Spatial characteristics of the dayside auroral ionosphere observed by Incoherent Scatter Radar

Abstract. 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 fast elevation scans to obtain both altitudinal and latitudinal information of the ionospheric parameters electron density N_e, electron temperature T_e, and ion temperature T_i. 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, the F-region electron temperature T_e is significantly enhanced on open field lines and peaks just poleward of the OCB reaching up to 4 degrees Northward. In particular, T_e is highest between 11–13 MLT where the ESR is most likely below the cusp. During this interval, the gradient in T_e from closed to open field lines peaks. Additionally, N_e 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, N_e decreases with increasing latitude into the polar cap, especially pre-noon. Further, examining the ratio of N_e in the E and F regions, we observe that the ratio peaks on closed field lines pre-noon, consistent with high energy precipitation in the early morning. In addition, the variability in the ion temperature T_i 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.