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

Abstract

AbstractWe 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 (Ti) 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 Ti 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+.