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

Separation of oceanic and continental crustal field signatures using Slepian functions

Models of the crustal magnetic field are typically represented using spherical harmonic coefficients. Rather than spherical harmonics, spherical Slepian functions can be employed to produce a locally and also globally orthogonal basis in which to optimally represent the available data in a region at a given degree. The region can have any arbitrary shape and size. The Slepian functions can be tailored to be either band- or space-limited, allowing a trade-off between spectral and spatial concentration in the region and leakage beyond. Another advantage is that only N Slepian coefficients are required to be solved for to optimally concentrate the energy of the Slepian functions into the region of interest (N = (L+1)2R/4π ; where N is the Shannon Number and R is the size of the region as a fraction of the full sphere) . We use N Slepian functions to optimally separate a crustal field model into its oceanic and continental regions in order to investigate the spectral content of each. Spherical harmonic coefficients are transformed into Slepian coefficients, separated into the appropriate regions and transformed back to spherical harmonic coefficients representing the space-limited extent of the oceans and continents. The spectral power of each region is examined over degrees L = 16-72. We show that both regions display different power levels at discrete bandwidths. For example, the oceanic signal dominates at degrees 16-30, while the continental signal is stronger at degrees 45-65. We compare different crustal models to illustrate that the derived signals are robust

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

Use of Swarm gradient field data to improve lithospheric field models

The Swarm mission, launched in November 2013, consists of three identical satellites designed to measure the magnetic field to the highest resolution ever. One of the Swarm mission's unique aspects is the ability to measure the magnetic field at approximately the same location using two satellites (Swarm A and Swarm C) which travel close to one another at the same altitude. Using measurements of the gradient of the field between the satellites (i.e. across-track) removes much of the external magnetic field's influence in the data, leaving the contribution from the steady internal field, each time the satellites pass over the same location [Ref. 1]. In combination with measurements of the satellites’ along-track differences this adds a new capability that can be exploited to produce models of the crustal magnetic field with higher accuracy than ever before. As the mission accumulates more data at lower altitudes, our understanding of the Earth’s magnetic field will continue to improve. We apply a Slepian decomposition technique to the new BGS lithospheric field model to analyse the relative contributions to the magnetic field from the ocean and continents, which may be useful for geological applications

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

Using Ensemble KalmanFiltering to improve magnetic field models during vector satellite data ‘gaps’?

Kalmanfiltering can be used to combine data optimally from different sources assuming that the error or variance of each data type is suitably understood. Typically a physical model is combined with occasional real measurements. Ensemble KalmanFilters (EnKF) extend this idea by making multiple simulations with randomly perturbed models drawn from probability distribution of fixed variance. Here we use EnKFto combine steady core surface flow models of the fluid outer core with magnetic field models derived from periods when no vector satellite data were available. We test if there is an optimal combination of flow and field that minimises the overall root-mean-square misfit to a ‘true’ magnetic field calculated after the resumption of satellite vector measurements

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

Real-time geomagnetic data from a Raspberry Pi magnetometer network in the UK

In 2014, BGS and the University of Lancaster won an STFC Public Engagement grant to build and deploy 10 Raspberry Pi magnetometers to secondary schools across the UK. The system uses a Raspberry Pi computer as a logging and data transfer device, connected to a set of three orthogonal miniature fluxgate magnetometers. The system has a nominal sensitivity of around 1 nanoTesla (nT), in each component direction (North, East and Down). This is around twenty times less sensitive than a current scientific-level instrument, but given its relatively low-cost, at about £250 per unit, this is an excellent price-to-performance ratio given we could not improve the sensitivity unless we spent a lot more money. The magnetic data are sampled at a 5 second cadence and sent to the AuroraWatch website at Lancaster University every 2 minutes. The data are freely available to view and download. The primary aim of the project is to encourage students from 14-18 years old to look at how sensors can be used to collect geophysical data and integrate it together to give a wider understanding of physical phenomena. A second aim is to provide useful data on the spatial variation of the magnetic field for analysis of geomagnetic storms, alongside data from the BGS observatory and University of Lancaster’s SAMNET variometer network. We show results from the build, testing and running of the sensors including some recent storms and we reflect on our experiences in engaging schools and the general public with information about the magnetic field. The information to build the system and logging and analysis software for the Raspberry Pi is all freely available

Geophysical signals in the 3-50 Hz ELF band

The British Geological Survey have been operating two high-frequency induction coil magnetometers (100 Hz cadence) at their observatory in Eskdalemuir (Scottish Borders) [54.3°N; 3.2°W] since Jun 2012. They have an approximate sensitivity of 0.07 pTat 0.1—50 Hz. The geophysical signals within the dataset encompass the Schumann Resonances (SR) generated by energy from equatorial lightning refracting in the Earth-ionosphere cavity [Ref. 1], the Ionospheric Alfvén Resonances (IAR) of the upper ionosphere cavity and magnetospheric pulsations (0.001-10 Hz) triggered by geomagnetic storms. We can also observe several subharmonics of the UK 50 Hz power grid. On occasion, local lightning strikes can also be observed directly in the data. These signals range in amplitude from 0.5 pT for the IAR to 5 pT for the SR and over 250 pTfor lightning strikes. In this poster, we give examples of spectrograms showing each type of geophysical signal and describe their occurrence statistics and typical amplitude ranges