Climatology of polar ionospheric density profile in comparison with mid-latitude ionosphere from long-term observations of incoherent scatter radars: A review

Abstract

Although the horizontal density structures of the polar ionosphere have been extensively studied, there are relatively few studies on the vertical density profiles. In this review, we present the climatology of polar ionospheric density profiles not only in the F-region but also in the E-region and topside ionosphere, in comparison with the mid-latitude ionosphere, using long-term incoherent scatter radar (ISR) observations at Millstone Hill, Tromso, and Svalbard. The observed density profiles during 1995-2015 are analyzed to study on the variations with local time, season, and solar/geomagnetic activity. The diurnal variations of the F-region density are much smaller in the polar region than at mid-latitude, particularly in summer. At Svalbard, there is a characteristic double-peak structure of the polar ionosphere in the diurnal variation in winter and equinox only for high solar activity. The diurnal variation of hmF2 decreases with increasing latitude and eventually disappear at Svalbard for low solar activity. The hmF2 in the polar ionosphere however is greatly enhanced as solar activity increases. The distinctive irregularity in the mid-latitude F1-layer nearly disappears in the polar region, especially at Svalbard. The seasonal variations of the F-region density are less evident in the polar ionosphere especially for low solar activity. The polar E-region density shows characteristic nighttime peaks induced by auroral precipitation but it does not necessarily increase with solar activity. The topside ionospheric density variations are much stronger in the polar region than in the mid-latitude for high solar activity. Finally, it is found that the polar ionospheric density profiles more strongly respond to increasing solar and magnetic activities than the mid-latitude ionosphere.