Comparative study of polar cap electron density measurements and E-CHAIM modeling

Abstract

The electron density in the Earth’s topside ionosphere has been studied with ground-based incoherent scatter radars (ISRs), Langmuir Probes (LP) placed on low-altitude satellites and many other instruments and techniques. These measurements have been continuously digested into statistical models of the electron density distribution. These models are used for forecasting of radio wave propagation in the ionosphere. The success of ionospheric models depends on the overall coverage by instruments and quality of their measurements. Joint observations with multiple instruments, however, have been rarely considered while it is important to assess whether they report consistently comparable values of the electron density. One example of a concern is an early suspicion in the LP experimentation in space that an underestimation effect can occur because of the contamination of the current-collecting surfaces. This thesis addresses several aspects of the electron density measurements in the ionosphere with two instruments, ISRs and LP instruments onboard Swarm satellites, and modelling with the recently developed Empirical Canadian High Arctic Ionospheric Model (E-CHAIM). The study focuses on the Resolute Bay (Nunavut, Canada) area, located at extreme high latitudes where the ionosphere is very dynamic and poorly investigated. The first objective of the work was to evaluate the consistency of LP instruments on the Swarm A and C satellites flying one after another at the same altitude with a time separation of 7-10 seconds and spatial separation of ∼100 km. Occasional inconsistencies between the reported values were identified, and those were related to the occurrence of patches with enhanced electron density (polar cap patches). It was concluded that the polar cap patches are more frequent in the night sector, especially in summer and winter. Secondly, the long-term trends in the electron density reported by the satellites at two flight heights of ∼450 km (Swarm A and C) and ∼510 km (Swarm B) were investigated. A strong solar cycle effect was identified, in agreement with predictions by the E-CHAIM model. Comparison of the model output with the Swarm data showed typically larger values, up to 30%. To further assess the electron densities measured by the Swarm LP instruments, a point- by-point comparison with ISR measurements of the electron density was performed for about 200 conjunction points. It was shown that Swarm values are lower than those measured by the radars by ∼35%, on average. The agreement between the satellite-radar data is better for the electron densities between 5×10^10 m−3 and 40×10^10 m−3 . The conclusion on the electron density underestimation for Swarm LP instruments is, overall, consistent with that reported for middle latitudes in the past, but the effect is much stronger at high latitudes. Moreover, at high latitudes, the underestimation effect becomes progressively stronger as the electron density increases. Finally, predictions of the E-CHAIM model electron densities over Resolute Bay were compared with measurements by the ISR radars with the goal of assessing the quality of model predictions at various heights. It was shown that for the middle part of the F layer, around its maximum, E-CHAIM shows reasonable agreement with measurements. The ratio of the predicted density to the observed density was mostly between 0.5 and 1.5, with 1.0 indicating perfect agreement. The best agreement was found in the summer. At the topside altitudes, the model was found to underestimate electron densities, particularly in the summer season. The worst agreement between the model and measurements was found for the ionospheric bottomside where the model often shows 2-3 times larger electron densities, especially in winter and spring. At the end of the thesis, suggestions for future research have been outlined.