Ionospheric climatology and variability from long-term and multiple incoherent scatter radar observations: variability

Long-term incoherent scatter radar (ISR) observations are used to study ionospheric variability for two midlatitude sites, Millstone Hill and St. Santin. This work is based on our prior efforts which resulted in an empirical model system, ISR Ionospheric Model (ISRIM), of climatology (and now variability) of the ionosphere. We assume that the variability can be expressed in three terms, the background, solar activity and geomagnetic activity components, each of which is a function of local time, season and height. So the background variability is ascribed mostly to the day-to-day variability arising from non solar and geomagnetic activity sources. (1) The background variability shows clear differences between the bottomside and the topside and changes with season. The Ne variability is low in the bottomside in summer, and high in the topside in winter and spring. The plasma temperature variability increases with height, and reaches a minimum in summer. Ti variability has a marked maximum in spring; at Millstone Hill it is twice as high as at St. Santin. (2) For enhanced solar activity conditions, the overall variability in Ne is reduced in the bottomside of the ionosphere and increases in the topside. For Te, the solar activity enhancement reduces the variability in seasons of high electron density (winter and equinox) at altitudes of high electron density (near the F2-peak). For Ti, however, while the variability tends to decrease at Millstone Hill (except for altitudes near 200 km), it increases at St. Santin for altitudes up to 350 km; the solar flux influence on the variability tends to be stronger at St. Santin than at Millstone Hill.