Changes In Geomagnetic Field Research Articles

Cracking the code of Earth's magnetic mysteries: ancient secrets unveiled by byzantine bricks reconfirmed by Mesopotamian ceramics

The study of the magnetization of artifacts discovered at archaeological sites, such as ceramics or hearth remnants, is known as archaeomagnetism. This technique is based on the idea that some materials get magnetized in the direction and intensity of the Earth's magnetic field at the time when heated or exposed to extreme heat. Development in the field has been made to improve magnetometer devices and sample size, as well as measuring techniques of thermal remanent magnetization. It helped to study the variation of the complex Earth’s magnetic field or geomagnetic field (GMF) for both geophysical and archaeological dating benefits. The rapid change in GMF within a few hundred or decades, as being of genuine origin or methodological and sample unsuitability has been a matter of debate. Data from about 40 years ago derived from well-dated Byzantine churches that first confirmed such geomagnetic spikes is discussed along with recent archaeomagnetic data from Mesopotamian inscribed bricks and Levantine slags.

Linkages Between Geomagnetic Activity and Blood Pressure.

This review aims to critically examine and presentevidence for and against potential linkages between geomagnetic activity and its effects on blood pressure (BP). Four databases were searched for peer-reviewed papers written in English: PubMed, Web of Science, EMBASE, and Biomedical Reference Collection. Retrieved titles were first screened for potential relevance followed by an abstract review for further clarifications if warranted. The preponderance of the reported evidence is consistent with the concept that space weather and related events that cause sufficiently large changes in the geomagnetic field (GMF) can impact BP. The associated BP change in most but not all cases is one in which both systolic blood pressure (SBP) and diastolic blood pressure increase, with SBP appearing to be more consistently involved. The magnitude of the reported BP increase ranges from about 3 to 8 mmHg depending on the intensity of the geomagnetic activity. The initiation of these BP changes has been variably reported to occur shortly before the GMF change or in synchrony with the abrupt change in the GMF. Such GMF-linked BP changes are not present in all persons and there appears to be increased sensitivity in women and in persons with co-existing hypertension. The utility of these findings in assessing or treating persons with known or suspected hypertension remains to be determined via future research. Further, research directed at determining the factors that determine responders from non-responders to GMF changes is warranted.

Simulated long-term evolution of the thermosphere during the Holocene – Part 2: Circulation and solar tides

Abstract. On timescales longer than the solar cycle, long-term changes in CO2 concentration and geomagnetic field have the potential to affect thermospheric dynamics. In this paper, we investigate the thermospheric dynamical response to these two factors during the Holocene, using two sets of ∼12 000-year control runs by the coupled thermosphere–ionosphere model, GCITEM-IGGCAS. The main results indicate that increased/decreased CO2 will enhance/weaken the thermospheric circulation throughout the Holocene, but this effect is non-linear. The cooling effect of CO2 in the thermosphere further provides plausible conditions for atmospheric tidal propagation and increases the thermospheric tidal amplitude. Geomagnetic variations induce hemispheric asymmetrical responses in the thermospheric circulation. Large changes in the circulation occur at high latitudes in the hemisphere with distant magnetic pole drift, inferring a crucial role of geomagnetic non-dipole variations in circulation changes. A positive correlation between the diurnal migrating tide (DW1) and geomagnetic dipole moment is revealed for the first time. The amplitude of DW1 in temperature will increase by ∼1–3 K for each 1×1022 A m2 increase in dipole moment.

Global Geomagnetic Field Evolution From 900 to 700 ka Including the Matuyama‐Brunhes Reversal

AbstractPolarity reversals and excursions are the most significant geomagnetic field changes generated in the liquid outer core of the Earth, therefore studying them helps understand geodynamo processes. This study examines the Matuyama‐Brunhes (MB) reversal using a new reconstruction of the global geomagnetic field based on paleomagnetic data, termed Global Geomagnetic Field Model for the MB reversal (GGFMB). GGFMB covers 900–700 ka, including late Matuyama and early Brunhes. This allows us to also investigate the Kamikatsura excursion (ca. 888 ka). The model is based on 38 high‐quality paleomagnetic sediment records with age control mostly independent of the magnetic signal. GGFMB suggests that the MB reversal began about ∼799 ka, when non‐dipole field components increased and the axial dipole component decreased. The transitional fields first appeared on Earth's surface in the high‐latitude southern hemisphere and equatorial regions. The minimum dipole strength was reached around 780 ka and the axial dipole changed sign. After ∼10 Kyr, the field stabilized in the normal polarity of the early Brunhes. The MB reversal lasted ∼29 Kyr (from 799 to 770 ka) and had slower rate of dipole decay than recovery as well as lower dipole moment for several millennia before than after the reversal. According to GGFMB, the dipole moment during the Kamikatsura excursion was approximately half that of the current field and it was a regional excursion observed only over eastern Asia and North America. Our sediment data collection is heavily biased toward the northern hemisphere, thus more southern hemisphere records are needed to demonstrate GGFMB's robustness in this region.

Eccentricity-paced geomagnetic field and monsoon rainfall variations over the last 870 kyr

Whether there are links between geomagnetic field and Earth's orbital parameters remains unclear. Synchronous reconstructions of parallel long-term quantitative geomagnetic field and climate change records are rare. Here, we present 10Be-derived changes of both geomagnetic field and Asian monsoon (AM) rainfall over the last 870kyr from the Xifeng loess-paleosol sequence on the central Chinese Loess Plateau. The10BeGM flux (a proxy for geomagnetic field-induced 10Be production rate) reveals 13consecutive geomagnetic excursions in the Brunhes chron, which are synchronized with the global records, providing key time markers for Chinese loess-paleosol sequences. The10Be-derived rainfall exhibits distinct ~100 kyr glacial-interglacial cycles, and superimposed precessional (~23 kyr) cycles that match with those in Chinese speleothem δ18O record. We find that changes in the geomagnetic field and AM rainfall share a common ~100 kyr cyclicity, implying a likely eccentricity modulation of both the geomagnetic field and climate.

On the Polar Bias in Ice Core 10Be Data

AbstractCosmogenic radionuclide records from polar ice cores provide unique insights into past cosmic ray flux variations. They allow reconstructions of past solar activity, space weather, and geomagnetic field changes, and provide insights into past carbon cycle changes. However, all these applications rely on the proportionality of the ice core radionuclide records to the global mean production rate changes. This premise has been long debated from a model and data‐perspective. Here, we address this issue through atmospheric mixing model experiments and comparison to independent data. We find that all mixing scenarios, which do not assume complete tropospheric mixing, result in a polar bias. This bias is more prominent for geomagnetic field changes than solar modulation changes. The most likely scenario, supported by independent geomagnetic field records and marine 10Be during the Laschamps geomagnetic field minimum, results in a dampening of geomagnetic field induced changes by 23%–37% and an enhancement of solar‐induced changes by 7%–8%. During the Holocene, we do not find conclusive evidence for a polar bias. We propose a correction function that allows deconvolving the glacial ice core record in order to restore proportionality to the global mean signal.

Characteristics of the Matuyama‐Brunhes Magnetic Field Reversal Based on a Global Data Compilation

AbstractMagnetic field reversals are irregular events in Earth's history when the geomagnetic field changes its polarity. Reversals are recorded by spot and continuous remanent magnetization data collected from lava flows and marine sediments, respectively. The latest field reversal, the Matuyama‐Brunhes reversal (MBR), is better covered by paleomagnetic data than prior field reversals, hence providing an opportunity to understand the physical mechanisms. Despite the quantity of data, a full understanding of the MBR is still lacking. The evolution of the MBR in time and space is explored in this work by compiling a global set of paleomagnetic data, both from sediments and volcanic rocks, which encompass the period 900–700 ka. After careful evaluation of data and dating quality, regional and global stacks of virtual axial dipole moment (VADM), virtual geomagnetic pole (VGP), and paleosecular variation index (Pi) are constructed from the sediment records using bootstrap resampling. Individual VADMs and VGPs calculated from lavas are compared to these stacks. Four phases of full‐vector field instability are observed in these stacks over the period 800–770 ka. The first three phases, observed at 800–785 ka, reflect a rapid weakening of the field coupled with low VGP latitude, after which the field returned to the reverse polarity of the Matuyama chron. The fourth phase, lasting from 780 to 770 ka, is when the field reversal process completed, such that the field entered the Brunhes normal polarity state. These findings point to a complex reversal process lasting ∼30 Kyr, with the reversal ending at ∼770 ka.

Reconstruction of the Virtual Geomagnetic Pole (VGP) path at high latitude for the last 22 kyr: The role of radial field flux patches as VGP attractor

Reconstruction of geomagnetic field changes has a strong potential to complement geodynamo modeling and improve the understanding of Earth's core dynamics. Recent works based on geomagnetic measurements pointed out that over the last two decades the position of the north magnetic pole has been largely determined by the influence of two competing flux lobes under Canada and Siberia.In order to understand if the waxing and waning of magnetic flux lobes have driven the path of geomagnetic paleopoles in the past, we present an augmented and updated record of the chronology and paleosecular variation of geomagnetic field for the last 22 kyr derived from sedimentary cores collected along the north-western margin of Barents Sea and western margin of Spitsbergen (Arctic). The path of the virtual geomagnetic pole (VGP) has been reconstructed over this time period and compared with the maps of the radial component of the geomagnetic field at the core-mantle boundary, obtained from the most recent models. The VGP path includes centuries during which the VGP position is stable and centuries during which its motion accelerates. We recognize both clockwise and counterclockwise VGP paths, mostly developing inside the surface projection of the inner core tangent cylinder in the Arctic region. The VGP path seems to follow the appearance of Br patches of normal magnetic flux, especially those located under Siberia and Canada areas, but also those that may cause peculiar paleomagnetic features such as the Levantine Iron Age Anomaly.

Post-Eruptive Persistent Cooling Beneath the Summit Crater of Usu Volcano as Revealed by Magnetic Repeat Surveys

Remarkable and continuous geomagnetic field change, suggesting remagnetization at a shallow depth, was detected through repeated geomagnetic field observation of the summit area of Mt. Usu Volcano from 2008 to 2021. Long-term cooling of the remnant magma, that intruded during the 1977–82 eruption, was considered responsible for the remagnetization. A magnetic dipole parallel to the present geomagnetic field well reproduced the observation. The modeled source was located near the Ginnuma crater on the south side of the previously inferred intrusive body beneath the Usu-Shinzan cryptodome. Meanwhile, no magnetic source was detected on the other side of the intrusion, implying asymmetric heat transport paths around the intrusion. Considering previous studies on seismicity, geodetic modeling, and resistivity structure, the magnetic source region is plausibly a high permeability zone through which heat from the intruded magma has been efficiently transported. In other words, the source region can be a key monitoring target for future eruptions as it may be linked to the subsurface magma system.

Multivariable statistical analysis between geomagnetic field, climate, and orbital periodicities over the last 500 KYR, and their relationships during the last interglacial

Relationships between geomagnetic field (GF) variations, paleoclimates, and Milankovitch cycles have increasingly attracted the attention of researchers mainly because of the assumption that GF may have acted as a climate forcing, suggesting that GF variations and orbital forcing may be linked in a complex synergistic way, especially, during interglacials and interstadials. We first performed an exhaustive analysis of the relationship between GF, a paleotemperature proxy, astroclimatic parameters, and the summer solstice mean daily insolation at 65 N over the last 500 kyr. We evaluated the relationship between the variables at the orbital scale, using wavelets, cross wavelets, and multiple cross wavelets. Our novel approach to the subject is the use of a new multiple cross wavelet algorithm, which allowed us to simultaneously analyze several data series and determine the relative phases between the signals. We confirm that the eccentricity modulates the GF behavior, and that the precession could have been related to some short-term GF variations. Both orbital parameters seem to be precursors of GF changes. Also, it is especially interesting to investigate the role of the GF reversion events because they could have acted as an indirect climate forcing, and we focused on the relationship among forcings during Marine Isotope Stage (MIS) 5. During this period, two GF reversions (Blake and post-Blake) could have been associated with relative climate cooling. The consequent lack or weakening of shielding associated with GF events and downward insolation, both influenced by orbital forcing, could finally have promoted a relative global climate cooling during MIS 5. • New multiple cross wavelets of RPI, δ 18 O, insolation, and orbital forcing. • GF is modulated by eccentricity in the trend, by precession in short-term changes. • Orbital forcing seems to be precursor of GF variations over the last 500 kyr. • Cooling is promoted by lows RPI and insolation, both influenced by orbital forcing.

Large Geomagnetically Induced Currents at Equator Caused by an Interplanetary Magnetic Cloud

AbstractHere, we report a rare observation of an extremely large and rapid change of geomagnetic field (dB/dt), a proxy for the geomagnetically induced currents (GICs), at the equator caused by a sudden drop in solar wind density at the front boundary of a magnetic cloud (MC) during the great 31 March 2001 storm. The horizontal component at the Indian equatorial station, Tirunelveli, recorded a sharp decline of ∼350 nT in just 5 min with a peak dB/dt exhibiting a concerning value of 136 nT/min, a possible GIC risk to the electric power systems. The responsible physical mechanisms were examined through magneto‐hydrodynamic model simulations and found that a prompt penetration of strong westward‐overshielding electric fields and ionospheric currents at the equator play a dominant role. This study provides some new insights into the extent of extreme geomagnetic field changes that can occur at the equatorial region due to solar wind density reduction at MC, which can have potential impacts on the electric power grid systems.

Detection of Periodic Disturbances in LOFAR Calibration Solutions

The Earth’s ionosphere is a highly variable medium on a wide range of spatio-temporal scales. The responsiveness of plasma to the geomagnetic field and its changes gives rise to anisotropy, which may introduce wave-like characteristics while scanning the ionosphere with a line-of-sight towards a radio source. Previous studies of LOw Frequency ARray (LOFAR) calibration phase solutions report that the estimated beta parameter of a structure function calculated over 6–8 h of astronomical observation timespan has a range of values from 1.6 to 2.0, with an average of 1.89. Such difference between the observations could result from transient wave-like disturbances within the data. This study aims to present a method of signal processing of ionospheric calibration datasets that allows the extraction of a transient wave-like signal and discuss its possible origin. We use complex Morlet wavelet analysis applied to two 8 h observations corresponding to very quiet geomagnetic conditions. We find a wave-like signal in the interferometric Total Electron Content data even during periods of no geomagnetic activity. We suggest it results from the relative velocity changes between the LOFAR line-of-sight and a convection pattern in the ionospheric F layer. Establishing the relationship between quiet time ionosphere, geomagnetic field changes and LOFAR’s calibration solutions may prove beneficial to determination of the dominant signals in the more disturbed conditions, which we leave for future study.

Study on Long-Term Variation Characteristics of Geomagnetic Cutoff Rigidities of Energetic Protons Caused by Long-Term Variation of Geomagnetic Field

As a parameter that quantitatively measures the shielding effect of the Earth’s magnetic field on energetic particles, the geomagnetic cutoff rigidities will change significantly with the long-term change of geomagnetic field. In this article, by using the numerical simulation method, combined with the variation trend of the Earth’s main field International Geomagnetic Reference Field (IGRF) model and the energetic particles’ windows (EPWs), the variation characteristics of the cutoff rigidities of geomagnetic energetic particles in the vertical direction from 1965 to 2025 were studied. The study found that there is a close relationship between the cutoff latitude corresponding to the cutoff rigidities and the background magnetic field intensity, but they are not in a simple linear relationship. The changes of the cutoff rigidities and geomagnetic field in the northern and southern hemispheres are asymmetric. The weakening of the geomagnetic field will push the cutoff latitudes toward the near equatorial region in the southern hemisphere, but for northern hemisphere the phenomenon is not always credible. In the northern hemisphere, the areas of EPWs will decrease at a relative rate of 0.03%/yr, but in the southern hemisphere, it will increase at a rate of about 0.05%/yr to 0.12%/yr. The long-term trajectory of the positions of the EPWs’ central points do not correspond to geomagnetic poles, nor the magnetic dip poles, relatively speaking, its position is closer to the geomagnetic poles. The study can be used for a long-term forecasting of cutoff rigidities caused by background geomagnetic field variations.

Satellite magnetic data reveal interannual waves in Earth’s core

SignificanceThe physics responsible for most of the interannual geomagnetic field changes, continually recorded by satellites for 20 years, is a long-standing open issue. By analyzing magnetic data, we detect Magneto-Coriolis waves in the Earth's outer core that account for a significant part of this signal. We further propose theoretical advances in the physical characterization of these waves, enabling a deeper understanding of the dynamics behind the geomagnetic signal. It should allow one to better sketch the heterogeneous magnetic field deep within the core, shedding further light on the mechanisms that sustain the geodynamo. Our interpretation does not require the presence of a stratified layer at the top of the core, with potent consequences regarding the Earth's thermal history.

Towards Understanding the Interconnection between Celestial Pole Motion and Earth's Magnetic Field Using Space Geodetic Techniques.

The understanding of forced temporal variations in celestial pole motion (CPM) could bring us significantly closer to meeting the accuracy goals pursued by the Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG), i.e., 1 mm accuracy and 0.1 mm/year stability on global scales in terms of the Earth orientation parameters. Besides astronomical forcing, CPM excitation depends on the processes in the fluid core and the core–mantle boundary. The same processes are responsible for the variations in the geomagnetic field (GMF). Several investigations were conducted during the last decade to find a possible interconnection of GMF changes with the length of day (LOD) variations. However, less attention was paid to the interdependence of the GMF changes and the CPM variations. This study uses the celestial pole offsets (CPO) time series obtained from very long baseline interferometry (VLBI) observations and data such as spherical harmonic coefficients, geomagnetic jerk, and magnetic field dipole moment from a state-of-the-art geomagnetic field model to explore the correlation between them. In this study, we use wavelet coherence analysis to compute the correspondence between the two non-stationary time series in the time–frequency domain. Our preliminary results reveal interesting common features in the CPM and GMF variations, which show the potential to improve the understanding of the GMF’s contribution to the Earth’s rotation. Special attention is given to the corresponding signal between FCN and GMF and potential time lags between geomagnetic jerks and rotational variations.

A comprehensive calibration method for non-orthogonal error and scale factor error of triaxial Helmholtz coil.

In order to calibrate the non-orthogonal error and scale factor error of triaxial Helmholtz coil, a comprehensive error calibration method is proposed in this paper. The comprehensive error model is established first, and then the method to generate a frequency-controllable spatial rotating magnetic field whose magnetic field intensity is constant based on triaxial Helmholtz coil is introduced. Based on the fact that the magnetic field intensity generated by the current through the coil is constant, the objective function is constructed and the error parameters are solved using the least-squares fitting method. Simulation experiments and field tests prove the effectiveness and feasibility of this method. The most obvious finding to emerge from this study is that the proposed method is not only suitable for the electromagnetic shielding room environment but also suitable for the field tests in the presence of a background geomagnetic field far away from urban electricity interference. The root mean square error value is reduced to 0.7 nT from 298.2 nT in the electromagnetic shielding room, and it is reduced to 0.8 nT from 299.0 nT in the wild. Compared to the two-step calibration method, the proposed method reduces the sampling time by controlling the frequency of the rotating magnetic field and reduces the influence of the changes of the background geomagnetic field on the experimental results. This method has better anti-interference ability. Because the modulus of the background geomagnetic field has no effect on this method, it gets rid of the dependence on large current or background geomagnetic field shielding devices. Comparative experiments and repetitive experiments verify the practicability and stability of this method.

International cooperation of S. I. Subbotin Institute Geophysics, NAS of Ukraine for 2010—2020

The geological structure of the lithosphere of the main tectonic structures has been refined for the territory of Ukraine and adjacent regions of Slovakia, Poland, Romania, Russia, as well as Bulgaria, the Antarctic Peninsula (West Antarctica) and Southeast Asia, and new data have been obtained on geophysical impacts that can affect the environment. A geodynamic scenario has been developed for the formation of large-scale folding of the Fore- Dobrudzja Trough, the South Ukrainian monocline and the Ingul block of the Ukrainian Shield, caused by tectonic events associated with the closing of the Paleotethys and Neotethys oceans in the Mesozoic. In the Pripyat-Dnieper-Donets Basin, the structure of the earth’s crust and upper mantle can reflect different intensities of rifting, from its passive stage in the Dnieper Graben to active rifting in the Pripyat Trough. An analysis of the geoelectric structure of the Earth’s crust in the Ukrainian Eastern Carpathians indicates that seismic events occur mainly in resistive solid rock domainswhich surrounded by aseismic high conductive zones consisting of at least partially melted material. The present-day mutual position of the Ukrainian shield and Fennoscandia stabilized 1720—1660 Ma. The age, distribution, orientation and composition have been studied for the LatePalaeoproterozoicdykes in the Volyn, Ingul and Azov blocks of the Ukrainian Shield. Eastern Crimea and the Sorokin Trough are fragments of a tectonic wedge formed after the Paleocene. The geothermal conditions of the Intra-Carpathian region are due to subduction during the closure of the Pannonian sea basin and the collisional interaction of the Eurasian plate with the microplates system of this region. In Bulgaria, most earthquakes occur outside high-resistive domains. The tectonic stages are reconstructed for the formation of the northern part of the Antarctic Peninsula in the Mesozoic-Cenozoic. The relationship has been established between the geomagnetic field and climate change, with it being different for the Northern and Southern Hemispheres. The results have been obtained within the framework of 25 international projects and 6 temporary international target teams of S. I. Subbotin Institute of Geophysics, NAS of Ukraine consisting of researchers from 23 countries. The results are presented in 53 publications, 38 of which are indexed in the Web of Scienct database, and 32 papers are published in 20 international journals and special publications of 10 countries with different impact factors (from 0,101 to 4,214), whose average impact factor is 3,341, and the total one is 66,815.

Investigation on longitudinal and decadal variations of the equatorial electrojet using a physical model

A detailed investigation on longitudinal and decadal variations of general characteristics (amplitude, width, peak altitude) of the equatorial electrojet (EEJ) is carried out using a physics-based model of electrojet. A few important aspects that emerge from this investigation include (i) changes in the half-width of EEJ with longitude and it’s dependence on the rate of change of dip-angle with latitude, (ii) changes in EEJ strength as large as 10–20% over the Peruvian and Brazilian sectors from 1960 to 2020 associated with significant changes in the local geomagnetic field, (iii) wave-4 structure in the EEJ current density and integrated EEJ current. In addition, it is shown that the peak EEJ current density over different longitudes inversely depends on the strength of geomagnetic field. More importantly, relationships of EEJ parameters (such as half-width, peak current density, integrated current) with ionospheric electric field, electron density, and geomagnetic field are brought out. These relationships are verified using EEJ observations carried out earlier using sounding rocket and ground based magnetometers. This comprehensive account of the longitudinal and decadal variations of EEJ along with the expressions to calculate EEJ characteristics can be used in observational and modelling studies to understand the low-latitude ionospheric electrodynamics in greater detail.

InSAR- and PIM-Based Inclined Goaf Determination for Illegal Mining Detection

The determination of the depth and boundary of the goaf is of great significance for the detection of illegal mining. However, determining the current location of unknown goafs mainly relies on low-efficiency, time-consuming, and labor-intensive physical detection methods such as geomagnetic field changes. Due to their large coverage and high degree of automation, research on remote sensing methods has been conducted to locate mining activities by monitoring surface deformation. This paper proposes a method that relies on the principle of the probability integration method (PIM) and on synthetic aperture radar interferometry (InSAR) to retrieve the location of an underground goaf. First, the relationship between ground subsidence and the location of the mined-out area was established according to PIM; then, the location of the mined-out area was obtained by the surface deformation acquired by InSAR. The proposed method does not rely on complex nonlinear models and has complete parameters; therefore, it has higher engineering application value. A test site in the Fengfeng mining area and 11 Radarsat-2 images were used to verify the proposed method. The experimental results showed that the average relative error of the proposed method is 6.35%, which is 27.56% higher than that of similar algorithms based on complex nonlinear models. Compared to algorithms that ignore the coal seam dip, the accuracy is improved to 98.27%.

Geomagnetically Induced Currents and Harmonic Distortion: High Time Resolution Case Studies

AbstractHigh time resolution (1–5 s) magnetometer, geomagnetically induced current (GIC), and mains harmonic distortion data from the Halfway Bush substation in Dunedin, New Zealand, are analyzed. A recently developed technique using very low frequency (VLF) radio wave data provides high‐resolution measurements of mains harmonic distortion levels. Three case studies are investigated, each involving high rates of change of local geomagnetic field, but with different timescales of magnetospheric driver mechanisms, and different substation transformer configurations. Two cases of enhanced GIC during substorm events are analyzed, and one case of a storm sudden commencement. Time delays between magnetic field fluctuations and induced transformer currents are found to be ~100 s for substorm events, but only ~20 s for the storm sudden commencement containing higher‐frequency variations. Boxcar averaging of the magnetic field fluctuations using running windows of ±2 min leads to spectral power profiles similar to those of GIC profiles, with reduced power at frequencies >0.003 Hz (periods <5 min). Substantially lower mains harmonic distortion levels were observed after the removal of the single phase bank transformer, HWB T4, from the high‐voltage configuration at Halfway Bush. No systematic time delay was found between GIC variations and mains harmonic distortion levels. The power spectra of magnetic field fluctuations and GIC variations during the sudden storm commencement with no harmonic distortion showed low levels of low‐frequency power (<0.003 Hz, periods >5 min). This low‐frequency component of the magnetic field power spectrum appears necessary for mains harmonic distortion to occur.