Analysis of the seasonal and solar effect on the vertical magnetic transfer function at Eskdalemuir Observatory, Scotland

SUMMARY Geomagnetic observations at Eskdalemuir observatory in Southern Scotland reveal reduced amplitudes in the vertical component variations compared with the horizontal components for periods of less than an hour. A subsurface high conductivity feature has previously been suggested to account for this anomaly. However, past studies have overlooked the effect of seasonal source changes and impact of solar activity on external geomagnetic field variations. The vertical magnetic transfer function — referred to as the tipper — relates temporal variations in the vertical magnetic field to those in the horizontal magnetic field and is sensitive to lateral electrical conductivity contrasts in the subsurface. Quantifying the seasonal variations in the tipper helps to identify times when external field variations minimally bias tipper estimates, thereby providing a more accurate representation of subsurface conductivity. Ionospheric current systems, particularly during geomagnetic storms, may violate the plane wave assumption underlying tipper estimation at mid-latitudes. This may allude to a more complex source geometry responsible for magnetic field variations. Our study quantifies and proposes a correction for space weather-driven external field contributions to observations for periods shorter than 1 hr. Using high-quality digital magnetic field data with a 1-min sampling rate from 2001 to 2019, we estimate the tipper at Eskdalemuir, revealing seasonal differences that increase with periods between 1000 s and 10 000 s. After finding that tipper estimates during the 2016 time-series are least affected by seasonal effects, we used 1-s time-series and a simple empirical model to quantify the daily variability of the tipper. The model consists of annual and semi-annual terms plus a term proportional to either the $F10.7$ cm solar flux or geomagnetic $A_p$ index. Neither model fits the data to within the expected error, but the model that uses $A_p$ has better fit. Tipper estimates from temporary site deployments are affected by these seasonal external variations, and we correct those obtained at sites near Eskdalemuir during a recent field experiment using this model.