is much larger in the slow than in the fast solar wind.