Modelling and Observations of the Ionospheric Alfven Resonator at Eskdalemuir, UK

The Ionospheric Alfv´en Resonator (IAR) occurs when Alfv´en waves are partially reflected from the highly conductive lower layers of the ionosphere and above the F-region peak where the Alfv´en velocity reaches a maximum, resulting in a resonance. This thesis concerns the frequencies of the IAR observed in the Eskdalemuir magnetic observatory, which is located in southern Scotland, UK. I have extracted the IAR frequencies for nine years of data from 2013 - 2021 and calculated the harmonic frequency separation (Δf). I then compared this with a model which uses modelled Alfv´en velocity profiles and a time of flight calculation to model Δf. The model and data show similar trends, with Δf being greater during periods where the plasma mass density is lower. Next, I looked at the effects of the non-uniform Alfv´en velocity profile on the individual IAR harmonics. To model the IAR harmonics, I solved a onedimensional wave equation using the Alfv´en velocity profiles. I have modelled the harmonics using different lower boundary conditions, and compared these to the data and previous uniform models. By looking at the offset of the fundamental frequency and Δf, the results of this work show that the lower boundary of the IAR electric field is closest to a node.

Lastly, I have looked at situations where the data deviates strongly from the model. I have observed more rapid variations in the IAR frequencies which may be a result of Travelling Ionospheric Disturbances (TIDs). Modelling of the effect of a TID on IAR frequencies shows that TID signatures should be observable in IAR measurements, and I have compared a perturbed IAR observation with ionosonde data. I have also examined the occurrence of IAR fine structure and splitting of IAR harmonics over the nine years of data.