Minor ion composition in the polar ionosphere

Ion composition measurements from the EXOSD Suprathermal Ion Mass Spectrometer (SMS) are presented. Ions other than H+, notably O+, He+, O++ N+ and N++, are found to constitute a significant (>0.1) and at times dominant (>0.5) component of the thermal ion population in the high‐altitude polar ionosphere. Their relative abundance and occurrence are highly variable. Ion flux ratios in the range of 0.1–0.5 are typical for 0+/H+, 0.1–0.3 for He+/H+, 0.1–0.3 for 0++/O+, and 0.05–0.1 for N++/N+. Our observations show that (1) ions other than H+, notably He+, O+, O++ and N+, often constitute a significant component (>0.1) of the thermal ion population in the high‐altitude polar ionosphere; (2) doubly charged oxygen and nitrogen (O++ and N++) are sometimes present with fluxes up to 0.1 of the singly‐charged (O+ and N+) ion fluxes; and (3) the He+ and O++ fluxes are sometimes comparable to the H+ and O+ fluxes.