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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]

Measurements from six longitudinally separated magnetic observatories, all located close to the 53° mid-latitude contour, are analysed. We focus on the large geomagnetic disturbance that occurred during 7 and 8 September 2017. Combined with available geomagnetically induced current (GIC) data from two substations, each located near to a magnetic observatory, we investigate the magnetospheric drivers of the largest events. We analyse solar wind parameters combined with auroral electrojet indices to investigate the driving mechanisms. Six magnetic field disturbance events were observed at mid-latitudes with dH/dt > 60 nT/min. Co-located GIC measurements identified transformer currents >15 A during three of the events. The initial event was caused by a solar wind pressure pulse causing largest effects on the dayside, consistent with the rapid compression of the dayside geomagnetic field. Four of the events were caused by substorms. Variations in the Magnetic Local Time of the maximum effect of each substorm-driven event were apparent, with magnetic midnight, morning-side, and dusk-side events all occurring. The six events occurred over a period of almost 24 h, during which the solar wind remained elevated at >700 km s−1, indicating an extended time scale for potential GIC problems in electrical power networks following a sudden storm commencement. This work demonstrates the challenge of understanding the causes of ground-level magnetic field changes (and hence GIC magnitudes) for the global power industry. It also demonstrates the importance of magnetic local time and differing inner magnetospheric processes when considering the global hazard posed by GIC to power grids.