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A variety of magnetosphere‐ionosphere current systems and waves have been linked to geomagnetic disturbance (GMD) and geomagnetically induced currents (GIC). However, since many location‐specific factors control GMD and GIC intensity, it is often unclear what mechanisms generate the largest GMD and GIC in different locations. We address this challenge through analysis of multi‐satellite measurements and globally distributed magnetometer and GIC measurements. We find embedded within the magnetic cloud of the 23–24 April 2023 coronal mass ejection (CME) storm there was a global scale density pulse lasting for 10–20 min with compression ratio of ∼10 ${\sim} 10$. It caused substantial dayside displacements of the bow shock and magnetopause, changes of 6RE $6{R}_{E}$ and 1.3−2RE $1.3-2{R}_{E}$, respectively, which in turn caused large amplitude GMD in the magnetosphere and on the ground across a wide local time range. At the time this global GMD was observed, GIC measured in New Zealand, Finland, Canada, and the United States were observed. The GIC were comparable (within factors of 2–2.5) to the largest ever recorded during ≥ ${\ge} $14 year monitoring intervals in New Zealand and Finland and represented ∼ ${\sim} $2‐year maxima in the United States during a period with several Kp≥ ${\ge} $7 geomagnetic storms. Additionally, the GIC measurements in the USA and other mid‐latitude locations exhibited wave‐like fluctuations with 1–2 min period. This work suggests that large density pulses in CME should be considered an important driver of large amplitude, global GMD and among the largest GIC at mid‐latitude locations, and that sampling intervals ≤10s ${\le} 10s$ are required to capture these GMD/GIC. Plain Language Summary: We explore how disturbances in the Earth's magnetic field, known as geomagnetic disturbances (GMD), and the resulting geomagnetically induced currents (GIC) in power systems are influenced by different electrical currents and waves in near‐Earth space. One challenge is the lack of easily accessible data on GIC over long periods, which makes it hard to figure out what factors are most responsible for changes in GIC in different places. Also, there is limited research combining data from satellites with data collected on the ground to figure out exactly how GMD and GIC are generated. To tackle these issues, we looked at data collected by multiple satellites in different parts of near‐Earth space along with data from ground magnetometers and GIC measurements distributed around the world. Our results suggest that density pulses from coronal mass ejections, a particular type of structure in the solar wind, are important in causing significant disturbances in the Earth's magnetic field globally and contribute to some of the largest GIC seen in the mid‐latitude region of United States. We emphasize the importance of taking measurements with high sampling rates (≤10s) $(\le 10s)$ to accurately capture these disturbances and the resulting GIC. Key Points: A density pulse embedded in a coronal mass ejection drives global geomagnetic disturbances (GMD) and geomagnetically induced currents (GIC)Measured GIC's comparable to or exceed reference values in several regions, including 58.1 A in the mid‐latitude region of United StatesLarge‐amplitude density pulses are an important driver of GIC and GMD for mid‐latitude regions with large populations [ABSTRACT FROM AUTHOR]

Large geomagnetic storms are a space weather hazard to power transmission networks due to the effects of Geomagnetically Induced Currents (GICs). GIC can negatively impact power transmission systems through the generation of even‐order current and voltage harmonics due to half‐cycle transformer saturation. This study investigates a decade of even‐order voltage total harmonic distortion (hereon referred to as Even‐Order Total Harmonic Distortion (ETHD)) observations provided by Transpower New Zealand Ltd., the national system operator. We make use of ETHD measurements at 139 locations throughout New Zealand, monitored at 377 separate circuit breakers, focusing on 10 large geomagnetic disturbances during the period 2013–2023. Analysis identified 5 key substations, which appeared to act as sources of ETHD. The majority of these substations include single phase transformer banks, and evidence of significant GIC magnitudes. The ETHD from the source substations was found to propagate into the surrounding network, with the percentage distortion typically decaying away over distances of 150–200 km locally, that is, at a rate of −0.0043 %km−1. During the study period some significant changes occurred in the power network, that is, removal of the Halfway Bush (HWB) single phase bank transformer T4 in November 2017, and decommissioning of the New Plymouth substation in December 2019. Decommissioning of these two assets resulted in less ETHD occurring in the surrounding regions during subsequent geomagnetic storms. However, ETHD still increased at HWB with increasing levels of GIC, indicating that three phase transformer units were still susceptible to saturation, albeit with about 1/3 of the ETHD percentage exhibited by single phase transformers. Plain Language Summary: Space weather, triggered by solar storms, can lead to significant variations in the Earth's magnetic field. The magnetic variations are termed geomagnetic disturbances. Changing magnetic fields induce electric fields at the Earth's surface, causing unwanted currents to flow in electrical power transmission networks. These Geomagnetically Induced Currents (GICs), pose a hazard to network operations as they can cause networks to become destabilized, leading to blackouts and damage to transformers. One negative impact of GIC in a power network is the production of harmonics of the power transmission frequency (typically 50 or 60 Hz); such harmonics contributed to the blackout of the Québec power system in March 1989. In our paper we study harmonic distortion occurrence in the New Zealand power grid using measurements from 139 substations across the country during 10 geomagnetic storms. Analysis identified 5 key substations, which appeared to act as sources of harmonic distortion. The majority of these substations include a particular transformer design which is known to be more susceptible to GIC issues, as well as evidence of significant GIC. Key Points: A decade of even‐order harmonic distortion data are used to investigate the impact of space weather on a power networkIn New Zealand four key substations containing single phase transformers act as sources of enhanced harmonic distortionThe harmonic distortion is found to propagate into the nearby power network over 150–200 km distances, decaying at a rate of −0.0043 %km−1 [ABSTRACT FROM AUTHOR]

Past studies found that large‐amplitude geomagnetically induced current (GIC) related to magnetospheric Ultra Low Frequency (ULF) waves tend to be associated with periods >120 s at magnetic latitudes >60°, with comparatively (a) smaller GIC amplitudes at lower latitudes and shorter wave periods and (b) fewer reports of waves associated with GIC at lower latitudes. ULF wave periods generally decrease with decreasing latitude; thus, we examine whether these trends might be due, in part, to the undersampling of ULF wave fields in commonly available measurements with 60 s sampling intervals. We use geomagnetic field (B), geoelectric field (E), and GIC measurements with 0.5–10 s sampling intervals during the 29–31 October 2003 geomagnetic storm to show that waves with periods <∼120 s were present during times with the largest amplitude E and GIC variations. These waves contributed to roughly half the maximum E and GIC values, including during times with the maximum GIC values reported over a 14‐year monitoring interval in New Zealand. The undersampling of wave periods <120 s in 60 s measurements can preclude identification of the cause of the GIC during some time intervals. These results indicate (a) ULF waves with periods ≤120 s are an important contributor to large amplitude GIC variations, (b) the use of 0.1–1.0 Hz sampling rates reveals their contributions to B, E, and GIC, and (c) these waves' contributions are likely strongest at magnetic latitudes <60° where ULF waves often have periods <120 s. Plain Language Summary: Geomagnetic field variation can induce geoelectric fields in the Earth that interfere with the operation of grounded electric power transmission systems. There are many sources of geomagnetic field variation, including plasma waves in the near‐Earth space environment. Past studies have found that wave periods between 120 and 600 s in high latitude regions led to the largest electrical currents in power systems. The waves responsible for driving these electrical currents have periods that decrease with decreasing latitude. We examine whether these trends may be partly due to the wide use of measurements with sampling rates that cannot resolve wave periods below 120 s. We use measurements sampled every 0.5–10 s during a major geomagnetic storm to show that wave periods shorter than 120 s can drive large amplitude geoelectric field and electrical current variations. These results suggest (a) waves with periods <120 s are an important contributor to electrical currents in power systems, (b) the use of sampling intervals of 1–10 s reveals their contributions, and (c) these waves' contributions are likely strongest at middle and low latitude regions where wave periods are often <120 s. Key Points: Waves with periods <∼2 min occur at magnetic latitudes <60° throughout the 29–31 October 2003 stormThe waves drive or contribute significantly to geomagnetically induced currents that are comparable to 14‐year peak values0.1–1.0 Hz sampling rates are required to determine wave contributions to currents at magnetic latitudes <60° [ABSTRACT FROM AUTHOR]

High 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. Key Points: High time resolution magnetic field, geomagnetically induced currents, and mains harmonic distortion data are analyzed for three case eventsObserved delays of 20–128 s between magnetic field and induced currents, but no delay between induced current and harmonic distortionEnhanced spectral power in induced current at frequencies <0.003 Hz (period >5 min) are required to produce harmonic distortion [ABSTRACT FROM AUTHOR]

Large geomagnetic storms are a known space weather hazard to power transmission networks due to the effects of geomagnetically induced currents (GICs). However, research in this area has been hampered by a lack of GIC observations. Previous studies have noted that New Zealand is unusually fortunate in having a comparatively dense, high quality, set of GIC measurements, spanning >60 transformers in >20 substations. However, due to operational reasons these observations are clustered in the mid and lower South Island. In this paper we analyze space weather‐induced GIC impact patterns over the entire country by using a different set of sensors that monitor levels of harmonic distortion, with even and odd harmonics measured separately. GICs lead to half cycle transformer saturation and is one of the few ways in which even harmonics are produced in a well‐run power transmission network. We make use of harmonic distortion measurements at 377 circuit breakers made at 126 separate locations. Focusing on the intense geomagnetic storm activity during 6–9 September 2017, we show how the even harmonic distortion observations provide a useful new picture of GIC‐stressed transformers. These observations demonstrate how GIC effects can be monitored by using even harmonic distortion in locations where no GIC measurements are present (e.g., the most of the North Island). We understand that harmonic distortion measurements are fairly common in electrical networks and could provide a new tool for space weather researchers. Key Points: Harmonic distortion measurements made at 126 locations in New Zealand are used to locate stressed transformers during geomagnetic stormsGeomagnetically induced current effects on transformers can be identified through commonly measured even harmonic distortion dataEven harmonic distortion is observed in unexpectedly low latitude locations during the 6–9 September 2017 geomagnetic storm period [ABSTRACT FROM AUTHOR]

Abstract: Several periods of geomagnetically induced currents (GICs) were detected in the Halfway Bush substation in Dunedin, South Island, New Zealand, as a result of intense geomagnetic storm activity during 6 to 9 September 2017. Unprecedented data coverage from a unique combination of instrumentation is analyzed, that is, measurements of GIC on the single‐phase bank transformer T4 located within the substation, nearby magnetic field perturbation measurements, very low frequency (VLF) wideband measurements detecting the presence of power system harmonics, and high‐voltage harmonic distortion measurements. Two solar wind shocks occurred within 25 hr, generating four distinct periods of GIC. Two of the GIC events were associated with the arrival of the shocks themselves. These generated large but short‐lived GIC effects that resulted in no observable harmonic generation. Nearby and more distant magnetometers showed good agreement in measuring these global‐scale magnetic field perturbations. However, two subsequent longer‐lasting GIC periods, up to 30 min in duration, generated harmonics detected by the VLF receiver systems, when GIC levels continuously exceeded 15 A in T4. Nearby and more distant magnetometers showed differences in their measurements of the magnetic field perturbations at these times, suggesting the influence of small‐scale ionospheric current structures close to Dunedin. VLF receiver systems picked up harmonics from the substation, up to the 30th harmonic, consistent with observed high‐voltage increases in even harmonic distortion, along with small decreases in odd harmonic distortion. [ABSTRACT FROM AUTHOR]

Geomagnetically induced current (GIC) observations made in New Zealand over 14 years show induction effects associated with a rapidly varying horizontal magnetic field (d BH/d t) during geomagnetic storms. This study analyzes the GIC observations in order to estimate the impact of extreme storms as a hazard to the power system in New Zealand. Analysis is undertaken of GIC in transformer number six in Islington, Christchurch (ISL M6), which had the highest observed currents during the 6 November 2001 storm. Using previously published values of 3,000 nT/min as a representation of an extreme storm with 100 year return period, induced currents of ~455 A were estimated for Islington (with the 95% confidence interval range being ~155-605 A). For 200 year return periods using 5,000 nT/min, current estimates reach ~755 A (confidence interval range 155-910 A). GIC measurements from the much shorter data set collected at transformer number 4 in Halfway Bush, Dunedin, (HWB T4), found induced currents to be consistently a factor of 3 higher than at Islington, suggesting equivalent extreme storm effects of ~460-1,815 A (100 year return) and ~460-2,720 A (200 year return). An estimate was undertaken of likely failure levels for single-phase transformers, such as HWB T4 when it failed during the 6 November 2001 geomagnetic storm, identifying that induced currents of ~100 A can put such transformer types at risk of damage. Detailed modeling of the New Zealand power system is therefore required to put this regional analysis into a global context. [ABSTRACT FROM AUTHOR]

Transpower New Zealand Limited has measured DC currents in transformer neutrals in the New Zealand electrical network at multiple South Island locations. Near-continuous archived DC current data exist since 2001, starting with 12 different substations and expanding from 2009 to include 17 substations. From 2001 to 2015 up to 58 individual transformers were simultaneously monitored. Primarily, the measurements were intended to monitor the impact of the high-voltage DC system linking the North and South Islands when it is operating in 'Earth return' mode. However, after correcting for Earth return operation, as described here, the New Zealand measurements provide an unusually long and spatially detailed set of geomagnetically induced current (GIC) measurements. We examine the peak GIC magnitudes observed from these observations during two large geomagnetic storms on 6 November 2001 and 2 October 2013. Currents of ~30-50 A are observed, depending on the measurement location. There are large spatial variations in the GIC observations over comparatively small distances, which likely depend upon network layout and ground conductivity. We then go on to examine the GIC in transformers throughout the South Island during more than 151 h of geomagnetic storm conditions. We compare the GIC to the various magnitude and rate of change components of the magnetic field. Our results show that there is a strong correlation between the magnitude of the GIC and the rate of change of the horizontal magnetic field ( H′). This correlation is particularly clear for transformers that show large GIC current during magnetic storms. [ABSTRACT FROM AUTHOR]