Significant geoelectric fields are produced by the interaction of rapidly varying magnetic fields with the conductive Earth, particularly during intense geomagnetic activity. Though usually harmless, large or sustained geoelectric fields can damage grounded infrastructure such as high-voltage transformers and pipelines via Geomagnetically Induced Currents (GICs). A key aspect of understanding the effects of space weather on grounded infrastructure is through the spatial and temporal variation of the geoelectric field. Globally, there are few long-term monitoring sites of the geoelectric field, so in 2012 measurements of the horizontal surface field were started at Lerwick, Eskdalemuir and Hartland observatories in the UK. Between 2012 and 2020, the maximum value of the geoelectric field observed was around 1 V/km in Lerwick, 0.5 V/km in Eskdalemuir and 0.1 V/km in Hartland during the March 2015 storm. These long-term observations also allow comparisons with models of the geoelectric field to be made.  We use the measurements to compute magnetotelluric impedance transfer functions at each observatory for periods from 20 to 30,000 seconds. These are then used to predict the geoelectric field at the observatory sites during selected storm times that match the recorded fields very well (correlation around 0.9). We also compute geoelectric field values from a thin-sheet model of Britain, accounting for the diverse geological and bathymetric island setting. We find the thin-sheet model captures the peak and phase of the band-passed geoelectric field reasonably well, with linear correlation of around 0.4 in general. From these two modelling approaches, we generate geoelectric field values for historic storms (March 1989 and October 2003) and find the estimates of past peak geoelectric fields of up to 1.75 V/km in Eskdalemuir. However, evidence from high voltage transformer GIC measurements during these storms suggests these estimates are likely to represent an underestimate of the true value. p, li { white-space: pre-wrap; }

Magnetic observatory data are widely used in the derivation of time-varying magnetic field models, often in combination with satellite magnetic data, when available. Traditionally the definitive observatory results are used, the availability of which can often lag those of the satellite data by months and even years. The recently defined quasi-definitive observatory data type has been introduced to meet the need to provide observatory data suitable for use in field modeling in a more rapid time frame and for producing Level 2 products planned for the upcoming European Space Agency Swarm mission. A method for producing quasi-definitive data is presented and the essential steps described. To evaluate the method, provisional data published on a next day basis since 2000 are tested against definitive data at five INTERMAGNET observatories. The means and standard deviations of the differences between the candidate quasi-definitive and definitive data are within the accuracy of 5 nT set by INTERMAGNET. Since the tested data were published on-line on a next day basis, they also easily meet the INTERMAGNET requirement of availability within three months. These results demonstrate that prompt production of quasi-definitive data is possible for observatories that already perform to the standards set by INTERMAGNET.

Abstract. The oil industry uses geomagnetic field information to aid directional drilling operations when drilling for oil and gas offshore. These operations involve continuous monitoring of the azimuth and inclination of the well path to ensure the target is reached and, for safety reasons, to avoid collisions with existing wells. Although the most accurate method of achieving this is through a gyroscopic survey, this can be time consuming and expensive. An alternative method is a magnetic survey, where measurements while drilling (MWD) are made along the well by magnetometers housed in a tool within the drill string. These MWD magnetic surveys require estimates of the Earth's magnetic field at the drilling location to correct the downhole magnetometer readings. The most accurate corrections are obtained if all sources of the Earth's magnetic field are considered. Estimates of the main field generated in the core and the local crustal field can be obtained using mathematical models derived from suitable data sets. In order to quantify the external field, an analysis of UK observatory data from 1983 to 2004 has been carried out. By accounting for the external field, the directional error associated with estimated field values at a mid-latitude oil well (55 • N) in the North Sea is shown to be reduced by the order of 20%. This improvement varies with latitude, local time, season and phase of the geomagnetic activity cycle. By accounting for all sources of the field, using a technique called Interpolation In-Field Referencing (IIFR), directional drillers have access to data from a "virtual" magnetic observatory at the drill site. This leads to an error reduction in positional accuracy that is close to matching that of the gyroscopic survey method and provides a valuable independent technique for quality control purposes.

In this study, using one-minute definitive data published by a number of INTERMAGNET observatories, we apply a number of time-and frequency-domain techniques to characterise the global, natural geomagnetic signal and isolate the artificial noise at an individual observatory. With the aim of developing an analytical tool that can be used to identify observatory noise against the natural signal, we report on the suitability of these techniques to detect common observatory noise types.