Investigating geoelectric tides at three geomagnetic observatories in the UK

Electric fields are generated in the Earth’s rocks and oceans by electromagnetic induction: for example through steady tidal motion of conducting sea water through the geomagnetic field or from the rapidly changing external magnetic fields in the ionosphere and magnetosphere. Continuous geoelectric field monitoring began at the three UK magnetic observatories in 2013 alongside the standard geomagnetic field measurements that have been ongoing for a century in some cases. Each observatory lies in a substantially different setting in relation to the seas surrounding the British Isles and hence the new data allow investigation of tidally generated signals. More generally, these new electric field measurements also provide ground-truth data to test the accuracy of geoelectric field estimates, calculated using the magnetic field data and models of the ground conductivity structure, for space weather applications. In this work, using 5 years of geoelectric field recordings, I investigate the effects of strongly periodic phenomena, revealing signals with solar and lunar origins. Firstly, we use Superposed Epoch Analysis at each station. I find the results for Hartland observatory (Devon) are consistent with the findings of previous experiments in the English Channel, with regard to the magnitude of solar and lunar semi-diurnal (S2 and M2) variations and offer the first results on the tidal influence at Eskdalemuir (Scottish Borders) and Lerwick (Shetland Islands) observatories. Secondly, I employ frequency-domain techniques to find the dominant frequencies present in the geoelectric field data, again identifying well-known solar quiet-time variations (Sq) and its harmonics caused by ionospheric current systems and the dominant motion-induced M2 (lunar) tidal period at each station. Since there are difficulties in carrying out conventional Fourier analysis because of gaps in the data, I experiment using Lomb-Scargle Periodogram to investigate the frequency spectrum, as this method is designed to cope with unevenly spaced time-series data. I investigate the relationship between the North-South and East-West geoelectric field components and tidal records from the closest gauge stations to each site using time-domain and frequency- domain techniques. I also attempt to remove low-frequency component of the geoelectric field variations using Empirical Mode Decomposition (EMD) to isolate the externally induced signal. The outputs of this project include clean-up geoelectric field time-series and methodology for removing the tidal signals in geoelectric field data to isolate the space weather influence.