Response of the equatorial topside ionosphere to 27‐day variations in solar EUV input during a low solar activity period using C/NOFS

Abstract

The ion velocity meter (IVM), a part of the CINDI instrument on board the C/NOFS spacecraft, is used to make ion density (Ni) and temperature (Ti) measurements in the low‐latitude topside ionosphere in the 400–850 km altitude range during the low‐solar activity period 2009–2010. Averages of the Ni measurements are then compared to the daily F10.7 values and the Mg II index values that serve as proxies for solar EUV input to the ionosphere. Before this comparison the IVM ion densities are normalized to a constant 550 km altitude level using the International Reference Ionosphere. Results from daytime data during December 2009–March 2010 indicate the presence of 30–40% Ni variations with an approximately 27‐day period. Cross correlation between the density measurements and the daily averaged EUV proxies shows that the ion density variations lag solar EUV by approximately 1 to 3 days in phase. We also note that there is low correlation of the EUV proxies with ion temperature.