Observations of the interaction between Schumann and Ionospheric Alfvén Resonances

Long-term masurements of the high-frequency magnetic field (0.1-100 Hz) have been made at Eskdalemuir (ESK) Observatory (55.3°N, -3.2°E; L = 3.5) in the UK since September 2012. We analyse five years of dynamic spectrograms to examine the occurrence and behaviour of the Schumann (SR) and Ionospheric Alfvén Resonances (IAR), as well as Pc1-type pulsations. The resonances, observed as diffuse bands, arise from reflections of energy both within the earthionosphere cavity and from the non-linear conductivity gradient of the ionosphere. Schumann Resonances occur continuously but we find that IAR are observed to arise at local night time in ~50% of days in the ESK data. Typically, IAR are found at frequencies of 1-8 Hz, but we find them extending out to 30 Hz and strongly interacting with the first three Schumann Resonances around 9% of the time. These interactions include constructive and destructive interference, non-linear frequency changes over the span of several hours and polarity enhancements. In addition, the magnitude of the IAR does not decline rapidly with frequency as often proposed. We find the IAR and interactions with SR are strongly controlled by season and geomagnetic activity. As current theoretical models do not account for many of these observations, we suggest further work is needed to understand how and why they arise.