Effect of Temperature and Vertical Drift on Helium Ion Concentration Over Arecibo During Solar Maximum

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 incremental T i are 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 + .