Explore the vast range of titles available.

Understanding global space weather effects is of great importance to the international scientific community, but more localized space weather predictions are important on a national level. In this study, data from a ground magnetometer at Valentia Observatory is used to characterize space weather effects on the island of Ireland. The horizontal component of magnetometer observations and its time derivative are considered, and extreme values of these are identified. These extremes are fit to a generalized extreme value distribution, and from this model return values (the expected magnitude of an observation within a given time window) are predicted. The causes of extreme values are investigated both in a case study, and also statistically by looking at contributions from geomagnetic storms, substorms, and sudden commencements. This work characterizes the extreme part of the distribution of space weather effects on Ireland (and at similar latitudes), and hence examines those space weather observations which are likely to have the greatest impact on susceptible technologies.

We need a typical method of directly measuring geomagnetically induced current (GIC) to compare data for estimating a potential risk of power grids caused by GIC. Here, we overview GIC measurement systems that have appeared in published papers, note necessary requirements, report on our equipment, and show several examples of our measurements in substations around Tokyo, Japan. Although they are located at middle latitudes, GICs associated with various geomagnetic disturbances are observed, such as storm sudden commencements (SSCs) or sudden impulses (SIs) caused by interplanetary shocks, geomagnetic storms including a storm caused by abrupt southward turning of strong interplanetary magnetic field (IMF) associated with a magnetic cloud, bay disturbances caused by high-latitude aurora activities, and geomagnetic variation caused by a solar flare called the solar flare effect (SFE). All these results suggest that GIC at middle latitudes is sensitive to the magnetospheric current (the magnetopause current, the ring current, and the field-aligned current) and also the ionospheric current.

The preliminary impulse of the sudden commencement is simply explained by the generation of the compressional wave due to sudden compression of the dayside magnetopause and mode conversion from the compressional wave to the Alfvén wave in the magnetosphere. However, this simple model cannot explain a time delay of the peak displacement and longer duration time in the higher latitudes in the pre-noon and post-noon sectors of the polar region. Based on the global magnetohydrodynamic simulation of the magnetosphere–ionosphere system reveals that this peculiar behavior of the geomagnetic variation of the preliminary impulse is associated with temporal deformation of the ionospheric field-aligned current distribution of the preliminary impulse into a crescent shape; its lower-latitude edge extends toward the anti-sunward direction, and its higher-latitude edge almost stays on the same longitude near noon. Numerical simulations revealed that the deformation of the field-aligned current distribution is derived from different behaviors of the two current systems of the preliminary impulse. The first current system consists of the field-aligned current connected to the field-aligned current of the preliminary impulse in the lower latitude side of the ionosphere, the cross-magnetopause current, and the magnetosheath current (type L current system). The cross-magnetopause current is the inertia current generated in the acceleration front of the solar wind due to the sudden compression of the magnetosheath. Thus, the longitudinal speed of the type L current system in the ionosphere is the solar wind speed in the magnetosheath projected into the ionosphere. In contrast, the current system of the preliminary impulse connected to the field-aligned current of the preliminary impulse at higher latitude (type H current system) consists of the upward/downward field-aligned current in the pre-noon/post-noon sector, respectively, and dawn-to-dusk field-perpendicular current along the dayside magnetopause. The dawn-to-dusk field-perpendicular current moves to the higher latitudes in the outer magnetosphere over time. The field-aligned current of the type H current system is converted from the field-perpendicular current due to convergence of the return field-perpendicular current heading toward the sunward direction in the outer magnetosphere; the return field-perpendicular current is the inertia current driven by the magnetospheric plasma flow associated with compression of the magnetopause behind the front region of the accelerated solar wind. The acceleration front spreads concentrically from the subsolar point. Consequently, as the return field-perpendicular current is converted to the field-aligned current of the type H current system, it does not move much in the longitudinal direction over time because the dawn-to-dusk field-perpendicular current of the type H current system moves to the higher latitudes. Therefore, the high-latitude edge of the current distribution of the preliminary impulse in the ionosphere moves only slightly. Finally, we clarified that the conversion between field-perpendicular current and field-aligned current of the type L current system mainly occurs in the region where the Alfvén speed starts to increase toward the Earth. A region with a steep gradient of the Alfvén speed like the plasmapause is not always necessary for conversion from the field-perpendicular current to the field-aligned current. We also suggest the possible field-aligned structure of the standing Alfvén wave that may occur in the preliminary impulse phase.

Solar variability can lead to significant disturbances, such as coronal mass ejections (CMEs). A CME impacting the Earth's magnetosphere often causes geomagnetic storms that affect not only the magnetosphere but also the ionosphere, the upper atmosphere, and even the ground. During extreme events, rapidly changing geomagnetic fields can create strong geomagnetically induced currents (GICs) at ground. These GICs can severely impact human technology, causing damage to high‐voltage power transformers and leading to power outages, as well as corrosion in oil and gas pipelines. On 10 May 2024, the most intense geomagnetic storm since the Halloween 2003 storm impacted Earth's environment, causing auroras to appear at much lower latitudes than usual in both the northern and southern hemispheres. This study investigates the effects of geomagnetically induced electric fields (GIEs), and hence GICs, during the sudden storm commencement (SSC) of the geomagnetic storm on 10 May 2024, over Europe, using the European quasi‐Meridional Magnetometer Array ground magnetometers. Despite the magnetometer array being placed in the late afternoon (18:00 LT), the combined influence of a strong solar wind dynamic pressure amplitude (P∼22nPa) $(P\\sim 22nPa)$ and a significant, long‐lasting southward interplanetary magnetic field (IMF) (Bz,IMF∼−25 ${B}_{z,IMF}\\sim -25$nT) resulted in strong SSC amplitudes (∼180 ${\\sim} 180$nT) at mid‐low latitudes (λ∼57° $\\lambda \\sim 57{}^{\\circ}$N). Results suggest that the CME‐driven shock inclination in the meridional plane leads to high GIE driven only at high latitudes. In addition, the decomposition of the SSC disturbance field at ground into ionospheric (DP field) and magnetospheric (DL field) origin contribution should commend input to GIEs (and hence to GICs) from both DL and DP fields, rather than ionospheric current alone.

The local time variation of geomagnetic sudden commencements (SCs) has not been taken into account in the Siscoe’s linear relationship which connects the SC amplitude with the corresponding dynamic pressure variation of the solar wind. By considering the physical background of SC, we studied which local time is best to extract the information of the solar wind dynamic pressure and concluded that the SC amplitude at 4–5 h local time of middle- and low-latitude stations most directly reflects the dynamic pressure effect. This result is used to re-check the order of magnitude of the largest 3 SCs observed since 1868.

Interactions of solar wind dynamic pressure (SWDP) discontinuities with Earth's magnetosphere cause geomagnetic Sudden Commencements (SCs). Typically, positive/negative SCs occur at low latitudes due to enhancements/reductions in SWDP. However, anomalous dawn‐side SCs of opposite polarity were recently reported during the 10 May 2024, superstorm [Nilam et al., 2025, https://doi.org/10.1029/2025GL117032]. This study examines SC responses to positive and negative pressure discontinuities on the May 10 and October 10 storms under similar storm phases and local times. Both events consistently revealed anomalous dawn‐side low latitude SCs opposite to those at other longitudes. We suggest the main impulse of the Disturbance Polar (DP) field extending equatorward as the most likely source. Under highly compressed background magnetosphere conditions, field‐aligned currents associated with DP fields can shift to lower L‐shells, producing such anomalous SCs at dawn‐side low latitudes. These findings provide new insights into dawn‐side magnetosphere–ionosphere coupling during intense storms.

We have examined the most intense external (magnetospheric and ionospheric) and internal (induced) |dH/dt| (amplitude of the 10 s time derivative of the horizontal geomagnetic field) events observed by the high-latitude International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers between 1994 and 2018. While the most intense external |dH/dt| events at adjacent stations typically occurred simultaneously, the most intense internal (and total) |dH/dt| events were more scattered in time, most likely due to the complexity of induction in the conducting ground. The most intense external |dH/dt| events occurred during geomagnetic storms, among which the Halloween storm in October 2003 featured prominently, and drove intense geomagnetically induced currents (GICs). Events in the prenoon local time sector were associated with sudden commencements (SCs) and pulsations, and the most intense |dH/dt| values were driven by abrupt changes in the eastward electrojet due to solar wind dynamic pressure increase or decrease. Events in the premidnight and dawn local time sectors were associated with substorm activity, and the most intense |dH/dt| values were driven by abrupt changes in the westward electrojet, such as weakening and poleward retreat (premidnight) or undulation (dawn). Despite being associated with various event types and occurring at different local time sectors, there were common features among the drivers of most intense external |dH/dt| values: preexisting intense ionospheric currents (SC events were an exception) that were abruptly modified by sudden changes in the magnetospheric magnetic field configuration. Our results contribute towards the ultimate goal of reliable forecasts of dH/dt and GICs.

The ionospheric convection electric field is often assumed to be a potential field. This assumption is not always valid, especially when the ionosphere changes on short time scales T≲5$T\\lesssim 5$min. We present a technique for estimating the induction electric field using ground magnetometer measurements. The technique is demonstrated on real and simulated data for sudden increases in solar wind dynamic pressure of ∼${\\sim} $ 1 and 10 nPa, respectively. For the real data, the ionospheric induction electric field is 0.15 ±$\\pm $0.015 mV/m, and the corresponding compressional flow is 2.5 ±$\\pm $0.3 m/s. For the simulated data, the induction electric field and compressional flow reach 3 mV/m and 50 m/s, respectively. The induction electric field can locally constitute tens of percent of the total electric field. Inclusion of the induction electric field increased the total Joule heating by 2.4%. Locally the Joule heating changed by tens of percent. This corresponds to energy dissipation that is not accounted for in existing models. Plain Language Summary In the study of ionospheric dynamics, it is often assumed that the ionospheric electric field is a potential field. This means the contribution from induction is neglected. The induction electric field is described by Faraday's law and relates to temporal changes in the magnetic field. This assumption only holds when the ionospheric dynamics change slowly. In this study, we present a technique for calculating the ionospheric induction electric field using measurements of the magnetic field on the ground. We demonstrate the technique on real and simulated data of a dynamic event, that is, a sudden commencement. We find that the induction electric field, on a global scale, is small compared to the potential electric field. Inclusion of the induction electric field increased the total energy dissipation, that is, Joule heating, by only a couple of percent but resulted in local variations of tens of percent. Furthermore, we quantified and visualized the compression flow which is the compression and expansion of the magnetic field related to the temporal evolution of a dynamic ionospheric event. Key Points A method for estimating the induction electric field using ground magnetometer measurements is presented Locally, the estimated induction electric field can constitute tens of percent of the total electric field The spatial pattern of ionospheric Joule heating is shown to be highly affected by the induction electric field, even during weak induction

Geomagnetically Induced Currents (GICs) are a severe space weather hazard, driven through coupling between the solar wind and magnetosphere. GICs are rarely measured directly, instead the ground magnetic field variability is often used as a proxy. Recently space weather models have been developed to forecast whether the magnetic field variability (R) will exceed specific, extreme thresholds. We test an example machine learning‐based model developed for the northern United Kingdom. We evaluate its performance (discriminative skill and calibration) as a function of magnetospheric state, solar wind input and magnetic local time. We find that the model's performance is highest during active conditions, for example, geomagnetic storms, and lowest during isolated substorms and “quiet” intervals, despite these conditions dominating the training data set. Correspondingly, the performance is high when the solar wind conditions are elevated (i.e., high velocity, large total magnetic field strength, and the interplanetary magnetic field oriented southward), and at a minimum when the north‐south component of the magnetic field is highly variable or around zero. Regarding magnetic local time, performance is highest within the dusk and night sectors, and lowest during the day. The model appears to capture multiple modes of magnetospheric activity, including substorms and viscous interactions, but poorly predicts impulsive phenomena (i.e., storm sudden commencements) and longer timescale coupling processes. Future models of mid‐latitude magnetic field variability will need to effectively use longer time intervals of unpropagated (i.e., observations from L1) solar wind to more completely describe the magnetospheric conditions and response.

Changes in the Earth's geomagnetic field induce geoelectric fields in the solid Earth. These electric fields drive Geomagnetically Induced Currents (GICs) in grounded, conducting infrastructure. These GICs can damage or degrade equipment if they are sufficiently intense—understanding and forecasting them is of critical importance. One of the key magnetospheric phenomena are Sudden Commencements (SCs). To examine the potential impact of SCs we evaluate the correlation between the measured maximum GICs and rate of change of the magnetic field (H′) in 75 power grid transformers across New Zealand between 2001 and 2020. The maximum observed H′ and GIC correlate well, with correlation coefficients (r2) around 0.7. We investigate the gradient of the relationship between H′ and GIC, finding a hot spot close to Dunedin: where a given H′ will drive the largest relative current (0.5 A nT−1 min). We observe strong intralocation variability, with the gradients varying by a factor of two or more at adjacent transformers. We find that GICs are (on average) greater if they are related to: (a) Storm Sudden Commencements (SSCs; 27% larger than Sudden Impulses, SIs); (b) SCs while New Zealand is on the dayside of the Earth (27% larger than the nightside); and (c) SCs with a predominantly East‐West magnetic field change (14% larger than North‐South equivalents). These results are attributed to the geology of New Zealand and the geometry of the power network. We extrapolate to find that transformers near Dunedin would see 2000 A or more during a theoretical extreme SC (H′ = 4000 nT min−1).