Sensitivity of geomagnetically induced currents to varying auroral electrojet and conductivity models

Geomagnetically induced currents (GIC) are created by the interaction of rapid changes in the magnitude of the magnetic field with the conductive subsurface of the Earth. The changing magnetic field induces electric currents, which are particularly strong along boundaries between regions of contrasting conductivity structure such as the land and sea. A technique known as the ‘thin-sheet approximation’ can be used to determine the electric field at the Earth’s surface, which in turn allows the calculation of GIC in the earthing connections of high-voltage nodes within a power grid. The thin-sheet approximation uses a spatially varying conductance over the region of interest on a 2D surface, combined with a 1D layered model of upper lithosphere conductance. We produce synthetic models of the auroral electrojet in different locations over the United Kingdom (UK) and investigate the effects of varying the 2D thin-sheet model. We assess different two-dimensional surface conductance models and vary the underlying 1D conductivity models to simulate the effects of resistant through to conductive lithosphere. With an advanced network model of high-voltage electrical distribution grid, we compute the expected GIC at each node in the system given the input surface electric fields from the various synthetic electrojets and conductivity models. We find that the electrojet location is the primary control on the size of GIC, with conductivity being a second-order effect in general, though it can be locally important

Investigation of global lightning using Schumann resonances measured by high frequency induction coil magnetometers in the UK

In June 2012, the British Geological Survey (BGS) Geomagnetism team installed two high frequency (100 Hz) induction coil magnetometers at the Eskdalemuir Observatory, in the Scottish Borders of the United Kingdom. The induction coils permit us to measure the very rapid changes of the magnetic field in the Extremely Low Frequency (ELF) range in a passband from around 0.1 Hz to 100 Hz. The Eskdalemuir Observatory is one of the longest running geophysical monitoring sites in the UK (in operation since 1904) and is located in a rural valley with a quiet magnetic environment. BGS intend the coils to become part of our long term scientific monitoring of the magnetic field, in this case for ionospheric and agnetospheric research. The data are freely available on request and we are interested in collaboration with other institutes and researchers

Spectral analysis of regional main field and secular variation in CHAOS-4 using spherical Slepian functions

Magnetic models such as CHAOS-4 represent the global field using Spherical Harmonic (SH) functions weighted by a set of numbers known as Gauss coefficients. This representation allows values of the field to be calculated at any location and altitude above the core-mantle boundary, but has limitations when attempting to isolate the contribution to the field from specific areas or regions. Spherical Slepian functions provide an alternative mathematical basis to represent the field [Ref. 1]. They have the advantage of allowing an area of interest to be optimally described in a spatio-spectral sense. In addition, spherical Slepian functions can also be used to separate and decompose the Gauss coefficients from a SH magnetic field model into the components that represent the contribution to the model from individual regions of the globe [Ref. 2]. We investigate the spectral and spatial changes of the main magnetic field of CHAOS-4q [Ref. 3] at the Earth's surface between spherical harmonic degrees 12-35 in eight different regions across the globe: the Americas; Africa; Australia; Eurasia; Antarctica; the Pacific Ocean; the Atlantic Ocean and the Indian Ocean between 1997 and 2011

Efficient analysis and representation of geophysical processes using localized spherical basis functions

While many geological and geophysical processes such as the melting of icecaps, the magnetic expression of bodies emplaced in the Earth's crust, or the surface displacement remaining after large earthquakes are spatially localized, many of these naturally admit spectral representations, or they may need to be extracted from data collected globally, e.g. by satellites that circumnavigate the Earth. Wavelets are often used to study such nonstationary processes. On the sphere, however, many of the known constructions are somewhat limited. And in particular, the notion of `dilation' is hard to reconcile with the concept of a geological region with fixed boundaries being responsible for generating the signals to be analyzed. Here, we build on our previous work on localized spherical analysis using an approach that is firmly rooted in spherical harmonics. We construct, by quadratic optimization, a set of bandlimited functions that have the majority of their energy concentrated in an arbitrary subdomain of the unit sphere. The `spherical Slepian basis' that results provides a convenient way for the analysis and representation of geophysical signals, as we show by example. We highlight the connections to sparsity by showing that many geophysical processes are sparse in the Slepian basis.Comment: To appear in the Proceedings of the SPIE, as part of the Wavelets XIII conference in San Diego, August 200

Evidence‑based uncertainty estimates for the International Geomagnetic Reference Field

The International Geomagnetic Reference Field (IGRF) is a multi-institute model of the Earth’s magnetic field, compactly described by sets of up to 195 spherical harmonic (Gauss) coefficients to degree and order 13, which allows the continuous evaluation of the field at any location and time on or above the surface. It is developed from satellite and ground-based magnetometer data and describes the large-scale variation of the magnetic field in space and time under quiet conditions. While much effort has been made on improving the forecast of the secular variation of the field over the 5-year intervals between release and renewal, less emphasis has been placed on understanding the spatial errors from a user point of view. In this study, we estimate the large-scale time-invariant spatial uncertainty of the IGRF based on the globally averaged misfit of the model to ground-based measurements at repeat stations and observatories between 1980 and 2021. As the ground measurements are reduced to quiet-time values, the external field is minimized for the purposes of this study. We find the 68.3% confidence interval is 87 nT in the North (X) component, 73 nT in the East (Y) component and 114 nT in vertical (Z) component. Due to the Laplacian distribution of the residuals, the standard deviations are larger at 144, 136 and 293 nT, respectively

The BGS magnetic field candidate models for the 12th generation IGRF

We describe the candidate models submitted by the British Geological Survey for the 12th generation International Geomagnetic Reference Field. These models are extracted from a spherical harmonic ‘parent model’ derived from vector and scalar magnetic field data from satellite and observatory sources. These data cover the period 2009.0 to 2014.7 and include measurements from the recently launched European Space Agency (ESA) Swarm satellite constellation. The parent model’s internal field time dependence for degrees 1 to 13 is represented by order 6 B-splines with knots at yearly intervals. The parent model’s degree 1 external field time dependence is described by periodic functions for the annual and semi-annual signals and by dependence on the 20-min Vector Magnetic Disturbance index. Signals induced by these external fields are also parameterized. Satellite data are weighted by spatial density and by two different noise estimators: (a) by standard deviation along segments of the satellite track and (b) a larger-scale noise estimator defined in terms of a measure of vector activity at the geographically closest magnetic observatories to the sample point. Forecasting of the magnetic field secular variation beyond the span of data is by advection of the main field using core surface flows

Building a Raspberry Pi school magnetometer network in the UK

As computing and geophysical sensor components have become increasingly affordable over the past decade, it is now possible to design and build a cost-effective system for monitoring the Earth’s natural magnetic field variations, in particular for space weather events. Modern fluxgate magnetometers are sensitive down to the sub-nanotesla (nT) level, which far exceeds the level of accuracy required to detect very small variations of the external magnetic field. When the popular Raspberry Pi single-board computer is combined with a suitable digitiser it can be used as a low-cost data logger. We adapted off-the-shelf components to design a magnetometer system for schools and developed bespoke Python software to build a network of low-cost magnetometers across the UK. We describe the system and software and how it was deployed to schools around the UK. In addition, we show the results recorded by the system from one of the largest geomagnetic storms of the current solar cycle

Automatic detection of ionospheric Alfvén Resonances using signal and image processing techniques

Ionospheric Alfvén Resonances (IAR) are a type of ELF wave phenomenon in the 0.5-10 Hz frequency range and are detectable in magnetic field variations as measured by induction coil magnetometers. They are believed to be excited by electric fields from lightning strikes leaking from the troposphere into the ionosphere, causing the oscillation of magnetic field lines through the upper atmosphere. In September 2012, the British Geological Survey installed two orthogonal coils in Eskdalemuir in the Scottish Borders. The coils reliably record variations of the magnetic field in the frequency band from 0.1 - 100 Hz in the north-south and east-west components. IAR appear in the data almost on a daily basis. Investigation of the IAR events in the induction coil data show that they appear as a series of fringes in spectrograms (i.e. plots of power at time versus frequency) between 0.5 and 7 Hz, occasionally extending beyond the first Schumann resonance to 10 Hz. They typically appear after the onset of local night time, expanding in frequency width until midnight and fading a few hours before local dawn. There are usually between five and fifteen fringes. Their appearance varies with season – occurring most often in northern hemisphere winter. Given the complexity of the analysis and the volume of data, a new protocol for detecting and classifying key parameters was developed using image and signal processing techniques. In the first instance, individual spectra for time-series of 100 seconds are analysed for peak power in the frequencies between 0.5 and 10 Hz. Hence the frequency for each IAR is determined (i.e. the signal processing part). This generates an image of ‘spot’ frequencies for each IAR for each day. Next, a series of image processing filters are applied to the ‘spot’ images to connect the IAR occurrences together in a coherent manner. No manual intervention is required. The method picks out the IAR across an entire day of data, allowing statistics related to frequency (f), peak-to-peak frequency (∆f) and number of peaks etc. to be automatically determined from the entire dataset. We show the details and examples of the new method and the results from the first eighteen months of data

The International Geomagnetic Reference Field (IGRF) generation 12: BGS candidates and final models

The International Geomagnetic Reference Field (IGRF) model is a reference main field magnetic model updated on a quinquennial basis. The latest revision (generation 12) was released in January 2015. The IGRF-12 consists of a definitive model (DGRF2010) of the main field for 2010.0, a model for the field at 2015.0 (IGRF2015) and a prediction of secular variation (IGRF-12 SV) for the forthcoming five years until 2020.0. The remaining coefficients of IGRF-12 are unchanged from IGRF-11. Nine candidates were submitted from various international teams for consideration to the IGRF Taskforce led by Erwan Thebault (Nantes) and Chris Finlay (DTU Space). The final models were computed from all candidates using a Huber weighting in space scheme. In this poster, we outline the modelling steps for the three BGS candidate models and compare them to the other submitted candidates and the final official models released as IGRF-12

Merging fluxgate and induction coil data collected from Eskdalemuir geophysical observatory to produce low-noise, one-second data

This report describes the use of one-second MFS-07 induction coil and DMI FGE89J fluxgate magnetometer magnetic field data recorded at Eskdalemuir Geophysical Observatory in order to compute an "improved" one-second time series for frequencies higher than specified fluxgate operating band. The DMI instrument is a true one-minute system though it can produce one-second data which are relatively noisy in the 0.2 - 0.5 Hz band. The induction coils are sensitive for periods between 0.1 Hz and 50 Hz allowing their high frequency response to complement the low frequency to DC response of the DMI instrument. A numerical technique is adapted from published algorithms developed for merging one-second data recorded at Niemegk observatory in Germany. Our goal is to combine the long-term stability of the DMI fluxgate magnetometer with the low-noise of the induction magnetometer to capture the natural magnetic field for frequencies down to 0.5Hz. Magnetic field data with long-term stability in the low frequency end and also providing information at high frequencies are useful in the study of space weather storms. We have determined the improvement by examining the merged time series, computing the coherence and phase of the one-second merged data with that of the induction coil and fluxgate magnetometer data for the two horizontal components as well as plotting the spectrograms of fluxgate and merged frequencies of interest. We repeated our analysis using data from an instrument capable of meeting the INTERMAGNET one-second definitive data standard, the LEMI-025 fluxgate magnetometer, to compute a separate merged one-second time series as an independent check. We find that a high coherence exists between the two merged time series (LEMI-025/induction coil and FGE89J/induction coil), without phase difference. Spectrograms of the merged time series reveal micro-pulsations that are otherwise masked by the inherent noise from either fluxgate instruments at the frequency band where Pc1 pulsations occur (0.2-0.5Hz). The Pc1 pulsation in the one-second merged time series from Eskdalemuir observatory is also identified in the equivalent spectrogram of the Niemegk observatory from the same day. This proves that the numerical technique created to capture natural field variations in the 0.2-0.5Hz band can be applied effectively for this UK observatory