IPY Observations of Ionospheric Yearly Variations from High- to Middle-Latitude Incoherent Scatter Radars

Abstract

Upper atmospheric research during the 2007 International Polar Year features unprecedented yearlong observations by incoherent scatter radars. This valuable data set, collected under very low solar and magnetic activity conditions from a set of four radars spanning middle (Millstone Hill), auroral (Poker Flat), and higher latitudes (Sondrestrom and Svalbard), enables us to address a fundamental question of background climatology on ionospheric yearly variations. The main focus of this paper is on the noon-time baseline annual and semiannual variations, with an emphasis on the less-known height dependency, for those geophysically important areas. The radar measurements of ion drifts allow us to examine dynamic effects which have not been well addressed previously. High-latitude ionospheric climatology shows some interesting features, in particular, the two semiannual peaks of electron density N_e. The two peaks, neither of which is near summer solstice, are essentially of equinoctial symmetry at very high latitude sites, where downward ion drifts are strong in summer thus causing N_e reduction. The symmetry vanishes at the midlatitude site. Clear phase progression is seen from the topside to the F2 peak at the midlatitude site. Comparisons of Millstone Hill data from this period and prior solar minima indicate decreases in the ion temperature throughout the F2 peak and the topside ionosphere. These decreases do not seem to be associated with solar flux and magnetic activity effects.