Solar cycle variability of nighttime topside helium ion concentrations over Arecibo

Abstract

The incoherent scatter radar mode and data extraction method used in topside experiments at the Arecibo Observatory are discussed. Helium ion concentrations in the lower topside ionosphere over Arecibo are presented for low and high solar flux periods of the same season (October). Data from 8–9 October 1988, 21–22 October 1995, 26–27 October 1997, and 12–13 October 2001 are compared. The data from October 1988 and 2001 are representative of high solar flux conditions, while the other two periods (1995 and 1997) are from low solar flux conditions. All data sets represent geomagnetic quiet periods over Arecibo and serve as examples of typical equinox topside conditions. For solar minimum the altitude distribution of the helium ions usually has a maximum near the O+ to H+ transition altitude () during the night, usually around 550–650 km. The He+ number densities tend to be quite low, 2 or 3 × 103 cm−3 or <15% of the topside plasma density at the peak of the layer. For the higher solar flux case, however, He+ forms a more distinct layer at an altitude that is slightly above at night. The O+‐to‐He+ transition altitude () is reported to be below after the postsunset collapse in these solar‐maximum conditions, meaning that He+ is the dominant species. We find that the relative abundance of He+ can reach 60–65% of the topside plasma at the peak of this layer during these equinox conditions or up to 4 × 104 cm−3, with as low as 700 km and the He+ to H+ transition () extending over 1000 km at night.