Ground Magnetometer Research Articles

Observations of Localized Horizontal Geomagnetic Field Variations Associated With a Magnetospheric Fast Flow Burst During a Magnetotail Reconnection Event Detected by the THEMIS Spacecraft

AbstractOn 20 December 2015, three Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft detected a nightside magnetotail reconnection event in the early main phase of a major geomagnetic storm. The spacecraft (P5, P4, and P3) had their footprints located over North America near the Gillam ground magnetometer station in Canada. Multipoint observations, both in space and from the ground, allow for an examination of the spatiotemporal characteristics of the disturbance on the ground and the associated physical drivers in the magnetosphere and ionosphere. This study shows that the horizontal geomagnetic field d/dt localized (on the scale of 100–300 km) feature observed at Gillam ground magnetometer site was caused by an isolated substorm onset near that station driven by a nightside magnetotail reconnection event detected by three THEMIS spacecraft that were located near the central plasma sheet. A close inspection of equivalent ionospheric current and current amplitude maps derived from ground magnetometer measurements using the spherical elementary current system technique indicates that the location of the localization lies roughly between the upward and downward field aligned current system, which is consistent with other earlier studies. This event represents the first reported observation of ground d/dt localization that is directly linked to nightside magnetotail fast flow bursts and reconnection event during the onset phase of a major Geomagnetic disturbance (GMD).

Near‐Earth Reconnection Contributing to Recovery Phase of Geomagnetic Storm

AbstractRecent observations show very near‐Earth reconnection (∼8–13RE) could efficiently power the ring current during the main phase of geomagnetic storms, but whether the recovery phase might be contributed remains unclear. During the recovery phase of the May 2024 major geomagnetic storm, intense auroral brightening and geomagnetic disturbances were observed at midnight, indicative of particle injections. Current wedges observed by mid‐latitude ground magnetometers around midnight suggest dipolarizing flux bundles (DFBs). The latitude of the auroral brightening was clearly lower than usual, suggesting near‐Earth reconnection (NERX) was closer to Earth than during substorms (∼20–30RE). GOES‐18 at midnight detected magnetic field and plasma signatures consistent with DFBs, following an extremely thin current sheet likely compressed by strong upstream dynamic pressure. These results indicate NERX could have been close enough for resultant DFBs to penetrate geosynchronous orbit and contribute to the ring current during the recovery phase. This scenario deserves further examination in future.

Automatic Detection and Classification of Aurora in THEMIS All‐Sky Images

AbstractWe report a novel machine‐learning algorithm for automatically detecting and classifying aurora in all–sky images (ASI) that is largely trained without requiring ground–truth labels. By including a small number of labeled images, we are able to automatically label all of the approximately 700 million images in the Time History of Events and Macroscale Interactions during Substorms (THEMIS) ASI data set from 2008 to 2022. We use a two–stage approach. In the first stage, we adapt the Simple framework for Contrastive Learning of Representations (SimCLR) algorithm to learn latent representations of THEMIS all–sky images. We then finetune a classifier network on the latent representations our model learns of the manually labeled Oslo aurora THEMIS (OATH) data set. We demonstrate that this two–stage approach achieves excellent classification results on data for which there is no current ML classification benchmark. The outcome of this work will facilitate efficient information retrieval for researchers interested in specific categories of aurora and will enable large scale statistical studies and machine learning analyses of THEMIS all–sky images that have not previously been possible. To demonstrate possible ways to utilize this database, we performed a statistical analysis of the occurrence rates of auroral labels with respect to solar wind parameters, interplanetary magnetic field vector, and geomagnetic indices. We further investigate the occurrence rates of auroral phenomena in the annotated data set and their geoeffectiveness by utilizing the co–located THEMIS ground magnetometer data set.

Investigating the Trip of a Transformer in Sweden During the 24 April 2023 Storm

AbstractGeomagnetically Induced Currents are unwanted currents that flow in large ground‐based conductive infrastructure and are a significant threat to bulk power grids. This susceptibility is increased at high latitudes due to the larger amplitude geomagnetic disturbances caused by the strong and dynamic auroral electrojet currents. In Sweden, there has been a record of disturbances connected to geomagnetically induced currents; the most documented was a blackout in Malmö, a city in southern Sweden, on 30 October 2003. However, on 24 April 2023, there was a transformer trip in the Bandsjö substation (near Sundsvall) around the time of enhanced space weather. In this paper, we investigate this event by studying the solar wind properties as well as geomagnetic disturbances. The study shows evidence that the transformer trip was caused by a strong geomagnetic disturbance in the morning sector. This was triggered by a high‐pressure sub‐structure within the interplanetary coronal mass ejection cloud, preceded by 2 hr of strong southward interplanetary magnetic field. Additionally, analysis of multiple ground magnetometers shows that the geomagnetic impact was highly spatially structured.

Impacts of Extreme Ultraviolet Late Phase of the Solar Flare on Ionospheric Electrodynamics

Previous investigations of ionospheric electrodynamical responses to solar flares primarily focused on the main phases (MPs) of solar flares. Typical solar irradiance models for driving global ionosphere models do not include the extreme ultraviolet (EUV) late phase (ELP) of flares, which was recently observed with new high-quality solar EUV spectra. Thus, it is still unclear how ionospheric electrodynamics respond to the flare ELP. Here, we analyzed the ionospheric electrodynamical response to the MP and ELP of the X9.3 flare on 2017 September 6, using observations from ground magnetometers, along with simulation results from an ionosphere–thermosphere coupled model. Observations indicated an intensification of the dayside eastward equatorial electrojet (EEJ) by approximately 12 nT at the ELP peak as compared to the quiet day reference. Additionally, the dayside eastward electric field increased due to the ELP, which is different from the reduction of dayside electric fields during MP. The upward E × B plasma drifts decreased by 2.5 m s–1 during MP but increased by 0.75 m s–1 during the ELP. Altitude-dependent responses of ionospheric conductivities to the ELP modulated the relative contribution of the E- and F-region wind dynamo to zonal electric fields, resulting in an overall increase in the daytime eastward electric fields. Furthermore, combined effects of electric fields and conductivities enhancements contributed to EEJ intensification during the ELP. This study enhances our understanding of how solar flares with ELP change global ionospheric electric fields and currents.

Identifying Ionospheric Small‐Scale Currents: A Spatial Correlation Study Using Closely‐Spaced Pairs of Ground Magnetometers

AbstractThe occurrence of small‐scale and intense ionospheric currents that can contribute to geomagnetically induced currents have recently been discovered. A difficulty in their characterization is that their signatures are often only observed at single widely spaced (typically 200–400 km) ground geomagnetic stations. These small‐scale structures motivate the examination of the maximum station separation required to fully characterize these small‐scale signatures. We analyze distributions of correlation coefficients between closely spaced mid‐latitude and auroral zone ground magnetometer stations spanning day to month long intervals to assess the separation distance at which geomagnetic signatures appear in only one station. Distributions were analyzed using periods that included low and high geomagnetic activity. We used data from pairs of magnetometer stations across North America within 200 km of each other, all of which were separated primarily latitudinally. Results show that while measurements remain largely similar up to separations of 200 km, large and frequent differences appear starting at around 130 km separation. Larger differences and lower correlations are observed during high geomagnetic activity, while low geomagnetic activity leads to frequent high correlation even past 200 km separation. Small but identifiable differences can appear in magnetometer data from stations as close as 35 km during high geomagnetic activity. We recommend future magnetometer array deployment in the auroral and sub‐auroral zone to have separations of 100–150 km. This enables the monitoring of large scale effects of geomagnetic storms, better temporal and spatial resolution of substorms, and observations of small‐scale current signatures.

On the Association of Substorm Identification Methods

AbstractSubstorms are a rapid release of energy that is redistributed throughout the magnetosphere‐ionosphere system, resulting in many observable signals, such as enhancements in the aurora, energetic particle injections, and ground magnetic field perturbations. Numerous substorm identification techniques and onset lists based on each of these signals have been provided in the literature, but often with no cross‐calibration. Since the signals produced are not necessarily unique to substorms and may not be sufficiently similar to be identified for each and every substorm, individual event lists may miss or misidentify substorms, hindering our understanding and the development and validation of substorm models. To gauge the scale of this problem, we use metrics derived from contingency tables to quantify the association between lists of substorms derived from SuperMAG SML/SMU indices, midlatitude magnetometer data, particle injections, and auroral enhancements. Overall, although some degree of pairwise association is found between the lists, even lists generated by applying conceptually similar gradient‐based identification to ground magnetometer data achieve an association with less than 50% event coincidence. We discuss possible explanations of the levels of association seen from our results, as well as their implications for substorm analyses.

Crucial future observations and directions for unveiling magnetopause dynamics and their geospace impacts

The dynamics of Earth’s magnetopause, driven by several different external/internal physical processes, plays a major role in the geospace energy budget. Given magnetopause motion couples across many space plasma regions, numerous forms of observations may provide valuable information in understanding these dynamics and their impacts. In-situ multi-point spacecraft measurements measure the local plasma environment, dynamics and processes; with upcoming swarms providing the possibility of improved spatiotemporal reconstruction of dynamical phenomena, and multi-mission conjunctions advancing understanding of the “mesoscale” coupling across the geospace “system of systems.” Soft X-ray imaging of the magnetopause should enable boundary motion to be directly remote sensed for the first time. Indirect remote sensing capabilities might be enabled through the field-aligned currents associated with disturbances to the magnetopause; by harnessing data from satellite mega-constellations in low-Earth orbit, and taking advantage of upgraded auroral imaging and ionospheric radar technology. Finally, increased numbers of closely-spaced ground magnetometers in both hemispheres may help discriminate between high-latitude processes in what has previously been a “zone of confusion.” Bringing together these multiple modes of observations for studying magnetopause dynamics is crucial. These may also be aided by advanced data processing techniques, such as physics-based inversions and machine learning methods, along with comparisons to increasingly sophisticated geospace assimilative models and simulations.

Statistical Study of Hot Flow Anomaly Induced Ground Magnetic Ultra‐Low Frequency Oscillations

AbstractPc5 ULF waves play an important role in transporting energy and particles in the coupled magnetospheric and ionospheric system. They are known to be initiated by dynamic pressure fluctuations upstream of the magnetopause, including those induced by hot flow anomalies (HFAs). However, the role of HFAs in generating magnetospheric and ground magnetic Pc5 ULF oscillations has not been investigated statistically yet. Thus, in this paper, we investigate the contribution of HFAs to ground magnetic Pc5 ULF oscillations and analyze how the characteristics of HFAs influence these oscillations, based on the coordinated observations between the THEMIS probes and the ground magnetometers at high latitudes during the years 2008, 2009 and 2019. We find that HFAs can serve as a notable source of ground magnetic Pc5 ULF oscillations, with about 18.9% of Interplanetary Magnetic Field (IMF) discontinuity‐induced HFAs associated with discernible enhancements in Pc5 ULF wave power, whereas spontaneous HFAs play a comparatively minor role in generating these oscillations. Furthermore, we observe that the cores of HFAs are likely to contribute more significantly to modulating the induced ground magnetic Pc5 ULF oscillations than their compressed boundaries. More dynamic pressure reductions within HFA cores correspond to stronger ground magnetic Pc5 ULF oscillations. Additionally, HFAs can propagate with the IMF discontinuity along the bow shock, continuously generating ground magnetic Pc5 ULF oscillations during their propagation. This research sheds light on the mechanisms underlying Pc5 ULF wave generation and underscores the role of HFAs in driving magnetospheric‐ionospheric interactions.

How Do Substorms Influence Hemispheric Asymmetries in Equivalent Currents?

AbstractIonospheric dynamics exhibits a distinct hemispheric asymmetry, influenced primarily by the Interplanetary Magnetic Field (IMF) By component, dipole tilt, or a combination of both. Previous studies have indicated a reduction in these asymmetries during substorms. In this study, we conduct a superposed epoch analysis using ground magnetometer data from the northern hemisphere to examine the impact of substorms on ionospheric current asymmetry. This analysis uses the assumption of mirror symmetry between the northern and southern hemispheres when IMF By and dipole tilt are reversed. We observe a significant reduction in nightside equivalent current asymmetry indicating the IMF By and dipole tilt have minimal influence on the substorm current. On the other hand, we find that substorms exert minimal or negligible effects on dayside currents. This difference in response between nightside and dayside currents emphasizes the need to incorporate nightside dynamics into existing climatological models, which presently rely mainly on upstream parameters due to a lack of robust parameters effectively representing them. Our findings provide important insights for future modeling efforts, highlighting the distinct interactions between substorms and ionospheric currents across different hemispheric regions.

The Ionospheric Leg of the Substorm Current Wedge: Combining Iridium and Ground Magnetometers

AbstractUtilizing magnetic field measurements made by the Iridium satellites and by ground magnetometers in North America we calculate the full ionospheric current system and investigate the substorm current wedge. The current estimates are independent of ionospheric conductance, and are based on estimates of the divergence‐free (DF) ionospheric current from ground magnetometers and curl‐free (CF) ionospheric currents from Iridium. The DF and CF currents are represented using spherical elementary current systems (SECS), derived using a new inversion scheme that ensures the current systems' spatial scales are consistent. We present 18 substorm events and find a typical substorm current wedge (SCW) in 12 events. Our investigation of these substorms shows that during substorm expansion, equivalent field‐aligned currents (EFACs) derived with ground magnetometers are a poor proxy of the actual FAC. We also find that the intensification of the westward electrojet can occur without an intensification of the FACs. We present theoretical investigations that show that the observed deviation between FACs estimated with satellite measurements and ground‐based EFACs are consistent with the presence of a strong local enhancement of the ionospheric conductance, similar to the substorm bulge. Such enhancements of the auroral conductance can also change the ionospheric closure of pre‐existing FACs such that the ground magnetic field, and in particular the westward electrojet, changes significantly. These results demonstrate that attributing intensification of the westward electrojet to SCW current closure can yield false understanding of the ionospheric and magnetospheric state.

A Quantitative Analysis of the Uncertainties on Reconnection Electric Field Estimates Using Ionospheric Measurements

AbstractCalculating the magnetic flux transfer across the open‐closed boundary (OCB) per unit time and distance—the reconnection electric field—is an important means of remotely monitoring magnetospheric dynamics. Ground‐based measurements of plasma convection velocities together with velocities of the OCB are commonly used to infer reconnection rates. However, this approach is limited by spatial coverage and often lacks robust uncertainty quantification. In this paper, we assimilate Super Dual Auroral Radar Network convection measurements, ground magnetometer data, and estimates of the conductance derived from the Imager for Magnetopause‐to‐Aurora Global Exploration satellite imagers, using the Local mapping of polar ionospheric electrodynamics (Lompe) framework over a region in North America. We present a new method to assess various contributions to uncertainties in the derived reconnection electric fields, including a novel approach to estimate uncertainties in conductance from global auroral imaging. Our method is demonstrated on a substorm event with an associated pseudobreakup during a period of favorable observational coverage. In this case study, the uncertainties in the reconnection electric field are ∼5–10 mV/m at the peak of substorm expansion, roughly 15% of the peak reconnection electric field. We find that the main contributor to the reconnection electric field estimates after substorm onset is the OCB motion, whereas during the pseudobreakup the main contributor is ionospheric plasma convection.

Estimating the Ionospheric Induction Electric Field Using Ground Magnetometers

AbstractThe 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 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 1 and 10 nPa, respectively. For the real data, the ionospheric induction electric field is 0.15 0.015 mV/m, and the corresponding compressional flow is 2.5 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.

Signatures of Dipolarizing Flux Bundles in the Nightside Auroral Zone

AbstractDipolarizing flux bundles (DFBs) have been suggested to transport energy and momentum from regions of reconnection in the magnetotail to the high latitude ionosphere, where they can generate localized ionospheric currents that can produce large nighttime geomagnetic disturbances (GMDs). In this study we identified DFBs observed in the midnight sector from ∼7 to ∼10 RE by THEMIS A, D, and E during days in 2015–2017 whose northern hemisphere magnetic footpoints mapped to regions near Hudson Bay, Canada, and have compared them to isolated GMDs observed by ground magnetometers. We found 6 days during which one or more of these DFBs coincided to within ±3 min with ≥6 nT/s GMDs observed by latitudinally closely spaced ground‐based magnetometers located near those footpoints. Spherical elementary current systems (SECS) maps and all‐sky imager data provided further characterization of two events, showing short‐lived localized intense upward currents, auroral intensifications and/or streamers, and vortical perturbations of a westward electrojet. On all but one of these days the coincident DFB—GMD pairs occurred during intervals of high‐speed solar wind streams but low values of SYM/H. The observations reported here indicate that isolated DFBs generated under these conditions influence only limited spatial regions nearer Earth. In some events, in which the DFBs were observed closer to Earth and with lower Earthward velocities, the GMDs occurred slightly earlier than the DFBs, suggesting that braking had begun before the time of the DFB observation.

Investigation of the influence of high-latitude ionospheric current systems on the results of impedance measurement in the Arctic on the example of the Kola Peninsula

We investigated the influence of high-latitude ionospheric current systems on the results of impedance measurements on the Kola Peninsula. Model calculations and joint analysis of satellite data and ground magnetometer data have shown the possibility of distortions in the interpretation of geophysical data due to the spatial vicinity of current systems in cases where rocks of the Earth’s crust have high electrical resistance.

Substorm‐Time Ground dB/dt Variations Controlled by Interplanetary Shock Impact Angles: A Statistical Study

AbstractIn this study, we investigate the effects caused by interplanetary (IP) shock impact angles on the subsequent ground dB/dt variations during substorms. IP shock impact angles have been revealed as a major factor controlling the subsequent geomagnetic activity, meaning that shocks with small inclinations with the Sun‐Earth line are more likely to trigger higher geomagnetic activity resulting from nearly symmetric magnetospheric compressions. Such field variations are linked to the generation of geomagnetically induced currents (GICs), which couple to artificial conductors on the ground leading to deleterious consequences. We use a sub‐set of a shock data base with 237 events observed in the solar wind at L1 upstream of the Earth, and large arrays of ground magnetometers at stations located in North America and Greenland. The spherical elementary current system methodology is applied to the geomagnetic field data, and field‐aligned‐like currents in the ionosphere are derived. Then, such currents are inverted back to the ground and dB/dt variations are computed. Geographic maps are built with these field variations as a function of shock impact angles. The main findings of this investigation are: (a) typical dB/dt variations (5–10 nT/s) are caused by shocks with moderate inclinations; (b) the more frontal the shock impact, the more intense and the more spatially defined the ionospheric current amplitudes; and (c) nearly frontal shocks trigger more intense dB/dt variations with larger equatorward latitudinal expansions. Therefore, the findings of this work provide new insights for GIC forecasting focusing on nearly frontal shock impacts on the magnetosphere.

Response of Global Ionospheric Currents to Solar Flares with Extreme Ultraviolet Late Phases

It is known that solar flares can affect the current system of the middle- and low-latitude ionosphere. Most earlier studies have focused on such effects during their impulsive phases. Recent studies have reported flares with a significant extreme ultraviolet (EUV) late phase, the effects of which on ionospheric currents have not yet been investigated. Here, we examine the solar quiet (Sq) currents and equatorial electrojets during two X-class flares with EUV late phases using data from more than 200 ground magnetometers. Our results indicate that the ionospheric currents could be significantly enhanced during the impulsive phase, while the effects of the EUV late phase may increase the global ionospheric currents, but are often weak and thus could be obscured by a change in solar wind conditions. In the X1.8 flare event on 2012 October 23, besides the solar flare effects, the currents were modulated by solar wind pressure. In the X1.3 flare event on 2014 April 25, the solar wind pressure was weak and stable, and the Sq currents were enhanced compared to nonflare conditions. We also found that even weak changes in the solar wind dynamic pressure, with magnitudes as low as ∼2 nPa, which are often ignored, may have an appreciable impact on the global ionospheric current system.

A Case Study of Pc1 Waves Observed at the Polar Cap Associated with Proton Precipitation at Subauroral Latitudes

The importance of ElectroMagnetic Ion Cyclotron (EMIC) ultra-low-frequency (ULF) waves (and their Pc1 counterparts) is connected to their critical role in triggering energetic particle precipitation from the magnetosphere to the conjugated ionosphere via pitch angle scattering. In addition, as a prominent element of the ULF zoo, EMIC/Pc1 waves can be considered a perfect tool for the remote diagnosis of the topologies and dynamic properties of near-Earth plasmas. Based on the availability of a comprehensive set of instruments, operating on the ground and in the top-side ionosphere, the present case study provides an interesting example of the evolution of EMIC propagation to both ionospheric hemispheres up to the polar cap. Specifically, we report observations of Pc1 waves detected on 30 March 2021 under low Kp, low Sym-H, and moderate AE conditions. The proposed investigation shows that high-latitude ground magnetometers in both hemispheres and the first China Seismo-Electromagnetic Satellite (CSES-01) at a Low Earth Orbit (LEO) detected in-synch Pc1 waves. In strict correspondence to this, energetic proton precipitation was observed at LEO with a simultaneous appearance of an isolated proton aurora at subauroral latitudes. This supports the idea of EMIC wave-induced proton precipitation contributing to energy transfer from the magnetosphere to the ionosphere.

Persistent Pitch Angle Anisotropies of Relativistic Electrons in the Outer Radiation Belts

AbstractPitch angle distributions (PADs) in the radiation belts are well characterized with sinn(α). By tracking the exponent “n,” termed pitch angle index, we are able to observe persistent and cross‐energy changes in PADs of Van Allen radiation belt electrons using Van Allen Probes particle observations. The PAD measurements are well fit down to a single satellite spin, and therefore can track spatially and temporally confined changes to determine the connection between particles and waves. With this method, we study long‐lasting and high‐energy anisotropic electron PADs during a quiet period over 2 days, 26 and 27 June 2013. One potential driver for these changes is electromagnetic ion cyclotron wave interaction with the particles. We use several ground magnetometer stations from Canadian Array for Realtime Investigations of Magnetic Activity and Finnish pulsation magnetometer network of Sodankylä Geophysical Observatory to observe waves during the 2 days of interest. The connection between the waves and particles is inconclusive, although there is some temporal overlap of the phenomenon.

Association of Equatorward Extended Auroral Streamers With Overshielding Conditions at Equatorial Latitudes: First Observations

AbstractWe report the first observations of the association between equatorward extending streamers and overshielding using the THEMIS all‐sky imagers and ground magnetometers. Because auroral streamers indicate plasma sheet flow bursts, these observations uncover the effect of flow bursts on overshielding. Results show that, in general, bright equatorward extended streamers were associated with an increase in equatorial electrojet (EEJ) on the nightside and a decrease in the dayside EEJ, indicating a striking correspondence between auroral streamers and overshielding conditions. Thus, the driving of overshielding at equatorial latitudes can be identified via bright equatorward extended streamers, indicating that flow bursts are an alternate means to discern the earthward injections that increase the region 2 field aligned currents and associated overshielding electric fields. Repetitive auroral streamers were associated with repetitive overshielding, resulting in a stepwise development of the dayside and nightside EEJ. The stepwise intensifications were also observed in the midlatitude positive bay and Pi2 pulsations. Our results could explain the occurrence of overshielding conditions at equatorial latitudes during substorms and nonsubstorm times without a northward turning of IMF‐Bz. As seen through streamers, the localized current structures (wedgelets) associated with flow bursts giving injection that leads to overshielding is titled northeast‐to‐southwest. Our results add a new element to the understanding of high‐to‐low latitude electrodynamical coupling by demonstrating the association between bright equatorward extended auroral streamers and enhanced shielding electric fields caused by earthward injections associated with flow bursts.