Spatial Characteristics of the Dayside Auroral Ionosphere Observed by Incoherent Scatter Radar

Observation-based characteristics of the dayside ionosphere are important for the knowledge of the coupling between the solar wind, magnetosphere and ionosphere. Therefore, this paper presents descriptions and quantitative analyses of characteristics of the polar dayside ionosphere during the winter. We use EISCAT Svalbard radar (ESR) fast elevation scans to obtain both altitudinal and latitudinal information of the ionospheric parameters electron density Ne, electron temperature Te, and ion temperature Ti. We determine the location of the open-closed field line boundary (OCB) and divide the ionosphere into three regions based on their position relative to the OCB: on closed field lines, along the OCB, and in the polar cap. We first show two case examples, illustrative of the method and the dynamic response of the ionosphere to variable solar wind. We then statistically investigate how the parameters vary from closed to open field lines across the OCB and with altitude in the three regions. Finally, we compare the obtained OCB latitudes with the ones obtained in previous studies. Overall, significant differences in the ionospheric parameters can be seen between the three latitude regions. In general, observed enhancements in Te peak in the F-region on open field lines just poleward of the OCB, reaching up to 4 degrees poleward. In particular, Te is highest between 11:00-13:00 MLT where the ESR is most likely below the cusp. During this interval, the gradient in Te from closed to open field lines peaks. Additionally, Ne appears to be slightly enhanced poleward of the OCB at most altitudes and maximizes just below 300 km on open field lines, increasing with a factor 1.2 from closed field lines. In the E-region, Ne decreases with increasing latitude into the polar cap, especially pre-noon. Further, we observe that the ratio between Ne in the E and F regions is larger on closed than on open field lines. In addition, the variability in the ion temperature Ti appears to be larger on open field lines. Together, these result contribute to a quantification of characteristics of the dayside auroral ionosphere with respect to both altitude and latitude.