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Abstract We present an analysis of helium ion (He⁺) fraction in an altitude range from about 400 km to around 700 km and its relationship to the ion temperature (T_i) and the vertical ion drift under solar maximum conditions. The data were obtained from the Arecibo incoherent scatter radar during 27 September to 1 October 2014 and 16–20 December 2014. The large He⁺fraction (>10%) lasts 15 hr per day during the winter solstice, which is 3 times larger than during fall equinox. This difference is caused by the more persistent downward ion drift in the winter. The incremental He⁺fraction and incrementalT_iare well anticorrelated, and the anticorrelation is more prominent during the daytime. These characteristics are associated with whether O⁺and He⁺are in diffusive equilibrium. During nighttime, we show that the vertical ion flow is downward causing the He⁺layer peak altitude to move to an altitude of 500 km from above 650 km. According to our analysis, He⁺fraction has to be larger than two thirds for diffusive equilibrium to occur above the He⁺peak height. Therefore, above the He⁺peak altitude, O⁺and He⁺cannot be in diffusive equilibrium with He⁺being the minor species. The vertical ion flow plays an important role in determining the diurnal variation and seasonal difference of He⁺distribution and whether He⁺is in a diffusive equilibrium with O⁺.