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Rapid increases in solar wind dynamic pressure, known as solar wind pressure pulses, compress the Earth's magnetosphere and can rapidly restructure the electrodynamics within. The propagation of pressure pulse effects into the magnetosphere is known as a geomagnetic sudden commencement (SC). SCs can be further subdivided into compressions which are rapidly followed by a geomagnetic storm (a sudden storm commencement, SSC) and those which are not (a sudden impulse, SI). In this paper, SSCs and SIs are compared and contrasted, and we examine in particular the differences between the pressure pulses that drive SSCs/SIs, and explore the physical conditions of the magnetosphere before pressure pulse arrival. Firstly, it is shown that SSCs are more likely to be driven by pressure pulses with higher magnitude and/or shorter rise time. Secondly, the magnetosphere is primed by stronger driving conditions and higher geomagnetic activity prior to SSCs than SIs. Finally, there is a solar cycle dependence in the occurrence and magnitude of solar wind pressure pulses. Plain Language Summary: The solar wind's dynamic pressure controls the size of the cavity the Earth's magnetic field forms in space. When the pressure increases rapidly, this is known as a solar wind pressure pulse. Pressure pulses can affect currents and magnetic fields within the Earth's space environment, and can have serious space weather implications. In this paper, the differences between pressure pulses that are followed by geomagnetic storms (large releases of energy in the Earth's system), and those that are not are investigated. The state of the Earth's system before the arrival of the pressure pulse is also considered, and its contribution to the resulting space weather effects. It is shown that larger pressure pulses are more likely to trigger a geomagnetic storm. Additionally, analysis presented shows that the Earth's system may already be active prior to the triggering of these storms. Finally, the occurrence and strength of pressure pulse events follows solar activity. Key Points: There is a solar cycle dependence to the occurrence and magnitude of sudden commencements (SCs)Sudden storm commencements (SSCs) are driven by higher magnitude and/or shorter duration pressure pulsesThe magnetosphere is primed by stronger solar wind/interplanetary magnetic field and higher geomagnetic activity for SSCs [ABSTRACT FROM AUTHOR]

Interplanetary (IP) shocks are one of the dominant solar wind structures that can significantly impact the Geospace when impinge on the Earth's magnetosphere. IP shocks severely distort the magnetosphere and induce dramatic changes in the magnetospheric currents, often leading to large disturbances in the geomagnetic field. Sudden enhancements in the solar wind dynamic pressure (PDyn) during IP shocks cause enhanced high‐latitude convection electric fields which penetrate promptly to equatorial latitudes. In response, the equatorial electrojet (EEJ) current exhibits sharp changes of magnitudes primarily controlled by the change in PDyn and the local time. In this paper, we further investigated the influence of shock impact angle on the EEJ response to a large number (306) of IP shocks that occurred during 2001–2021. The results consistently show that the EEJ exhibits a heightened response to the shocks that head‐on impact the magnetosphere (frontal shocks) than those with inclined impact (inclined shocks). The greater EEJ response during the frontal shocks could be due to a more intensified high‐latitude convection electric field resulting from the symmetric compression of the magnetosphere. Finally, an existing empirical relation involving PDyn and local time is improved by including the effects of impact angle, which can quantitatively better predict the EEJ response to IP shocks. Plain Language Summary: Solar Wind, a continuous stream of high‐energy particles emanating from the Sun, is ubiquitous in interplanetary (IP) space. In unison, the energetic and/or transient eruptions on the Sun often release bursts of fast solar wind. When this fast solar wind interacts with the ambient solar wind in the IP space, a shock front is formed known as IP shock. These IP shocks (if Earth‐directed) can impinge on the Earth's magnetosphere and transfer tremendous amounts of energy and momentum. As a result, the Earth's magnetic field is often severely disturbed. Severe geomagnetic disturbances are known to cause a myriad of space weather effects from the loss of satellites in space to damage of electrical power grids and transmission lines on ground. Historically, geomagnetic field disturbances are known to be more cataclysmic at high latitudes and lessen at latitudes below the auroral region. However, these disturbances again dramatically enhance at equatorial latitudes due to a unique ionospheric current system, known as equatorial electrojet (EEJ). This study provides new insights into the significant role of angle of shock impacting the magnetosphere and the resultant disturbances in EEJ. This study also derives an empirical relation to accurately estimate the EEJ disturbances during IP shocks. Key Points: The angle of interplanetary (IP) shock impact on the magnetosphere plays a significant role in controlling the equatorial electrojet (EEJ) responseThe magnitude of EEJ change due to the impact of frontal IP shocks is significantly larger than that for the inclined shocksAn improved empirical relation including the effects of impact angle is derived which provide better estimates of EEJ responses to IP shocks [ABSTRACT FROM AUTHOR]

Surface waves on Earth's magnetopause have a controlling effect upon global magnetospheric dynamics. Since spacecraft provide sparse in situ observation points, remote sensing these modes using ground‐based instruments in the polar regions is desirable. However, many open conceptual questions on the expected signatures remain. Therefore, we provide predictions of key qualitative features expected in auroral, ionospheric, and ground magnetic observations through both magnetohydrodynamic theory and a global coupled magnetosphere‐ionosphere simulation of a magnetopause surface eigenmode. These show monochromatic oscillatory field‐aligned currents (FACs), due to both the surface mode and its non‐resonant Alfvén coupling, are present throughout the magnetosphere. The currents peak in amplitude at the equatorward edge of the magnetopause boundary layer, not the open‐closed boundary as previously thought. They also exhibit slow poleward phase motion rather than being purely evanescent. We suggest the upward FAC perturbations may result in periodic auroral brightenings. In the ionosphere, convection vortices circulate the poleward moving FAC structures. Finally, surface mode signals are predicted in the ground magnetic field, with ionospheric Hall currents rotating perturbations by approximately (but not exactly) 90° compared to the magnetosphere. Thus typical dayside magnetopause surface modes should be strongest in the East‐West ground magnetic field component. Overall, all ground‐based signatures of the magnetopause surface mode are predicted to have the same frequency across L‐shells, amplitudes that maximize near the magnetopause's equatorward edge, and larger latitudinal scales than for field line resonance. Implications in terms of ionospheric Joule heating and geomagnetically induced currents are discussed. Plain Language Summary: Waves on the boundary of the magnetosphere, the magnetic shield established by the interplay of the solar wind with Earth's magnetic field, play a controlling role on energy flow into our space environment. While these waves can be observed as they pass over satellites in orbit, due to the small number of suitable satellites available it would be helpful to be able to detect these waves from the surface of the Earth with instruments that measure the northern/southern lights, motion of the top of our atmosphere, or magnetic field on the ground. However, we do not currently understand what the signs of these waves should look like in such instruments. In this paper we develop theory and use computer simulations of these boundary waves to predict key features one might expect to measure from the ground. Based on these predictions, we also discuss how the waves might contribute to the hazards of space weather. Key Points: Theory and global simulations of magnetopause surface waves' effects on the aurorae, ionosphere, and ground magnetic field are investigatedWe predict poleward‐moving periodic aurora, convection vortices, and ground pulsations, with larger latitudinal scales than Alfvén modesAmplitudes of all signals peak near the projection of the inner/equatorward edge of the magnetopause rather than the open–closed boundary [ABSTRACT FROM AUTHOR]

Geomagnetic disturbances caused by solar storms have the potential to create large‐scale geomagnetically induced currents in long conductors at the Earth's surface. These may disrupt the operation of electric power grids and cause blackouts. Ocean‐continent boundary regions are of particular concern because of the sharp contrast between the higher ocean electrical conductivity compared to the lower continental conductivity. This contrast may generate high‐amplitude geoelectric fields and cause power grids in coastal regions to be more vulnerable to space weather hazards. Previously, analytical calculations were used to estimate geomagnetically induced currents at ocean‐continent boundaries. However, for the analytical equations to be solvable, the physics and geometries of the problem were simplified. As a result of these simplifications and due to a lack of published measurements examining this issue in coastal regions, it is difficult to know for sure whether there are unique hazards to electric power grids at ocean‐continent boundaries. In this paper, a grid‐based, time domain modeling approach is used to solve the complete Maxwell's equations, which permits accommodation of (1) the complete physics of the propagating electromagnetic fields from disturbed ionospheric currents through the air and into the lithosphere and even into the ocean via the skin effect and (2) more realistic coastal geometries. Using this more robust approach, in the variety of scenarios studied in this paper, only a local enhancement of the electric fields was observed, which are expected to only pose potential hazards to power grids only a local scale near ocean‐continent boundaries. Key Points: Electromagnetic fields across ocean‐continent boundaries are solved via FDTD while accounting for a finite‐depth ocean and topographyAnalytical approaches are found to provide a close approximation to the more realistic FDTD results for the range of studied scenariosSurface electric field enhancements are highly localized along the coast, even for low frequencies and shallow oceans [ABSTRACT FROM AUTHOR]

The geomagnetic field is composed of a variety of sources that act on a wide range of timescales and amplitudes. The separation of magnetic storm effects from quiet variations is needed to accurately quantify impacts of space weather events. The extraction of such quiet contributions within geomagnetic measurements is achieved by the determination of baselines, which, ideally, is done by a simple algorithm which captures quiet sources suitably well, while being applicable to an extensive network of magnetic observatories independent of the period of time. In this work, we apply signal filtering techniques on the horizontal components of geomagnetic field measurements from low‐ and mid‐latitude observatories to determine baselines. The variations within the baseline are investigated for magnetically quiet periods between 1991 and 2019, focusing on long‐term trends, seasonal and local time dependencies, and day‐to‐day variability. The analysis confirms that the contributing quiet sources include the secular variation and the solar quiet (Sq) current system. The non‐negligible day‐to‐day variability, that is typical for Sq in low‐ and mid‐latitudes, is embedded within the baseline. Thus, the filter approach extracts quiet magnetic field variations well. Comparisons with other baseline methods show good agreements. We conclude that the filter approach can be used to determine baselines automatically during magnetically quiet periods without the need of further apriori information and is applicable on a wide network of magnetic observatories. It marks the first step for deriving magnetic indices for (near) real‐time space weather applications. Plain Language Summary: The Earth's intrinsic magnetic field is generated by the motion of molten rock within its interior and interacts with the constant flow of charged particles coming from the Sun. Measurements of the geomagnetic field strength on the surface not only include the intrinsic magnetic field but also phenomena that arise due to this interaction. Some of these phenomena show regular variations without major effects and some, like solar storms, are able to disrupt the geomagnetic field, affecting technological systems. In order to quantify how harmful disruptive events are, it is important to determine the regular variations first. In this paper, we determine the regular variations within the signal (baselines) by applying signal filtering techniques on geomagnetic field measurements. Our analysis shows that regular variations during undisturbed days in low‐ and mid‐latitude ranges are captured accurately. Key Points: A basic signal filtering approach is used to determine geomagnetic baselines during quiet periodsThe baselines capture the secular variation and solar quiet current systems accuratelyThe baseline method is applicable for low‐ and mid‐latitude magnetic observatories and does not require any apriori information [ABSTRACT FROM AUTHOR]

This paper presents updated methods for locating the Poleward and Equatorward Auroral Luminosity Boundaries (PALB and EALB) directly from IMAGE Far UltraViolet (FUV) images of the Northern Hemisphere auroral oval. Separate boundaries are determined from images measured at different FUV wavelengths. In addition, new methods for indirectly estimating the Open‐Closed magnetic field line Boundary (OCB) and the Equatorward Precipitation Boundary (EPB) locations are presented; these new boundaries are derived from a combination of the auroral luminosity boundary estimates with statistical latitudinal offsets derived from comparisons with low‐altitude spacecraft Particle Precipitation Boundaries (PPBs). Subsequently, we derive new circle model fits for all these boundary data sets, as well as new quality control criteria for these model fits. The suitability of circle fits for each of the data sets is discussed, and the OCB and PALB circle fits are validated against the Convection Reversal Boundary (CRB), as measured by low‐altitude in situ spacecraft. All the new boundary data sets, covering the epoch May 2000 to October 2002, are freely available online. Plain Language Summary: The ability to measure and model near‐Earth space, its response to driving forces in the interplanetary solar wind, and the impact of these forces on the Earth's upper atmosphere is the basis for the understanding and forecasting of space weather. Because there is a strong physical connection between the Earth's magnetosphere (the region of near‐Earth space dominated by the Earth's magnetic field) and ionosphere (the ionized region of the upper atmosphere) near the poles, observations of the ionosphere provide information about the magnetospheric state. Dynamic processes within the magnetosphere accelerate electrons and ions within the space environment giving them the energy to precipitate in the upper atmosphere. These high‐energy charged particles excite atmospheric atoms and molecules, creating aurora. The light from the aurora can be observed at ultraviolet wavelengths by space‐based imagers. In this study we identify the higher and lower latitude edges of the aurora and determine their relationship to important particle boundaries. Circles are then fitted to the spatial boundary variations to make sure they are defined across all longitudes. These data sets can be used to identify and track the spatial extent and dynamics of magnetospheric regions and boundaries. Key Points: Improved poleward and equatorward auroral luminosity boundaries are determined from IMAGE FUV measurementsNew Open‐Closed magnetic field line Boundary (OCB) and Equatorward Precipitation Boundary (EPB) data sets are derived from these observationsNew circle model fits are derived for all the boundary data sets [ABSTRACT FROM AUTHOR]

Using ground‐based magnetic field measurements and numerical simulations from the Thermosphere‐Ionosphere Electrodynamic General Circulation Model (TIEGCM), a first paper (Zhang, Yamazaki, et al., 2021, doi: https://doi.org/10.1029/2021JA029681; under review) introduced the potential roles of disturbance dynamo electric field due to subauroral polarization streams (SAPS) on the equatorial electrojet (EEJ) during a moderate geomagnetic storm on 1 June 2013. Our second study investigated the temporal responses of equatorial electrojet to SAPS. At noon, the residual EEJ (ΔEEJ) owing to SAPS flows westward, that is, counter equatorial electrojet (CEJ). The temporal variation of CEJ excited by the dynamo electric field was basically consistent with that by SAPS, and the effects of zonal wind were larger than those of meridional wind. The relative time delay of CEJ and SAPS was related to the propagation time of disturbance wind from mid‐latitudes to low‐latitudes. It took 2–3 hr for SAPS‐related disturbance wind to propagate to the equatorial region and change the polarity of EEJ. The influence of meridional winds on the temporal variations of ΔEEJ is related to the generation of eastward currents at mid‐latitudes, which can accumulate the positive charges at dusk terminator and then generate a westward electric field at lower latitudes. Plain Language Summary: The equatorial electrojet (EEJ) represents a ribbon of intense electric current flowing in the ionospheric E region (approximately 110 km) on the dayside along the dip equator. Its behaviors are controlled by the daytime ionospheric electric field and ionospheric conductivity. However, the temporal variations of EEJ in response to the subauroral polarization streams (SAPS), an interesting and important physical phenomenon at subauroral latitudes, are seldom explored and poorly understood. The understanding of EEJ behaviors and their associated physical drivers during SAPS periods can contribute to the modeling and forecasting of the equatorial space environment, and the understanding of the coupling between ionosphere‐thermosphere systems at high‐latitudes and dip equator. Key Points: ubauroral polarization streams‐induced counter equatorial electrojet (CEJ) shows a 2–3 hr delay with respect to SAPSThe time delay of CEJ attributes to the traveling atmospheric disturbances in zonal windsThe effects of meridional winds on CEJ depend greatly on the geomagnetic declination [ABSTRACT FROM AUTHOR]

In this paper, we have evaluated the role of interplanetary magnetic field (IMF) By on the asymmetry of the ring current during the main phase of geomagnetic storms. The mean H variations have been calculated using 31 ground magnetic stations over magnetic latitudes of 09–45° following the methodology of Li et al. (2011, https://doi.org/10.1029/2011JA016886). Further, the magnetic local time (MLT) variations in the H component at these stations w.r.t. the mean H were investigated for three cases of geomagnetic storms with varying southward IMF Bz and IMF By conditions. Significant ring current asymmetries were observed during the main phase of geomagnetic storms. The primary role of IMF Bz on the asymmetry of the ring current is observed from these cases. More importantly, the investigation brings out for the first time, the additional role of IMF By in influencing the MLT distribution of ring current observed at ground magnetic stations. Under southward IMF Bz conditions, it is shown based on SuperDARN and AMPERE data that IMF By can alter the MLT distribution of ring current under suitable conditions. The timescales of IMF By also play very important role in determining the asymmetry in the ring current. Under steady convection state, IMF By can rotate the convection cells based on its polarity, which in turn can change the MLT distribution of ring current observed by low‐latitude ground stations. This investigation, thus, brings out the important role of IMF By on the asymmetric MLT distribution of ring current under southward IMF Bz. Key Points: IMF By plays important role in asymmetry of the ring current observed at ground stations in addition to IMF BzSuperDARN convection cells and AMPERE‐derived FACs show the association of IMF By with MLT distribution of the ring currentUnder suitable conditions, IMF By can alter the MLT distribution of the ring current [ABSTRACT FROM AUTHOR]

It is generally accepted that extreme space weather events tend to be related to strong flares and fast halo coronal mass ejections (CMEs). In the present paper, we carefully identify the chain of events from the Sun to the Earth induced by all 12 X‐class flares that occurred in 2002. In this small sample, we find an unusual high rate (58%) of solar sources with a longitude larger than 74°. Yet all 12 X‐class flares are associated with at least one CME. The fast halo CMEs (50%) are related to interplanetary CMEs (ICMEs) at L1 and weak Dst minimum values (more than −51 nT), while five (41%) of the 12 X‐class flares are related to solar proton events (SPEs). We conclude that (i) all 12 analyzed solar events, even those associated with fast halo CMEs originating from the central disk region, and those ICMEs and SPEs were not very geo‐effective. This unexpected result demonstrates that the suggested events in the chain (fast halo CME, X‐class flares, central disk region, ICME, and SPE) are not infallible proxies for geo‐effectiveness. (ii) The low value of integrated and normalized southward component of the interplanetary magnetic field (Bz*) may explain the low geo‐effectiveness for this small sample. In fact, Bz* is well correlated to the weak Dst and low auroral electrojet activity. Hence, the only space weather impact at Earth in 2002 we can explain is based on Bz* at L1. Key Points: We discussed geo‐effectiveness of the 12 X‐class flare‐CMEs in 2002 based on the available observationsThey failed to evoke strong disturbances since weak interplanetary magnetic field southward componentsThe usual solar proxies (complex active region, X‐class flare, and fast halo CME) are not sufficient to forecast geo‐effectivity in 2002 [ABSTRACT FROM AUTHOR]

Medium‐scale ionospheric ionization structures are a persistent global feature of the Earth's ionosphere. Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation measurements are well suited to address the incomplete global observational picture of plasma density irregularities, including the global climatology in both bottomside and topside F region layers, and their structure in the vertical dimension. A climatological database of F region ionospheric irregularities and their characteristics has been developed through detection of total electron content perturbations by Global Positioning System receivers onboard COSMIC satellites. This paper presents global occurrence rates and detailed characteristics of equatorial to midlatitude medium‐scale irregularities under quiet geomagnetic conditions. The study covers 4 years, two during solar minimum (2008–2009) and two during the ascending phase of solar cycle 24 (2012–2013). Irregularities were found to occur frequently at high latitudes and during nighttime in equatorial to midlatitude regions in both bottom and topside F region layers. Longitudinal‐seasonal occurrence trends at equatorial and midlatitudes are consistent with previous irregularity climatology, which reaffirms that localized enhancements in plasma instability growth rates contribute to irregularity occurrence. Seasonal occurrence patterns also indicate a high occurrence of irregularities in regions corresponding to the solar terminator, confined primarily to altitudes below ~300 km. The local time‐altitude distributions of equatorial and midlatitude irregularity occurrence, amplitude, and scale size provide further insight into irregularity generation mechanisms, and include features consistent with "spread F" irregularities and traveling ionospheric disturbances. Key Points: COSMIC radio occultation TEC measurements are used to examine the global climatology of medium‐scale ionospheric plasma irregularitiesObservation of vertical irregularity structure and distribution provides new insight into irregularity occurrence and characteristicsIrregularities are persistently observed in nighttime equatorial to midlatitude regions, near the solar terminator, and at high latitudes [ABSTRACT FROM AUTHOR]

We show the global dynamics of spatial correlation of Pc2 wave activity can track the evolution of the 2015 St. Patrick's Day geomagnetic storm for an 8 hr time window around onset. The global spatially coherent response is tracked by forming a dynamical network from 1 s data using the full set of 100+ ground‐based magnetometer stations collated by SuperMAG and Intermagnet. The pattern of spatial coherence is captured by network parameters which in turn track the evolution of the storm. At onset interplanetary magnetic field (IMF) Bz > 0 and Pc2 power increases, we find a global response with stations correlated over both local and global distances. Following onset, whilst Bz > 0, the network response is confined to the day‐side. When IMF Bz < 0, there is a strong local response at high latitudes, consistent with the onset of polar cap convection driven by day‐side reconnection. The spatially coherent response as revealed by the network grows and is maximal when auroral (SuperMAG electrojet) and ring current (SuperMAG ring current) 1 min resolution geomagnetic indices peak, consistent with an active electrojet and ring‐current. Throughout the storm there is a coherent response both in stations located along lines of constant geomagnetic longitude, between hemispheres, and across magnetic local time. The network does not simply track average Pc2 wave power, it is characterized by network parameters which track the storm evolution. This is the first study to parameterize global Pc2 wave correlation and offers the possibility of statistical studies across multiple events and comparison with, and validation of, space weather models. Plain Language Summary: Space weather poses a risk to infrastructure including satellites and power systems. A key challenge within space weather is predicting the magnetospheric response. To better understand geomagnetic activity, we (for the first time) build a dynamical network to parameterize the Pc2 wave response. Closed magnetic field lines in the inner magnetosphere can support standing Alfven waves (a magnetic "harp") and these are measured on the ground as Pc waves which occupy distinct frequency bands. Pc waves are excited by a variety of processes related to space weather. Previous work has focused on chains of magnetometers that are at constant magnetic longitude which sample the different resonant frequencies of the "harp" (different field line lengths). Recently, SuperMAG has begun to offer second resolution data which allows higher frequency Pc2 waves to be resolved and studied globally. Our first results are a study of an intense isolated geomagnetic storm where we have identified network parameters and have shown that these track the distinct phases of the storm in terms of spatial coherence of Pc2 wave activity. Using these network parameters we can perform statistical studies across many storms and quantitatively benchmark space weather models with observations. Key Points: First global network analysis of Pc2 wave activity using 100+ magnetometers at 1 s cadenceNovel method to construct the dynamical network of globally coherent Pc2 wave activityNetwork parameters reveal the global magnetospheric response in the 2015 St. Patrick's Day storm [ABSTRACT FROM AUTHOR]

We present an automated method to identify high‐frequency geomagnetic disturbances in ground magnetometer data and classify the events by the source of the perturbations. We developed an algorithm to search for and identify changes in the surface magnetic field, dB/dt, with user‐specified amplitude and timescale. We used this algorithm to identify transient‐large‐amplitude (TLA) dB/dt events that have timescale less than 60 s and amplitude >6 nT/s. Because these magnetic variations have similar amplitude and time characteristics to instrumental or man‐made noise, the algorithm identified a large number of noise‐type signatures as well as geophysical signatures. We manually classified these events by their sources (noise‐type or geophysical) and statistically characterized each type of event; the insights gained were used to more specifically define a TLA geophysical event and greatly reduce the number of noise‐type dB/dt identified. Next, we implemented a support vector machine classification algorithm to classify the remaining events in order to further reduce the number of noise‐type dB/dt in the final data set. We examine the performance of our complete dB/dt search algorithm in widely used magnetometer databases and the effect of a common data processing technique on the results. The automated algorithm is a new technique to identify geomagnetic disturbances and instrumental or man‐made noise, enabling systematic identification and analysis of space weather related dB/dt events and automated detection of magnetometer noise intervals in magnetic field databases. Plain Language Summary: High‐frequency (second‐timescale) components of the surface geomagnetic field are not often included in studies on geomagnetically induced currents (GICs) because they do not pose a direct threat to technological infrastructure. However, high‐frequency intervals occur prior to and within some larger space weather events that can lead to GICs. Because these perturbations are very similar to signals that arise due to noise‐interference, we have developed an automated procedure to identify such high‐frequency intervals and predict the source of the signal as geophysical or noise‐type. It was found that common data processing techniques can reduce or remove high‐frequency geophysical disturbances, but do not remove all noise‐type intervals. Thus, the automated process provides an event list of 1‐hr event windows that contain high‐frequency disturbances and the classification of the signals within. This list can be used to identify hour windows of data that are undesirable for space weather research as well as events that contain high‐frequency geophysical disturbances that may provide insight to the small‐scale features of space weather events. Key Points: A new technique is presented to identify high‐frequency perturbations of the surface geomagnetic fieldA support vector machine predicts with high accuracy the source of high‐frequency events as geophysical or caused by noise interferenceHigh‐frequency magnetic signals with geophysical sources provide detail on small‐scale features of magnetosphere‐ionosphere current systems [ABSTRACT FROM AUTHOR]

Auroral brightening is one of the most common phenomena that occur during substorm onset and is usually recognized as a projection of the substorm‐associated magnetospheric plasma dynamics to the ionosphere. However, electromagnetic fields and plasma features associated with the substorm brightening arc have not been well understood. In this study, we present a comprehensive observation of the source plasma and field variations of a substorm brightening aurora in the inner magnetosphere. We performed a unique conjugate observation of a substorm brightening auroral arc observed by a ground‐based camera and by the Arase satellite in the magnetospheric source region at L ∼ 6. The event was observed at Tromsø (69.6°N, 19.2°E), Norway, on 12 October 2017. The brightening arc indicates east‐west structures with longitudinal scales of ∼0.5°–2.0°. Field‐aligned bi‐directional electrons with an energy range between 66 and 1,800 eV were detected by the satellite, simultaneously with the appearance of the brightening arc in the camera. These electrons were probably supplied from the auroral brightening region in the ionosphere, indicating that the satellite was on the same field line of the brightening aurora. The magnetic and electric field data show characteristic fluctuations and earthward Poynting flux around the time that the satellite crossed the aurora. Anti‐phase oscillations between the thermal pressure and the magnetic pressure are also reported. Based on these observations, we suggest the possibility that a ballooning instability occurred in the source region of the substorm brightening arc in the inner magnetosphere at L ∼ 6. Plain Language Summary: A frequently occurring source of variations in the magnetosphere is the substorm, a process that causes energy dissipation into the atmosphere. Substorm is presented as the development of aurorae at high latitudes in the ionosphere. The study of substorm processes helps in understanding the near‐Earth space environment and the space weather. Along Earth's magnetic field lines, the aurora at a latitude of ∼65°N can be traced to ∼4–7 Earth radii away from the Earth at the equatorial plane in space. Using a ground‐based auroral camera, we can construct the correspondence between auroral motion and field and plasma variation at the satellite. This study reports such a unique event of substorm brightening arc observed at Tromsø, Norway, on 12 October 2017. Satellite observed bi‐directional electrons prove the connection between aurora break‐up at ∼100 km altitude and its source region in the magnetosphere at ∼30,000 km away from Earth. Based on the magnetic wave spectrograms, auroral bead‐like structures and other observational results, we suggest the possibility that a ballooning plasma instability occurred in the source region of the substorm brightening arc in the inner magnetosphere. Key Points: Observation of plasma and field features in the source region of a sudden brightening auroral arc during a minor substorm onset at L ∼ 6Energization of particles, field‐aligned electrons, and electromagnetic field fluctuations were observed during the arc crossing by AraseSeveral observational facts indicate the possibility of ballooning instability occurring at this substorm onset [ABSTRACT FROM AUTHOR]

Cross‐satellite calibration of energetic particle fluxes is essential to understanding Earth's radiation belt dynamics and modeling the space radiation environment. Using the method of comparing phase space density (PSD) conjunctions in the same set of phase space coordinates, we perform a cross‐satellite calibration of energetic electron fluxes measured by the Low Earth Orbit Chinese FengYun‐3B satellite (FY‐3B) and the near‐equatorial Highly Eccentric Orbit Van Allen Probe‐A (VAP‐A), respectively. VAP‐A provides pitch angle resolved electron (directional) differential fluxes while FY‐3B provides omnidirectional electron differential fluxes. Before calculating the PSDs from FY‐3B high energy electron detector (HEED), a method is introduced to estimate the local pitch angle of FY‐3B based on the installation direction information of HEED and the T89c magnetic field model. We calculate the calibration factors by using the PSD conjunctions inferred from the two satellites for three fixed sets of (μ, K, L*) during the geomagnetic storm times. The calibration factors range from 0.76 to 0.86 varying with fixed sets of (μ, K, L*), which indicates that the electron differential fluxes from FY‐3B are higher than that from VAP‐A. During geomagnetic quiet times, we perform calibration by using independent Gaussian fitting analysis of the number of points with the unaveraged PSDs of FY‐3B and VAP‐A for three fixed sets of (μ, K, L*), with the corresponding calibration factors of 0.78, 0.72, and 0.46. The calibrated electron differential fluxes of FY‐3B measured at the high magnetic latitudes, can be used in the future modeling of the radiation belt electron dynamics. Key Points: Cross‐satellite calibrations of electron fluxes measured by FengYun‐3B satellite and Van Allen Probe‐A are conducted using electron phase space densities (PSDs)We compare PSD conjunctions and perform independent Gaussian fitting analysis in the fixed sets of (μ, K, L*)The cross‐satellite calibration factors are 0.76–0.86 and 0.46–0.78 during geomagnetic storm times and quiet times, respectively [ABSTRACT FROM AUTHOR]

Polar Cap (PC) indices, PCN North based on magnetic observations from Qaanaaq (THL) in Greenland and PCS South based on magnetic data from Vostok in Antarctica, are very useful indices for studies of solar wind‐magnetosphere interactions and for space weather monitoring and forecasts. PCN indices are issued from the Danish Space Research Institute (DTU Space) while PCS indices are issued from the Arctic and Antarctic Research Institute (AARI) in St Petersburg. Unfortunately, series of invalid PCS values have been provided from AARI and used in a number of publications issued since the PC index concept was endorsed by the International Association of Geomagnetism and Aeronomy (IAGA) in 2013. In spite of the disclosure of the failure in the PCS indices in 2018, publishing at the IAGA‐supported International Service of Geomagnetic Indices of the invalid indices has continued up to now (2022). The present contribution defines the invalidating features of the PCS indices in question, conveys examples, and discusses the applications in peer‐reviewed publications. Plain Language Summary: The polar cap south (PCS) index series submitted from the Arctic and Antarctic Research Institute (AARI) in St Petersburg since 2014 holds extended sections of invalid index values with errors up to magnetic storm level as documented in the present contribution. Indices of this PCS index series, still published, have been used in peer‐reviewed publications. Thus, it is suggested that the authors examine these works, of which a number (9) is listed here, to look for devaluating effects from using the invalid PCS indices. The submission from AARI in December 2021 of a new PCS index series has confirmed the failure of the former PCS series. Key Points: The present work has analyzed published Polar Cap South (PCS) index series and disclosed comprehensive occurrences of invalid index valuesThe investigations have provided a survey of peer‐reviewed publications being devaluated by having used invalid PCS index valuesThe reported analyses have shown that unclear resolutions issued by the International Association of Geomagnetism and Aeronomy (IAGA) have supported the use of PCS index series without questioning their validity [ABSTRACT FROM AUTHOR]

The nightside magnetic field at the middle and low latitude observatory is usually disturbed, decreasing in its north component (noted as Bx) during the periods of enhanced space weather activity. As indicated by the SYM‐H index, the decreasing field mainly originated from the magnetospheric westward ring current and the cross‐tail currents. In the meridian direction, the maximum of the disturbed field has been observed not always located at the geomagnetic dipole equator. One possible explanation is that the latitudinal position of the magnetotail current systems is affected by the orientation of solar wind to the Earth's geomagnetic dipole axis (noted as tilt angle). For a non‐zero tilt angle, the current deviates from the equatorial position (e.g., Malin & Mete Işikara, 1976; 10.1111/j.1365-246x.1976.tb07096.x). In this study, the mid‐low latitudinal distribution of the nightside disturbed field is systematically investigated every ∼1.5 hr, based on the magnetic surveys of the Swarm satellite constellation at polar orbits of ∼450 km altitude. The meridian distribution of Bx regularly appears like an upward‐opening parabolic shape with its vertex representing the magnetic minimum. In summer, the minimum is found generally located in the southern hemisphere, while in winter the pattern is reversed. The overall distribution demonstrates north‐south asymmetry. When the disturbed level is low, the deviation of the magnetic minimum from the dipole equator usually gets larger. The north‐south asymmetric distribution during storm times is further analyzed. In the initial phase, the magnetic minimum was shifted prominently (about 10°–30°) from the equator, then moved back (with the offset less than 10°) in the main phase and recovery phase in most cases. Moreover, the latitudinal deviation undergoes a diurnal variation, which is conspicuous in spring and autumn. It seems the asymmetric distribution is anti‐correlated to the contribution of the ring current, indicated by the SYM‐H index. The warping structure of the cross‐tail current sheet probably plays a predominant role in generating the north‐south asymmetry of the magnetic field disturbances for non‐zero dipole tilt angles. Monitoring the external magnetic configuration via the low‐Earth‐orbit satellites will provide fundamental evidence to identify the relative position of the ring current and the cross‐tail current, as well as the dynamic processes in the near magnetotail. Key Points: The nightside magnetic disturbed maximum at mid‐low latitudes annually shifts in meridian direction, which is notable when SYM‐H is quietThe disturbed maximum generally stayed close to the dipole equator in the main phase and the recovery phase of the geomagnetic stormThe cross‐tail current largely contributes to the near‐Earth disturbed fields, dominating the magnetic north‐south asymmetric distribution [ABSTRACT FROM AUTHOR]

Aurora provides an essential diagnostic to spatial and temporal variations of terrestrial space environment and is also an important proxy of solar and geomagnetic activities. Contemporary auroral observations have just continued for more than half a century. Visual auroral phenomena recorded in historical books provide key clues to understand the solar and geomagnetic activities in the long history prior to modern era. In this study, we compiled a new auroral catalog from ancient Korean historical books, including 2013 auroral records with day-level resolution from 1012 to 1811 CE, especially for the records searched from the Seungjeongweon Ilgi. The number of auroral records in this new catalog is greatly enlarged compared with previous lists. The occurrence of the aurora in the new catalog is generally consistent with previous data sets. This extended data set provides valuable support for various studies related to solar-terrestrial space weather and ancient climates in the past millennium. [ABSTRACT FROM AUTHOR]

Many modern outer radiation belt models simulate the long‐time behavior of high‐energy electrons by solving a three‐dimensional Fokker‐Planck equation for the drift‐ and bounce‐averaged electron phase space density that includes radial, pitch‐angle, and energy diffusion. Radial diffusion is an important process, often characterized by a deterministic diffusion coefficient. One widely used parameterization is based on the median of statistical ultralow frequency (ULF) wave power for a particular geomagnetic index Kp. We perform idealized numerical ensemble experiments on radial diffusion, introducing temporal and spatial variability to the diffusion coefficient through stochastic parameterization, constrained by statistical properties of its underlying observations. Our results demonstrate the sensitivity of radial diffusion over a long time period to the full distribution of the radial diffusion coefficient, highlighting that information is lost when only using median ULF wave power. When temporal variability is included, ensembles exhibit greater diffusion with more rapidly varying diffusion coefficients, larger variance of the diffusion coefficients and for distributions with heavier tails. When we introduce spatial variability, the variance in the set of all ensemble solutions increases with larger spatial scales of variability. Our results demonstrate that the variability of diffusion affects the temporal evolution of phase space density in the outer radiation belt. We discuss the need to identify important temporal and length scales to constrain variability in diffusion models. We suggest that the application of stochastic parameterization techniques in the diffusion equation may allow the inclusion of natural variability and uncertainty in modeling of wave‐particle interactions in the inner magnetosphere. Plain Language Summary: The Van Allen outer radiation belt is a region in near‐Earth space containing mostly high‐energy electrons trapped by the Earth's geomagnetic field. It is a region populated by satellites that are vulnerable to damage from the high‐energy environment. Many modern radiation belt models simulate the behavior of the high‐energy electrons with a diffusion model, which describes how electrons spread out from areas of higher concentration to areas of lower concentration. An important process in these models is radial diffusion, driven by ultralow frequency (ULF) waves, where electrons are drawn from the outer boundary and accelerated toward Earth, or pushed away from the outer radiation belt and lost to interplanetary space. Radial diffusion is generally characterized by a parameter that provides a single output from the specified inputs and does not allow for any variability in the physical process. In this study we present a series of numerical experiments on radial diffusion, which allow for natural variability in both time and space and see how modeling of radial diffusion is impacted. Our results find that better understanding of temporal and spatial variations of ULF wave interactions with electrons, and being able to characterize these variations to a good level of accuracy, is vital to produce a robust description of radial diffusion over long timescales in the outer radiation belt. Key Points: Ensemble numerical experiments of radial diffusion are performed using stochastic parameterization of diffusion coefficientsWhen reasonable temporal and spatial variability of diffusion is included, the temporal evolution of phase space density changesDiffusion depends on temporal and spatial scales of variability, and the size and shape of the distribution of diffusion coefficients [ABSTRACT FROM AUTHOR]

The geomagnetic storm (or "Carrington event") of 1–2 September 1859 is one of the largest geomagnetic disturbances on record. At the time, it caused widespread disruption to telegraph systems and was accompanied by aurorae seen overhead as far south as ∼29° magnetic latitude. The magnitude of the Carrington event means it remains a popular subject of study in the field of space weather, despite the sparse magnetic measurements available from the time. One set of measurements that is available is from the Rome observatory ("Collegio Romano," magnetic latitude ∼38.6°). Here we transcribe these horizontal magnetic field data and convert them to nanoteslas. We find that the device used at Rome had an operational range of around 305 nT. Despite going off‐scale during the storm, the magnetometer at Rome recorded changes of hundreds of nanoteslas per minute and tens of nanoteslas per second in the horizontal magnetic field. Apart from the tabulated data, we also examine the reported off‐scale deviation of 3,000 nT at Rome during the storm. While we could not explicitly locate this reported deviation in the tabulated data, we find that this deviation is comparable to magnetic variations seen at auroral latitudes for modern large magnetic storms, indicating that Rome was in the auroral oval during the morning of 2 September 1859. By comparing this large off‐scale deviation to modern geomagnetic data, we estimate that Rome may have experienced a maximum change of 420 nT min−1. Key Points: The horizontal magnetic data recorded at Rome during the August–September 1859 storms is tabulated and converted to nanoteslasThe reported horizontal deviation of ∼3,000 nT is examined and found to be consistent with an expanded auroral ovalMaximum changes of around 420 nT min−1 are estimated for Rome during the morning of 2 September 1859 [ABSTRACT FROM AUTHOR]

In this work, the occurrence probability of extreme geomagnetic storms is estimated by applying extreme value theory to the geomagnetic activity Aa index. The Aa index has 172 years observation time span, which is much longer than other geomagnetic indices, and thus is more suitable for analysis for rarely occurred extreme geomagnetic storms. We use two newly developed extreme value theory methods, block maxima method and peak over threshold, and find that the extreme geomagnetic storm that happened in March 1989 may happen once per century. This result implies that we should pay more attention to such extreme geomagnetic storms that can cause space weather hazards. Key Points: The probability of the Quebec‐like geomagnetic storms is about once per centuryThe negative shape parameter implies an upper bound for the Aa indexThe highest expected value for the Aa index is shown to be 392 nT [ABSTRACT FROM AUTHOR]

Simulating times of high geomagnetic activity are an important part of the continuing efforts to understand space weather and its impacts on humanity. Recent improvements to the Ring‐current Atmosphere interactions Model with Self‐Consistent magnetic field (RAM‐SCB) have been undertaken, with the purpose of expanding the accuracy and robustness of the model during these highly active times. The improvements include a number of changes to the functionality of both the RAM model and the self‐consistent magnetic field calculation. In addition, the models have been modernized and rewritten in an effort to make them more user friendly and understandable. The effect of these changes is shown by simulating the day of 17 March 2013, which saw Sym‐H drop below −100 nT and included a magnetosphere push‐in within geosynchronous orbit. The comparison between the previous model configuration and this new configuration is investigated by calculating a series of metrics for the relation between the observed and modeled Sym‐H, as well as the >10‐keV electron flux measured by the HOPE and MAGEIS instruments aboard RBSP‐B and the simulated electron flux. These metrics show that the improvements to the model have increased the accuracy of the model for the given simulation. Key Points: The RAM‐SCB inner magnetosphere model has been upgraded to track the magnetopause location and allow it to move inside the RAM‐SCB domainImprovements made to the self consistent magnetic field calculation give a more accurate solution for disturbed configurationsThe improved model shows improved accuracy and skill for SYM‐H and electron flux at Van Allen Probes for the 17 March 2013 geomagnetic storm [ABSTRACT FROM AUTHOR]

We report MMS observations of the ion‐scale flux transfer events (FTEs) that may involve two main X lines and tearing instability between the two X lines. The four spacecraft detected multiple isolated regions with enhanced magnetic field strength and bipolar Bn signatures normal to the nominal magnetopause, indicating FTEs. The currents within the FTEs flow mostly parallel to B, and the magnetic tension force is balanced by the total pressure gradient force. During these events, the plasma bulk flow velocity was directed southward. Detailed analysis of the magnetic and electric field and plasma moments variations suggests that the FTEs were initially embedded within the exhaust region north of an X line but were later located southward/downstream of a subsequent X line. The cross sections of the individual FTEs are in the range of ~2.5–6.8 ion inertial lengths. The observations suggest the formation of multiple secondary FTEs. The presence of an X line in the exhaust region southward of a second X line results from the southward drift of an old X line and the reformation of a new X line. The current layer between the two X lines is unstable to the tearing instability, generating multiple ion‐scale flux‐rope‐type secondary islands. Key Points: MMS observation of ion‐scale flux ropes in the reconnection outflow regionThe initial X line is embedded in the exhaust region downstream of a second X lineFlux transfer events (FTE) are formed between the two X lines due to tearing instability [ABSTRACT FROM AUTHOR]

Motivated by ongoing discussion regarding the magnetic configuration in the near‐Earth and midtail regions and its role in populating the inner magnetosphere during long‐duration steady magnetospheric convection (SMC) events, we analyze a rich collection of observations during ~10 hr of strong, steady solar wind driving. Auroral boundaries and regions of stretched and dipolarized magnetic field in the plasma sheet were monitored using solar electron loss cone anisotropy observed by low‐altitude spacecraft. Following a southward turning of the interplanetary magnetic field and a subsequent 3‐ to 4‐hr period of large‐scale substorm‐related reconfigurations and plasma injections, the near‐Earth magnetic configuration evolved into a nonstandard type, which lasted until the end of this SMC event (5 hr). During that time a dipolarized region with complicated Bz landscape persisted in the midtail, while the configuration was very stretched in the near tail. This was manifested as a highly depressed magnetic Bz component at geostationary orbit and as persistent nonadiabatic electron scattering at the periphery of the outer radiation belt. In addition, in situ observations suggest that a thin current sheet extended longitudinally toward the dawn terminator. In the return convection region near the terminator, observations of this azimuthal current sheet were sporadically interrupted/modulated by earthward convecting plasma structures, either remnants of reconnection‐produced plasma bubbles or flapping waves. The hybrid magnetotail configuration (dipolar in the midtail and stretched in the near tail) observed during this long‐duration SMC event poses a challenge for empirical magnetospheric modeling. Key Points: Magnetotail observations and remote sensing from low altitudes are used to monitor magnetic configuration for long steady convection eventExisting models fail to predict hybrid configuration (stretched inner tail and dipolarized midtail) formed after 3 hr of intense drivingUnusual azimuthal current sheet in the return convection region was sporadically modulated by earthward convecting plasma structures [ABSTRACT FROM AUTHOR]

We investigate ionospheric perturbations associated with traveling ionospheric disturbance (TID) during the geomagnetic storm on Memorial Day weekend (28 May) 2017. Results show the presence of both equatorward propagating large‐scale TIDs (LSTIDs) and poleward propagating medium‐scale TIDs. Equatorward moving TIDs are connected with enhanced auroral activity owing to geomagnetic storm conditions, while poleward TIDs are believed to be induced by local atmospheric gravity wave sources originating from convection activities near the West Coast of North America. Measurements from magnetometers located in the west, central, and east regions of North America are used to evaluate energy inputs from the auroral belt, and these observations confirm that equatorward LSTIDs are due to geomagnetic disturbance. The observed LSTID waves were characterized by some uncommon features, such as horizontal wavefront stretching from coast to coast, aligned in the NW to SE direction, and propagating to the southwest (equatorward) direction during the storm main phase period. In contrast, during the recovery phase and on other experimental control days, the observed medium‐scale TIDs were characterized with relatively smaller wavelengths aligned in the NW to SE direction and propagate primarily in the northeast (poleward) direction. Our results also reveal that LSTID waves appear to travel faster in the central continental region compared to LSTIDs in the western and eastern regions. Plain Language Summary: This study reveals the coupling of magnetosphere and ionosphere system as well as the interaction between the lower and upper atmosphere. Using the Global Navigation Satellite Systems (GNSS) receivers widely distributed over United States, networks of magnetometers over North America, and satellite measurements from the National Oceanic and Atmospheric Administration‐Geostationary Operational Environmental Satellite (NOAA‐GOES) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) missions, we investigated the traveling ionospheric disturbance (TID) characteristics during the 28 May 2017 Memorial Day weekend geomagnetic storm and some quiet geomagnetic periods. Our results show equatorward large‐scale TID owning to the 2017 Memorial Day weekend geomagnetic storm and poleward medium‐scale TID propagation owning to atmospheric convection activity over the continental United States. Key Points: Coupling between magnetosphere and ionosphere resulting from the 2017 Memorial Day weekend geomagnetic stormInteractions between lower and upper atmosphere owning to tropospheric weather convection activityEquatorward and poleward traveling ionosperic disturbances (TIDs) over North America during quiet and disturbed periods [ABSTRACT FROM AUTHOR]