Search

Topological materials occupy an important place in the quantum materials family due to their peculiar low‐energy electrodynamics, hosting emergent magneto‐electrical, and nonlinear optical responses. This manuscript reports on the optical responses for the magnetic topological nodal semimetal Co2MnGa, studied in a thin film geometry at various thicknesses. The thickness‐dependent optical conductivity is investigated, observing a substantial dependence of the electronic band structure on thickness. Additionally, details on the ultrafast response of the low energy excitations in the terahertz frequency are reported by employing optical pump‐terahertz probe (OPTP) spectroscopy. In particular, the photocarrier dynamics of Co2MnGa thin films is studied at varying pump fluence, pump wavelength, and film thickness, observing a negative THz photoconductivity which is assigned to a dynamical formation of large polarons in the material. [ABSTRACT FROM AUTHOR]

We formulate scalar electrodynamics in the braided L∞$L_\infty$‐algebra formalism and study its perturbative expansion in the algebraic framework of Batalin–Vilkovisky quantization. We also confirm that UV/IR mixing is absent at one‐loop order in this noncommutative field theory, and that the non‐anomalous Ward‐Takahashi identities for the braided gauge symmetry are satisfied. [ABSTRACT FROM AUTHOR]

Brain voltage plays a crucial role in indicating internal functions or diseases, and optical voltage imaging has gained intensive attention in recent years. Despite encouraging progress, current implementations encounter limitations pertaining to penetration depth, field of view (FOV), and photostability of indicators. To mitigate these challenges, a robust voltage‐sensitive dye (VSD)‐based whole‐field photoacoustic brain detection (WF‐PABD) platform is proposed, enabling direct evaluation of voltage dynamics across the whole brain, forming as PA‐VSD. WF‐PABD is equipped with a 512‐element ring‐array ultrasound detector capable of 360‐degree scanning, providing a large FOV (≈5 cm), high spatial resolution (≈110 µm), and rapid imaging acquisition. The proposed VSD remained ≈75% photostability after 30 min laser exposure, much greater than most calcium sensors. The optical voltage‐response mechanisms are validated and the capability of PA‐VSD to directly screen seizures is established. It is demonstrated that investigating connectivity among different brain regions allows to identify the precise location of active epileptic foci as well as the electrical conduction pathways and their directionality through fast temporal visualization. In summary, this study not only addresses the need for non‐invasive, high‐resolution, long‐term, and direct monitoring of brain voltage but also empowers exciting venues for PA applications in neuroscience. [ABSTRACT FROM AUTHOR]

Hot Flow anomalies (HFAs), one of the most well‐analyzed transient phenomena in the Earth's foreshock, are known as kinetic structures driven by tangential discontinuities (TDs). Recently, a 2‐dimensional (2D) magnetohydrodynamics (MHD) model reproduced HFAs with either a high‐ or low‐density core. Further investigation of an HFA with two cores observed by the Magnetospheric Multiscale (MMS) mission is reported. The observation via the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) mission suggests this MHD HFA is associated with a foreshock density hole‐like structure. The trailing flux tube in simulation may propagate with a TD in the foreshock. Our work suggests that HFAs with two low‐density cores can also be achieved in MHD process. Results show the total ram pressure can be an excellent diagnostic for the presence of transient structures, such as HFAs, at the bow shock. Plain Language Summary: The hot flow anomaly (HFA) is a typical foreshock transient on the upstream of Earth's bow shock. This phenomenon is characterized by heating and significant flow deflection inside its core region and is traditionally believed to be a kinetic structure associated with tangential discontinuities (TDs). More recently, HFA‐like structures have also been generated through magnetohydrodynamic (MHD) simulations. Here, we present an HFA with two low‐density cores captured by the Magnetospheric Multiscale (MMS) satellite. Observation from the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) mission suggests the observed double‐core HFA is driven by foreshock density hole‐like structure, which is propagating with the observed TD to the bow shock. The observed double‐core HFA is reproduced by a 2‐dimensional (2D) MHD model, in which two low‐density flux tubes are inputted simultaneously. Our observation and simulations show that the total ram pressure is an excellent indicator for the presence of transient structures at the bow shock. Key Points: Double‐core hot flow anomalies can be reproduced by 2‐dimensional magnetohydrodynamic model with two simultaneous low‐density flux tubesThe simulation results and the observation show general agreementThe total ram pressure is an excellent indicator for the formation of hot flow anomalies at the bow shock [ABSTRACT FROM AUTHOR]

A statistical study has been made of dynamic small‐scale auroral events in order to understand the drivers of the large variability in the electrodynamics of auroral arcs at fine scales. We used the Auroral Structure and Kinetics (ASK) instrument, located on Svalbard, in order to measure various electrodynamic properties of fine scale auroral arcs. We performed Spearman and Kendall statistical tests and found two significant correlations. The first is between the mean precipitation flux and the variability of flux, which we assume is because of the dynamic and bursty nature of the acceleration mechanism and its dependence on Alfvén waves. The second correlation is between the variability of the precipitating electron flux and the variability of the tangential component of the electric field close to the arc and perpendicular to the magnetic field. We propose that both variabilities occur because of the variability of the upward (field‐aligned) current sheet in and around the arc, which is dynamic and non‐uniform. The correlation between the two variabilities can therefore be explained by their common source. Key Points: We report a statistical analysis of small scale electrodynamics, to better understand the drivers of small scale variabilityThere is a significant correlation between average flux and variability of the flux in our eventsThere is a significant correlation between variability of tangential component of the electric field and variability in fluxWe propose that variability of the dynamic non‐uniform field aligned current sheet would be a source of variability in both electric fields and in flux, causing them to be correlated [ABSTRACT FROM AUTHOR]

Quantum error correction codes based on continuous variables play an important role for the implementation of quantum communication systems. A natural application of such codes occurs within quantum repeater systems which are used to combat severe channel losses and local gate errors. In particular, channel loss drastically reduces the distance of communication between remote users. Here, a cavity‐quantum electrodynamics (QED) based repeater scheme is considered to address the losses in the quantum channel. This repeater scheme relies on the transmission of a specific class of rotationally invariant error‐correcting codes. Several rotation‐symmetric bosonic codes (RSBCs) are compared for encoding the initial states of two remote users connected by a quantum repeater network against the convention of the cat codes and the performance of the system is quantified by using the secret key rate. In particular, the number of stations required to exchange a secret key over a fixed distance is determined and establish the resource overhead. For higher‐loss codes, the results show that a secret key rate (SKR) value of 0.01 bit per channel use can be achieved at a distance of 10000 km, with an elementary distance of 1.3 km. [ABSTRACT FROM AUTHOR]

The morphology and motion of auroras have been widely studied due to their indications on magnetospheric processes. Here, we report a new kind of "auroral curls," which have wavelengths in the mesoscale (∼100 km) and propagate azimuthally. Utilizing data from the Chinese Antarctic Zhongshan Station (the all‐sky imager and the high‐frequency radar), the Active Magnetosphere and Planetary Electrodynamics Response Experiment and the Defense Meteorological Satellite Program, we analyze an event occurred on 23 April 2019. We find these curls are fine structures in the poleward boundary of multiple arcs. Corresponding field‐aligned currents manifest as a series of longitudinally arranged pairs, while ionospheric flow velocities nearby oscillate with periods in the Pc 5 band. Observational evidence suggests these curls are connected with ultra‐low frequency (ULF) waves, which opens the possibility of using auroras to globally image ULF waves. Plain Language Summary: Auroras caused by precipitation of magnetospheric particles contain information about physical processes happened in the magnetosphere. In this letter, we report a new kind of auroral dynamic forms observed in Antarctica. These structures present both spatial and temporal periodic characteristics, which have similar scales with those of magnetospheric ultra‐low frequency (ULF) waves. We propose these auroral forms are connected with ULF waves, which provides a potential method to globally image ULF waves by analyzing properties of these auroras. Key Points: Azimuthally propagating "auroral curls" with mesoscale wavelengths were observed in AntarcticaThese curls are fine structures in the poleward boundary of multiple arcs formed by longitudinal‐arranged field‐aligned current pairsIonospheric flow velocities nearby oscillate with periods in the Pc 5 band, indicating connections with ultra‐low frequency waves [ABSTRACT FROM AUTHOR]

There has been ongoing debate about the efficacy and mechanism of action of neuromodulation devices in pain relief applications. It has recently been suggested that both issues may be resolved if electromagnetic theory is incorporated into the understanding and application of this technology, and we therefore undertook an in silico analysis to further explore this idea. We created a CAD replication of a standard neuromodulation electrode array with a generic linear 3/6 mm 8‐contact lead, developed a parameterized algorithmic model for the pulse delivered by the device and assigned material properties to biologic media to accurately reflect their electromagnetic properties. We then created a physical simulation of the device's output both in air and in the biophysical environment. The simulations confirmed the presence of an electromagnetic field (EM field). Variations in programming of the device affected the strength of the EM field by orders of magnitude. The biologic media all absorbed the EM field, an effect which was particularly pronounced in cerebrospinal fluid and muscle. We discuss the implications of all these findings in relation to the literature. We suggest that knowledge of electromagnetic theory and its application within the biophysical space is required for the optimal use of neuromodulation devices in pain relief applications. [ABSTRACT FROM AUTHOR]

Multipoles are paramount for describing electromagnetic fields in many areas of nanoscale optics, playing an essential role in the design of devices in plasmonics and all‐dielectric nanophotonics. Challenging the traditional division into electric and magnetic moments, toroidal moments are proposed as a physically distinct family of multipoles with significant contributions to the properties of matter. However, the apparent impossibility of separately measuring their response sheds doubt on their true physical significance. Here, the possibility of selectively exciting toroidal moments is confirmed without any other multipole. A set of general conditions is developed that any current distribution must fulfill to be entirely described by toroidal moments and prove the results in an analytically solvable case. The new theory allows to design and verify experimentally an artificial structure supporting a pure broadband toroidal dipole response in the complete absence of the electric dipole and other "ordinary" multipole contributions. In addition, a structure capable of supporting a novel type of non‐radiating source is proposed‐ a "toroidal anapole," originating from the destructive interference of the toroidal dipole with the unconventional electromagnetic sources known as mean square radii. The results in this work provide conclusive evidence on the independent excitation of toroidal moments in electrodynamics. [ABSTRACT FROM AUTHOR]

We introduce a new framework called Machine Learning (ML) based Auroral Ionospheric electrodynamics Model (ML-AIM). ML-AIM solves a current continuity equation by utilizing the ML model of Field Aligned Currents of Kunduri et al. (2020, https://doi.org/10.1029/2020JA027908), the FAC-derived auroral conductance model of Robinson et al. (2020, https://doi.org/10.1029/2020JA028008), and the solar irradiance conductance model of Moen and Brekke (1993, https://doi.org/10.1029/92gl02109). The ML-AIM inputs are 60-min time histories of solar wind plasma, interplanetary magnetic fields (IMF), and geomagnetic indices, and its outputs are ionospheric electric potential, electric fields, Pedersen/Hall currents, and Joule Heating. We conduct two ML-AIM simulations for a weak geomagnetic activity interval on 14 May 2013 and a geomagnetic storm on 7-8 September 2017. ML-AIM produces physically accurate ionospheric potential patterns such as the two-cell convection pattern and the enhancement of electric potentials during active times. The cross polar cap potentials (ΦPC) from ML-AIM, the Weimer (2005, https://doi.org/10.1029/2004ja010884) model, and the Super Dual Auroral Radar Network (SuperDARN) data-assimilated potentials, are compared to the ones from 3204 polar crossings of the Defense Meteorological Satellite Program F17 satellite, showing better performance of ML-AIM than others. ML-AIM is unique and innovative because it predicts ionospheric responses to the time-varying solar wind and geomagnetic conditions, while the other traditional empirical models like Weimer (2005, https://doi.org/10.1029/2004ja010884) designed to provide a quasi-static ionospheric condition under quasi-steady solar wind/IMF conditions. Plans are underway to improve ML-AIM performance by including a fully ML network of models of aurora precipitation and ionospheric conductance, targeting its characterization of geomagnetically active times. [ABSTRACT FROM AUTHOR]

Flexible piezoelectric sensors have been spotlighted as an essential human–machine interface (HMI) by obtaining high‐quality data from omnipresent biomechanical inputs. Because human voice is the most intuitive bio‐signal among them, flexible piezoelectric acoustic sensors (f‐PAS) have a potential to shift the paradigm of HMI technologies. Despite the reported outstanding performance such as high sensitivity and speaker recognition accuracy, the theoretical investigation of f‐PAS has been insufficient to realize future customized development, because sensing principles are fundamentally different from commercialized microphones. Here, a theoretical framework of self‐powered f‐PAS by using mechanical and electrical physics is introduced. First of all, the basic theory of f‐PAS is compared with the auditory system of human cochlear. Based on the biomimetic trapezoidal shape, the resonant frequencies are analyzed with various structural and material conditions. In addition, the piezoelectricity of f‐PAS is derived to predict the sensitivity and SNR prior to experiments. To investigate sensor properties under the medium condition that is similar to human ear, the acoustic responses depending on the states of matter are theoretically compared. Finally, the distance limit of f‐PAS is studied with the correlations between piezoelectricity and sound pressure, which would provide novel strategies of functional material design for future applications of f‐PAS. [ABSTRACT FROM AUTHOR]

Recently, interactions between putative axions and magnetic monopoles have been revisited by two of the authors.[1] It is shown that significant modifications to conventional axion electrodynamics arise due to these interactions, so that the axion–photon coupling parameter space is expanded from one parameter gaγγ$g_{a\gamma \gamma }$ to three (gaγγ,gaAB,gaBB)$(g_{a\gamma \gamma },g_{aAB},g_{aBB})$. Poynting theorem is implemented to determine how to exhibit sensitivity to gaAB$g_{aAB}$ and gaBB$g_{aBB}$ using resonant haloscopes, allowing new techniques to search for axions and a possible indirect way to determine if magnetically charged matter exists. [ABSTRACT FROM AUTHOR]

Solar eclipses present a rare glimpse into the impact of ionospheric electrodynamics on the magnetosphere independent of other well studied seasonal influences. Despite decades of study, we still do not have a complete description of the conditions for geomagnetic substorm onset. We present herein a mutual information based study of previously published substorm onsets and the past two decades of eclipses which indicates the likelihood of co‐occurrence is greater than random chance. A plausible interpretation for this relation suggests that the abrupt fluctuations in ionospheric conductivity during an eclipse may influence the magnetospheric preconditions of substorm initiation. While the mechanism remains unclear, this study presents strong evidence of a link between substorm onset and solar eclipses. Plain Language Summary: Geomagnetic substorms are a long‐studied phenomenon with significant potential for impact on human infrastructure and activities. Despite decades of research, a comprehensive description of what causes these violent eruptions of space plasma near earth has yet to be agreed upon. Although their evolution is well documented, the precise conditions required for substorms to manifest appear to be more complex than previously understood. We present evidence in this manuscript of a mutual dependence between solar eclipses and substorms, which suggests that changes to the upper atmosphere like those occurring during an eclipse may influence substorm development. Key Points: In a given 2 hr window between 2001 and 2021, a substorm occurs roughly 40% of the time, increasing to 67% during windows including an eclipseConditional Point‐wise Mutual Information analysis suggests the probability of eclipse‐substorm co‐occurrence is higher than random chanceThe mutual dependence between eclipses and substorms is likely the result of ionospheric conductivity feedback into the magnetosphere [ABSTRACT FROM AUTHOR]

Magnetoelectricity denotes the relationship between electric polarization and magnetization. In materials with an intrinsic magnetoelectric (ME) effect, the energy density comprises the polarization, magnetization, and ME energy densities. These three components of energy define local (subwavelength) characteristics of electromagnetic (EM) responses in multiferroic materials. In a subwavelength domain, coupling between the electric and magnetic dipole oscillations forms the ME field structures that are characterized by the violation of both spatial and temporal symmetry. Unlike multiferroics, bi(an)isotropic metamaterials are associated with an EM response characterized only by spatial symmetry breaking. This also applies to chiral materials. Since no "intrinsic magnetoelectricity" is assumed in such structures, any concepts about the stored ME energy are not applicable. This clearly points to the effect of nonlocality. That is why the basic concepts of bi(an)isotropy can only be analyzed by the EM far‐field characteristics. In this paper, it is argued that in the implementation of local (subwavelength) ME meta‐atoms and systems for near‐field probing of chirality, the concept on ME energy is crucial. Real ME energy can occur when ME fields in a singular subwavelength domain are characterized by a violation of both the symmetry of time reversal and spatial reflection. [ABSTRACT FROM AUTHOR]

Energy conditions and quasinormal modes (QNMs) for scalar perturbations of regular charged black holes within the framework of General Relativity coupled to non‐linear electrodynamics (NLE) are discussed. The frequencies are computed numerically adopting the WKB method, while in the eikonal limit an analytic expression for the spectra is obtained. The impact of the electric charge, the angular degree, and the overtone number on the spectra is investigated in detail. All frequencies are characterized by a negative imaginary part, and that each type of energy conditions imply a different quasinormal spectrum are found. [ABSTRACT FROM AUTHOR]

Recently, a variety of mechanisms have been discovered that extend the range of optical techniques for identifying and characterizing molecular chirality, beyond those associated with optical polarization. It is now evident that beams of light with a twisted wavefront, known as optical vortices, can also interact with chiral matter with a specificity determined by relative handedness. Exploring this chiral sensitivity of vortex light in its interactions with matter requires careful consideration of the symmetry properties that engage in such processes. Most of the familiar measures of chirality are directly applicable to either matter, or to light itself—but only to one or the other. To elicit the principles that determine the viability of distinctly optical vortex‐based forms of chiral discrimination invites a more universal approach to symmetry analysis, as is afforded by the common, fundamental physics of CPT symmetry. Taking this approach supports a comprehensive and straightforward analysis to identify the mechanistic origins of vortex chiroptical interactions. Careful inspection of selection rules for absorption also elicits the principles governing any identifiable engagement with vortex structures, providing a reliable basis to ascertain the viability of other forms of enantioselective vortex interaction. [ABSTRACT FROM AUTHOR]

In this paper, general uncharged accelerating BTZ black hole solutions are extracted and studied some of their properties. The analysis shows that spacetime's asymptotic behavior depends on four parameters: the cosmological constant, mass, acceleration, and topological constant. Then, the temperature of these black holes are studied and it is found that the temperature is always positive for AdS spacetime. Next, the study is extended for extracting charged accelerating BTZ black hole solutions in the presence of a nonlinear electrodynamics field known as conformally invariant Maxwell. The findings indicate a coupling between the electrical charge and other quantities of the accelerating BTZ black holes. The asymptotic behavior of charged accelerating BTZ black holes depends on five parameters: the cosmological constant, the electrical charge, mass, the acceleration parameter, and the topological constant. Then, the effects of charge, acceleration parameters, and the topological constant on the root of these black holes are studied. Finally, the temperature of these black holes in AdS spacetime is investigated. For these black holes, the temperature depends on the electrical charge, accelerating parameter, and the cosmological constant. The analysis indicates that the temperature of charged accelerating BTZ AdS black holes is always positive. [ABSTRACT FROM AUTHOR]

A quantum repeater scheme based on cavity‐quantum electrodynamics (QED) and quantum error correction of channel loss via rotation‐symmetric bosonic codes (RSBCs) is proposed to distribute atomic entangled states over long distances without memories and at high clock rates. In this scheme, controlled rotation gates, i.e., phase shifts of the propagating light modes conditioned upon the state of an atom placed in a cavity, provide a mechanism both for the entangled‐state preparations and for the error syndrome identifications. In order to assess the performance of this repeater protocol, an explicit instance of RSBCs—multicomponent cat codes—are studied quantitatively. It is found that the total fidelity and the success probability for quantum communication over a long distance (such as 1000 km) both can almost approach unity provided a small enough elementary distance between stations (smaller than 0.1 or 0.01 km) and rather low local losses (up to 0.1%) are considered. In a quantum key distribution application, secret key rates can become correspondingly high, both per channel use, beating the repeaterless bound, and per second thanks to the relatively high clock rates of the memoryless scheme. Based upon the cavity‐QED setting, this scheme can be realized at room temperature and at optical frequencies. [ABSTRACT FROM AUTHOR]

Non‐linear electrodynamics (NLED) is a generalization of Maxwell's electrodynamics for strong fields. It has significant implications for the study of black holes and cosmology and has been extensively studied in the literature, extending from quantum to cosmological contexts. In this work, two new ways to investigate these non‐linear theories are investigated. First, the Blandford‐Znajek mechanism is analyzed in light of this promising theoretical context, providing the general form of the extracted power up to second order in the black hole spin parameter a. It is found that, depending on the NLED model, the emitted power can be extremely increased or decreased, and that the magnetic field lines around the black hole seem to become vertical quickly. Considering only separated solutions, it is found that no monopole solutions exist and this could have interesting astrophysical consequences (not considered here). Last but not least, it is attempted to confine the NLED parameters by inducing the amplification of primordial magnetic fields ("seeds"), thus admitting non‐linear theories already during the early stages of the Universe. However, the latter approach proves to be useful for NLED research only in certain models. These (analytical) results emphasize that the behavior of non‐linear electromagnetic phenomena strongly depends on the physical context and that only a power‐law model seems to have any chance to compete with Maxwell. [ABSTRACT FROM AUTHOR]

This article discusses a possibility of deriving the basic equations of electrodynamics by means of the rational continuum mechanics principles and methods. We show that in order to give rise to the challenge, one needs to consider an elastic continuum of two‐spin particles of a special kind. We derive classical Maxwell's equations and discuss reasons limiting their application. After that, we propose more general equations that presumably provide a more correct description of the electromagnetic field. [ABSTRACT FROM AUTHOR]

The electromagnetic fields (D, H) represent contravariant components of an antisymmetric 4‐tensor, while the fields (E, B) represent covariant components of the same 4‐tensor. Both sets are identical in Lorentz frames. The governing equations for them represent equations of balance for the flux of D and the flux B across open surfaces and they are invariant under arbitrary analytic transformation of space and time. This convenient property has motivated mechanicians to reformulate mechanics and thermodynamics in terms of the symmetric 4‐tensor of energy‐momentum so as to exhibit, perhaps, the same invariance as electromagnetism: general relativity. The interpretation of electromagnetism in terms of fields carries the theory a long way. But it breaks down when absorption and emission of radiation by bodies is concerned. In this case the proper view is that of a photon gas. Also the elegant union of electromagnetism and thermodynamics meets practical difficulties for bodies with internal structure, like electric and magnetic dipoles or for mixtures of charged constituents. The description of such bodies is limited to velocities much smaller than that of light. Still in those bodies the entropy inequality implies severe and interesting restrictions on the dependence of material properties on the electromagnetic fields. It will be evaluated by using the Lagrange multiplier method. In the early days of electromagnetism there was much confusion among physicists and engineers about the different formulation of the fields and about proper dimensions and units. This dilemma is also briefly explained in the article. [ABSTRACT FROM AUTHOR]

This paper wants to draw attention to several issues in electrodynamic field theory and to make way for a rational continuum approach to the subject. The starting point are the balances for magnetic flux and electric charge, both in a very general formulation for volumes and for open surfaces, all of which can deform and be immaterial or material. The spatial point‐of‐view for the description of fields is favored and its advantages in comparison to the concept of material particles is explained. A straightforward answer to the question of how to choose units for the electromagnetic fields most suitably is also presented. The transformation properties of the electromagnetic fields are addressed by rewriting the balances in space–time notation. Special attention is paid to the connection between the two sets of electromagnetic fields through the so‐called Maxwell–Lorentz–æther relations. The paper ends with an outlook into constitutive theory of matter under the influence of electromagnetic fields and a discussion on curious developments in context with Maxwell's equations. [ABSTRACT FROM AUTHOR]

The dynamics of an incompressible, isotropic elastic continuum are discussed. Starting from the Lorentz‐invariant motion of defects in elastic continua, MacCullagh's aether theory (1839) of an incompressible elastic solid is reconsidered. Since MacCullagh's theory, based on linear elasticity, cannot describe charges, particular attention is given to a topological defect that causes large deformations and, therefore, requires a nonlinear description. While such a twist disclination can take the role of a charge, the deformation field of a large number of these defects produces a microstructure of deformation related to a Cosserat continuum (1909). On this microgeometric level, a complete set of quantities can be defined that satisfies equations equivalent to Maxwell's. From a broader perspective, the elastic continuum approach appears to be intriguing, yet it has considerable difficulties in describing Dirac's large numbers, another unresolved mystery in fundamental physics. Despite this shortcoming, I think, however, that this paper may stimulate further research. [ABSTRACT FROM AUTHOR]

Previous studies have shown that Strong Thermal Emission Velocity Enhancement (STEVE) events occur at the end of a prolonged substorm expansion phase. However, the connection between STEVE occurrence and substorms and the global high-latitude ionospheric electrodynamics associated with the development of STEVE and non-STEVE substorms are not yet well understood. The focus of this paper is to identify electrodynamics features that are unique to STEVE events through a comprehensive analysis of ionospheric convection patterns estimated from SuperDARN plasma drift and ground-based magnetometer data using the Assimilative Mapping of Geospace Observations (AMGeO) procedure. Results from AMGeO are further analyzed using principal component analysis and superposed epoch analysis for 32 STEVE and 32 non-STEVE substorm events. The analysis shows that the magnitude of cross-polar cap potential drop is generally greater for STEVE events. In contrast to non-STEVE substorms, the majority of STEVE events investigated are accompanied by with a pronounced extension of the dawn-cell into the pre-midnight subauroral latitudes, reminiscent of the Harang reversal convection feature where the eastward electrojet overlaps with the westward electrojet, which tends to prolong over substorm expansion and recovery phases. This is consistent with the presence of an enhanced subauroral electric field confirmed by previous STEVE studies. The global and localized features of high-latitude ionospheric convection associated with optical STEVE events characterized in this paper provide important insights into cross-scale magnetosphere-ionosphere coupling mechanisms that differentiate STEVE events from non-STEVE substorm events. [ABSTRACT FROM AUTHOR]

The launch of the Twin Rockets to Investigate Cusp Electrodynamics‐2 (TRICE‐2) took place on the morning of 08 December 2018. The two rockets of this campaign each sampled the low‐altitude Northern Hemisphere cusp region at approximately the apex of each rocket's trajectory (1,042 and 757 km for the High flyer and Low flyer, respectively). Ion and electron electrostatic analyzers (ESAs) on board each rocket measured in situ particle populations throughout the flights. Energy‐flux spectrograms of ions and electrons from each ESA clearly show the passage of each rocket through the cusp region. This work examines the locations of these entrance/exit points in relation to cusp models as provided in and compiled from the published literature, along with a discussion of model variables that have been optimized to best fit the cusp region boundary sampling locations by TRICE‐2. The results of this study set the stage for understanding the upcoming NASA Small Explorer Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites orbital plane intersections with (i.e., "trajectory cuts" through) the cusp region. Key Points: The low‐altitude cusp region was directly sampled by instruments onboard the two Twin Rockets to Investigate Cusp Electrodynamics‐2 rocketsCusp models have been compiled from published studies, with free parameters representing the low‐latitude boundary layer and mantleCusp model free parameters have been optimized to best match the observed cusp region location [ABSTRACT FROM AUTHOR]

Vector displacements expressed in spherical coordinates are proposed. They correspond to electromagnetic fields in vacuum that globally rotate about an axis and display many circular patterns on the surface of a ball. The fields satisfy the set of Maxwell's equations, and some connections with magnetohydrodynamics can also be established. The solutions are extended with continuity outside the ball. In order to avoid peripheral velocities of arbitrary magnitude, as it may happen for a rigid rotating body, they are organized to form successive encapsulated shells, with substructures recalling ball‐bearing assemblies. A recipe for the construction of these solutions is provided by playing with the eigenfunctions of the vector Laplace operator. Some applications relative to astronomy are finally discussed. [ABSTRACT FROM AUTHOR]

Resonant scattering of electromagnetic (EM) waves by small particles is considered as one of the basic problems in metamaterial science. At present, special subwavelength resonators are considered as structural elements in chiral and bianisotropic metamaterials. There is a general consensus that these small scatterers behave like "artificial atoms" with strong electrical and magnetic responses and an interconnection between these responses. However, the observed effect of magnetoelectric (ME) coupling in these meta‐atoms is not associated with the near‐field manipulation properties caused by intrinsic magnetoelectricity. This arises the question whether ME point scatterers of EM radiation really exist. In this paper, we show that there are mesoscopic structures with electric and magnetic dipole‐carrying excitations that behave like point scatterers with their inherent magnetoelectricity. In such subwavelength resonators, coherent oscillations of the electric polarization and magnetization can be considered as quasistatic oscillations described by electrostatic (ES) and magnetostatic (MS) scalar wave functions. The ME resonance effect arises from the coupling of two, ES and MS, oscillations. The near fields of these resonators, called the ME near fields, are characterized by simultaneous violation of time reversal and inversion symmetry. In study of ME fields and EM problems associated with these fields, we put forward the concept of ME‐field electrodynamics. [ABSTRACT FROM AUTHOR]

In this paper, the nonlinear magnetized black holes of massive gravity's rainbow is investigated. In order to achieve this, the double‐logarithmic electromagnetic field is used as a matter source and the new anti‐de Sitter black hole solution is derived. The effects of the massive graviton, rainbow functions, and nonlinearity of the electromagnetic field on the black hole's inner and outer horizons are also studied. The black hole's thermodynamic properties are studied, and the mathematical expressions for temperature, heat capacity, and Gibbs free energy are found. The extended first law and Smarr's formula are derived as well. Using the behavior of thermodynamic quantities as a starting point, the local and global thermodynamic stabilities are also studied. [ABSTRACT FROM AUTHOR]

This paper analyses magnetosphere‐ionosphere (M‐I) coupling from a perspective that is independent of inertial reference frame, and delineates how physical theories of M‐I coupling are affected by the principle of relativity. For the first time in the context of M‐I coupling, we discuss the literature from the 1970s on how the low‐velocity limit of the theory of special relativity is applied to electrodynamics. In most M‐I coupling theories, a particular low‐velocity limit applies, known as the "magnetic limit." Two important consequences of this literature are: (a) significant displacement currents in Maxwell's equations break the Galilean invariance of the equations and (b) magnetic fields are not generated by currents created by a net charge density in motion. We show how reference frame‐independent descriptions of M‐I coupling require that ion‐neutral relative velocities and ion‐neutral collisions are key drivers of the physics. Currents are independent of reference frame whereas electric fields depend on reference frame. Starting with the same momentum equations that are typically used to derive Ohm's law, it is possible to express the perpendicular ionospheric current as depending on collisions between ions and neutrals, and electrons and neutrals, without reference to electric fields. Ignoring the relative motion between ions and neutrals results in errors exceeding 100% for estimates of high latitude Joule heating during significant geomagnetic storms, when ion‐neutral velocity differences are largest near the initiation of large‐scale ion convection. Plain Language Summary: Interactions between the magnetized and ionized solar wind, the magnetospheric cavity surrounding Earth, and Earth's ionized upper atmosphere (ionosphere) can create rapid (∼1 km/s) large‐scale motions of the ionosphere during periods known as geomagnetic storms. We use the principle of relativity (PR) to gain insight into the complex physics of these interactions. Relativity states that the physics governing geospace must be independent of the velocity of an observer making measurements of the system. We write key equations governing interactions of the magnetosphere‐ionosphere system in terms of quantities that do not depend on the observer's motion. In doing so, we find that some previous theories had over‐emphasized the importance of a large‐scale electric field that grows during storms, while neglecting important physics related to collisions between the ionized portion of the atmosphere and the un‐ionized "neutral" component that contains much more mass. These collisional interactions create upper atmospheric heating and expansion, and cause large‐scale currents to flow between the ionosphere and magnetosphere, resulting in a multitude of impacts to our technological society. Using the PR, and isolating the physics that is independent of observer motion, led us to a deeper understanding of key physical processes during storms. Key Points: Relativistic transformations applied to electrodynamics are analyzed in the context of magnetosphere‐ionosphere (M‐I) couplingWe present an "Ohm's law" relating horizontal ionospheric currents to quantities that do not vary with inertial reference frameElectrodynamic theories of M‐I coupling that do not account for the relative motion of ions and neutrals are not quantitatively accurate [ABSTRACT FROM AUTHOR]

We present statistical analyses of whistler‐mode waves observed by Acceleration, Reconnection, and Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS). Although some observations showed that the lunar whistler‐mode waves have similarities to the terrestrial chorus emissions, it remains unknown whether the banded structure typically seen in chorus is common to the lunar waves. In this study, we automatically detected whistler‐mode waves from 9 years of ARTEMIS data and classified them into four types of spectral shapes: lower band only, upper band only, banded, and no‐gap. We first show that a magnetic connection to the lunar surface is a dominant factor in the wave generation. The occurrence rate of whistler‐mode waves is 10 times larger on the magnetic field line connected to the Moon. Then we compared the field line connected events according to the position of the Moon and the condition of the field‐line foot point (day/night and existence of magnetic anomalies). The results show that (a) almost no banded event is observed in any circumstances, suggesting that generation mechanisms for the two band structure of the terrestrial chorus are largely ineffective around the Moon and (b) the wave occurrence rate depends on the foot point conditions, presumably affected by electrostatic/magnetic reflections deforming the velocity distribution of the resonant electrons. Thus, our results provide implications for the two band structure formation and new insights into fundamental processes of the Moon‐plasma interaction. Key Points: We present a statistical study on Moon‐related whistler‐mode waves classified into four types of spectral shapes using Acceleration, Reconnection, and Turbulence and Electrodynamics of the Moon's Interaction with the Sun dataBanded waves are extremely rare, suggesting that two‐band structure formation is ineffective around the MoonWhistler‐mode wave spectra are highly variable resulting from spatial and temporal variability of the lunar plasma environment [ABSTRACT FROM AUTHOR]

In 1933‐1934 Born and Infeld constructed the first non‐linear generalization of Maxwell's electrodynamics that turned out to be a remarkable theory in many respects. In 1935 Heisenberg and Euler computed a complete effective action describing non‐linear corrections to Maxwell's theory due to quantum electron‐positron one‐loop effects. Since then, these and a variety of other models of non‐linear electrodynamics proposed in the course of decades have been extensively studied and used in a wide range of areas of theoretical physics including string theory, gravity, cosmology and condensed matter (CMT). In these notes I will overview general properties of non‐linear electrodynamics and particular models which are distinguished by their symmetries and physical properties, such as a recently discovered unique non‐linear modification of Maxwell's electrodynamics which is conformal and duality invariant. I will also sketch how non‐linear electromagnetic effects may manifest themselves in physical phenomena (such as vacuum birefringence), in properties of gravitational objects (e.g. charged black holes) and in the evolution of the universe, and can be used, via gravity/CMT holography, for the description of properties of certain conducting and insulating materials. In 1933‐1934 Born and Infeld constructed the first non‐linear generalization of Maxwell's electrodynamics that turned out to be a remarkable theory in many respects. In 1935 Heisenberg and Euler computed a complete effective action describing non‐linear corrections to Maxwell's theory due to quantum electron‐positron one‐loop effects. Since then, these and a variety of other models of non‐linear electrodynamics proposed in the course of decades have been extensively studied and used in a wide range of areas of theoretical physics including string theory, gravity, cosmology and condensed matter (CMT). In these notes the author will overview general properties of non‐linear electrodynamics and particular models which are distinguished by their symmetries and physical properties, such as a recently discovered unique non‐linear modification of Maxwell's electrodynamics which is conformal and duality invariant. There will follow also also a sketch how non‐linear electromagnetic effects may manifest themselves in physical phenomena (such as vacuum birefringence), in properties of gravitational objects (e.g. charged black holes) and in the evolution of the universe, and can be used, via gravity/CMT holography, for the description of properties of certain conducting and insulating materials. [ABSTRACT FROM AUTHOR]

An accurate description of the state of the ionosphere is crucial for understanding the physics of Earth's coupling to space, including many potentially hazardous space weather phenomena. To support this effort, ground networks of magnetometer stations, optical instruments, and radars have been deployed. However, the spatial coverage of such networks is naturally restricted by the distribution of land mass and access to necessary infrastructure. We present a new technique for local mapping of polar ionospheric electrodynamics, for use in regions with high data density, such as Fennoscandia and North America. The technique is based on spherical elementary current systems (SECS), which were originally developed to map ionospheric currents. We expand their use by linking magnetic field perturbations in space and on ground, convection measurements from space and ground, and conductance measurements, via the ionospheric Ohm's law. The result is a technique that is similar to the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique, but tailored for regional analyses of arbitrary spatial extent and resolution. We demonstrate our technique on synthetic data, and with real data from three different regions. We also discuss limitations of the technique and potential areas for improvement. Plain Language Summary: The ionosphere, where a small but significant fraction of the atmosphere is ionized, forms the edge of space. At only 100‐km altitude, it is the region in space which is by far best monitored by human instruments. Space scientists routinely use measurements that inform about specific aspects of the dynamics in the ionosphere, but not the whole picture. For example, magnetometers on ground measure one part of the electric current system while magnetometers on satellites measure another part. Radars measure the flow of charged particles in the ionosphere, while optical images and particle measurements can be used to estimate electric conductivity. In this paper, we present a technique that combines all these different types of measurements to give a complete picture of what takes place in the ionosphere. The technique is tailored for use in regions where the data density is high, and the spatial resolution and extent of the analysis region are flexible. Key Points: We present a technique to use disparate data types to produce local maps of polar ionospheric electrodynamicsΔB and convection measurements are related via ionospheric Ohm's law and spherical elementary current systemsWe demonstrate the technique on real and synthetic data, and discuss limitations and future development [ABSTRACT FROM AUTHOR]

We present statistical results of traveling ionospheric disturbances (TIDs) with origin near the geomagnetic equator during geomagnetic storms that occurred within the period of 2010–2018. Based on storm criteria of Kp > 4 and Dst ≤ −50 nT, we have analyzed total electron content perturbations derived from Global Navigational Satellite Systems observations within a latitude range of 40°S–60°N and longitude ranges of 20°–40°E and 50°–70°W representing the African and American sectors. Although the northern hemispheric part of the African sector has limited data coverage, results show that the launched TIDs do not exceed the latitudinal distance of 20°–25° from their origin during the analyzed period. A statistically similar result is observed over the American sector with launched poleward TIDs constrained largely within ±20°–30° around the geomagnetic equator. Where data are available, majority of these cases are linked to changes in ionospheric electrodynamics, especially the enhancement of equatorial electrojet (EEJ), although there are other observations that are not explained by EEJ variability. This indicates that there may be other physical mechanisms that play a role in launching TIDs at the geomagnetic equator during disturbed conditions. An important result is that large‐scale and medium‐scale TIDs have been found to occur predominantly during the main and recovery phases of geomagnetic storms, respectively, at least over the African sector. Key Points: Storm‐time‐related poleward traveling ionospheric disturbances (TIDs) originating from the geomagnetic equatorial region occur mainly during local daytimeLarge scale poleward TIDs are observed during the main phase while medium‐scale TIDs dominate the recovery phase, at least over the African sectorIncreased vertical E × B drift plays a crucial role in launching poleward TIDs [ABSTRACT FROM AUTHOR]

We investigate the proton precipitation caused by electromagnetic ion cyclotron (EMIC) waves and its impact on the ionosphere with the self‐consistent ring current‐atmosphere interactions model (RAM‐SCBE) coupled with an ionospheric particle transport model Global Airglow (GLOW). The EMIC wave diffusion process causes significant precipitation of tens of keV protons, particularly in the afternoon to midnight sector. These precipitating energetic protons further impact the ionosphere‐thermosphere and contribute to the ionization in the E/F regions. The integrated auroral conductance is significantly enhanced in the dusk‐to‐midnight sector. Although the EMIC waves do not directly interact with the ring current electrons, after the EMIC wave scattering included in the model, remarkable changes are found in the global distribution of precipitating electron flux. This means that the addition of proton precipitation driven by EMIC waves results in feedback effects on the ring current electron dynamics through the circulation system. Validation is also conducted by comparing the simulated precipitation flux and ionospheric electron density with observations. Key Points: The electromagnetic ion cyclotron (EMIC) wave diffusion process causes significant precipitation of tens of keV protons, particularly in the afternoon to midnight sectorEnergetic precipitating proton scattered by EMIC wave further impact ionosphere‐thermosphere and contribute to the ionization in E/F regionsEMIC waves produce large proton precipitation, changing ionospheric electrodynamics and feeding back to the magnetospheric dynamics [ABSTRACT FROM AUTHOR]

A new technique has been developed to determine the high‐latitude electric potential from observed field‐aligned currents (FACs) and modeled ionospheric conductances. FACs are observed by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), while the conductances are modeled by Sami3 is Also a Model of the Ionosphere (SAMI3). This is a development of the Magnetosphere‐Ionosphere Coupling approach first demonstrated by Merkin and Lyon (2010), https://doi.org/10.1029/2010ja015461. An advantage of using SAMI3 is that the model can be used to predict total electron content (TEC), based on the AMPERE‐derived potential solutions. 23 May 2014 is chosen as a case study to assess the new technique for a moderately disturbed case (min Dst: −36 nT, max AE: 909 nT) with good GPS data coverage. The new AMPERE/SAMI3 solutions are compared against independent GPS‐based TEC observations from the Multi‐Instrument Data Analysis Software (MIDAS) by Mitchell and Spencer (2003), and against Defense Meteorological Satellite Program (DMSP) ion drift data. The comparison shows excellent agreement between the location of the tongue of ionization in the MIDAS GPS data and the AMPERE/SAMI3 potential pattern, and good overall agreement with DMSP drifts. SAMI3 predictions of high‐latitude TEC are much improved when using the AMPERE‐derived potential as compared to Weimer's (2005), https://doi.org/10.1029/2005ja011270 model. The two potential models have substantial differences, with Weimer producing an average 77 kV cross‐cap potential versus 60 kV for the AMPERE‐derived potential. The results indicate that the 66‐satellite Iridium constellation provides sufficient resolution of FACs to estimate large‐scale ionospheric convection as it impacts TEC. Plain Language Summary: Plasma in the ionosphere convects across high latitudes in response to electric fields generated by solar wind driving of Earth's magnetosphere. Magnetic data from the 66‐satellite Iridium satellite constellation is routinely analyzed to measure the electric currents flowing between the ionosphere and magnetosphere. In this paper, we combined these observed field‐aligned currents with a model of the ionospheric conductance to estimate the corresponding electric field in the ionosphere. We used these electric field estimates to specify the plasma convection in a first‐principles ionosphere model, and compared the results against independent data in a moderately disturbed period. The results of our case study indicate that the technique works well, although plasma convection is not the only factor that causes uncertainty into predictions of the high‐latitude ionosphere. The data used here can be made available in near‐real‐time, so it is possible that our technique could be adopted for operational space weather prediction. Key Points: Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) field‐aligned currents have been combined with the Sami3 is Also a Model of the Ionosphere (SAMI3) model to estimate the high‐latitude potentialIndependent validation of the new technique using satellite drifts and ground‐based total electron content indicates good agreement overallThe SAMI3 model performs better in this case when using AMPERE‐derived potentials than when using the empirical Weimer potential [ABSTRACT FROM AUTHOR]

The Van Allen Probes spacecraft flew through the inner radiation belt and the plasmasphere, probing the extremely dynamic coupling between the thermosphere, the ionosphere and the subauroral magnetosphere. Examining the electrodynamics of this region using Van Allen Probes data required that the ~1 mV/m electric field (E-field) of interest be measured in a region where the E-field due to spacecraft motion was ~100 mV/m. This means that the E-field, the magnetic field, and the spacecraft velocity had to be measured to better than 1%. The instruments on board the Van Allen Probes have achieved this accuracy, delivering reliable near equatorial E-field measurements even below three Earth radii. The objective of this commentary is to summarize the methodology developed over the years to maximize the accuracy and scientific return of these double probe E-field measurements below L = 3. [ABSTRACT FROM AUTHOR]

The upward plasma drift and equatorial ionization anomaly (EIA) in the Earth's ionosphere are strongly influenced by the zonal electric field, which is generated by the wind dynamo. Specification and forecasting of thermospheric winds thus plays an important role in ionospheric weather prediction. In this study, we assess the impact of assimilating thermospheric wind observations from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) instrument on NASA's Ionospheric CONnection (ICON) explorer satellite on the ionospheric electrodynamics. Empirical localization functions (ELFs) of ICON/MIGHTI zonal and meridional winds are also computed and applied to our data assimilation experiments, enabling improved assimilation of the ICON/MIGHTI wind observations. A set of observing system simulation experiments (OSSEs) are performed using the National Center for Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCMX) with data assimilation provided by the Data Assimilation Research Testbed (DART) ensemble adjustment Kalman filter. The results show that assimilating the ICON/MIGHTI wind observations with the ELF improves the zonal and meridional wind root mean square error (RMSE) by 16%–18% and 7%–10%, respectively. The improved wind specification further improves the low‐latitude E × B vertical drift RMSE by about 18%. The response of electron densities is slower, and the overall impact is smaller. The improvement of the ionosphere F‐region maximum electron density (NmF2) between ±45° is about 8% after 18 days of data assimilation. Key Points: ICON/MIGHTI meridional and zonal wind data are assimilated into the WACCMX + DART to adjust the thermospheric wind stateThe adjusted thermospheric wind state can further improve the predictability of the plasma drift by 18% and NmF2 by 8%Empirical localization functions are used to better assimilate the ICON/MIGHTI meridional and zonal wind observations [ABSTRACT FROM AUTHOR]

Aside from the influence of forcing from above on the ionosphere during space weather, forcing from below also have significant influence on the ionosphere. This study investigates responses of the equatorial ionization anomaly (EIA) in the African and American longitudinal sectors to the combined effects of 2014 Sudden Stratospheric Warming (SSW) events and geomagnetic storms that coexisted with them. The study locations cover ±40° geomagnetic latitudes in both sectors. A multiinstrument approach with models was adopted. During the SSW events, a hemispherical asymmetry in TEC distribution was observed, with higher plasma ionization in the Northern Hemisphere (NH). Generally, in both sectors, EIA crests locations shifted to higher latitudes during peak phases of SSW, except in the SH of the African sector, where crests locations shifted to lower latitudes. Reversal of stratospheric zonal mean wind direction supported reversed fountain effect. TEC responded positively to SSW peak phases and daytime or nighttime orientation of Prompt Penetration Electric Field (PPEF) and PPEF strength played major role on TEC responses to storms. PPEF values were generally weak, but comparatively higher in the American sector. TEC were predominant in the American sector than the African sector due to the comparative higher electrodynamics over the American sector. EIA crests were generally located at higher latitudes on the days of SSW peaks than on the days of geomagnetic storms, except in the NH of the American sector. In both sectors, geomagnetic storms modified ionospheric irregularities by weakening or enhancing them, while the major SSW event weakened irregularities. Plain Language Summary: We investigated the responses of the EIA in the African and American sectors to the combined effects of 2014 SSW events and the geomagnetic storms that coexisted with them. We chose the study locations to cover ±40° geomagnetic latitudes in both sectors. We adopted a multiinstrument approach with models. Our results showed that the EIA crests locations in the African sector shifted from lower latitudes on pre‐SSW days to higher latitudes on minor SSW peak days in the NH. In the SH, these crests locations shifted from higher latitudes on pre‐SSW days to lower latitudes on SSW peak days, except on the day of the major SSW peak when the crest location shifted from lower latitude on pre‐SSW days to higher latitude on SSW peak days. In the American sector, EIA crest locations shifted from lower latitudes on pre‐SSW days to higher latitudes on the SSW peak days in both hemispheres. Reversal of stratospheric zonal mean wind direction supported reversed fountain effect. Both the strength and orientation of PPEF played major roles on TEC responses to storms. Furthermore, occurrence of geomagnetic storm modified ionospheric irregularities. Weakening of irregularities was noticed during days of the major warming in the American sector. Key Points: EIA crests locations expanded during 2014 arctic SSW event and the event weakened ionospheric irregularitiesReversal of stratospheric zonal mean wind direction supported reversed fountain effectIonospheric electrodynamics was higher over the American sector than over the African sector [ABSTRACT FROM AUTHOR]

In collisionless magnetic reconnection, the Hall electric field and the parallel electric field play important roles because they are responsible for acceleration of charged particles, especially electrons. Using particle‐in‐cell simulations, we study electrodynamic nature of the two electric fields in two‐dimensional collisionless reconnection. We find that the Hall electric field is predominantly (≳90%) electrostatic; its electromagnetic (induced) component is small but nonzero (≲10%). The parallel electric field or potential is contributed comparably by its electrostatic and induced components. In the electron inflow region, along the separatrix, and at the X‐line, the electrostatic component of the parallel electric field (or potential) is larger; while in the electron outflow region, the electromagnetic (induced) component is larger. An initial guide field does not change the electrodynamic nature of the Hall electric field but does change that of the parallel electric field. In the guide field case, the parallel electric field is contributed predominantly by the electromagnetic component from the electron inflow region to the X‐line and by the electrostatic component in the electron outflow region. Key Points: The Hall electric field in collisionless reconnection is predominantly electrostatic; its induced component is small but nonzeroThe parallel electric field (or potential) is contributed comparably by the electrostatic and induced componentsGuide field does not change the electrodynamic nature of the Hall electric field but changes that of the parallel electric field in details [ABSTRACT FROM AUTHOR]

In this work, the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model is used to investigate the responses of ionospheric electrodynamic processes to the solar flares at the flare peaks and the underlying physical mechanisms on September 6 and 10, 2017. Simulations show that solar flares increased global daytime currents and reduced the eastward electric fields during the daytime from the equator to middle latitudes. Furthermore, westward equatorial electric fields and equatorial counter electrojets occurred in the early morning. At the flare peak, these electrodynamic responses are predominantly related to the enhanced E‐region conductivity by flares, as the responses of neutral winds and F‐region conductivity to flares are negligible. Specifically, the Cowling conductance enhancement is not the major process causing the reduction of zonal electric fields. This electric field reduction is primarily associated with the decrease of the ratio between the field line‐integrated wind‐driven currents and the conductance. The flare‐induced conductivity enhancement is larger but the background wind speed is smaller in the E‐region than in the F‐region, as a result, the increase of total integrated wind‐driven currents is less than the conductance enhancement. Key Points: Flare‐induced conductivity enhancement, which is dominant over the decrease of eastward electric fields, increases global daytime currentsE‐region conductivity enhancement decreases the ratio between integrated wind‐driven currents and conductance and reduces electric fieldsThe conductance enhancement tends to induce westward electric fields and equatorial counter electrojets in the early morning [ABSTRACT FROM AUTHOR]

The stability of magnetism in reduced dimensions has become a major scientific agenda in the pursuit of implementing magnetic nanostructures as functional components in spintronic devices. Methods to probe and control magnetization states of such structures in a deterministic manner include use of spin polarized currents, photon absorption, and relatively recently, electric fields that tailor magnetoelectric coupling in multiferroic based structures. In theory, a short electric pulse is able to generate localized magnetic fields that can couple to the local magnetic dipoles electrodynamically. Here, using the Landau–Lifshitz–Gilbert formalism of magnetism dynamics combined with continuum Maxwell relations, the response of a ferromagnetic permalloy nanodisc to nanosecond electric field pulses is studied. The dynamics of the magnetic order of the nanodiscs during this process are examined and discussed. Ferromagnet nanodiscs, when below a critical size and in the absence of any external field, relax to a vortex phase as the ground state due to the demagnetizing field. Simulations demonstrate that the planar chirality of such a ferromagnet nanodisc can be switched via a time‐wise asymmetric electric field pulse on the order of a few ns duration that generates radially varying tangential magnetic fields. These fields couple to the vortex state of the nanodisc ferromagnet electrodynamically, revealing an effective and robust method to control chirality. [ABSTRACT FROM AUTHOR]

On December 08, 2018 the Twin Rocket Investigation of Cusp Electrodynamics 2 (TRICE 2) mission was successfully launched. The mission consisted of two sounding rockets, each carrying a payload capable of measuring electron and ion distributions, electric and magnetic fields, and plasma waves occurring in the northern magnetospheric cusp. This study highlights the ion and wave observations obtained by TRICE 2 in the cusp and observations from the magnetospheric multiscale (MMS) spacecraft at the low‐latitude magnetopause two hours prior to the TRICE 2 traversal of the cusp. Within the cusp, typical ion cusp features were observed, that is, energy‐latitude dispersion of injected magnetosheath plasma. However, a lower energy population was also measured near the equatorward edge of the cusp on open field lines. Pitch‐angle distributions of the low‐energy ions suggest that this population was magnetospheric in origin, and not from ionospheric upflows. Data from MMS show that counterstreaming ions were present in the outer magnetosphere and low‐latitude boundary layer at similar energies to those observed by TRICE 2 in the cusp. Correlations between the low‐energy ions within the cusp and broadband extremely low frequency waves suggest that the low‐energy magnetospheric ions that filled the flux tube may have undergone wave‐particle interactions. These interactions may cause pitch‐angle scattering of low‐energy magnetospheric ions closer to the loss cone, thereby allowing them to precipitate into the cusp and be measured by the TRICE 2 sounding rockets. Key Points: Low‐energy ions measured near the equatorward edge of the cusp by TRICE 2 are locally mirroring and propagating toward the ionosphereA similar counter‐streaming ion population is observed at the magnetopause by MMS two hours prior to the TRICE 2 cusp traversalIon and wave data in the cusp suggest wave‐particle interactions are pitch angle scattering low‐energy magnetospheric ions [ABSTRACT FROM AUTHOR]

During the Twin Rockets to Investigate Cusp Electrodynamics (TRICE‐2) High‐Flyer rocket's passage through the cusp the high frequency (HF) radio wave receiver observed three intervals of banded upper‐hybrid (UH) waves. The bands begin at the UH frequency (∼1.2–1.3 MHz), descending to as low as 1.1 MHz, with amplitudes of hundreds of mV/m. The spacing of the bands are ∼4.5–6 kHz and the number of bands ranges from 3 to 10. Simultaneously, the very low frequency (VLF) radio wave receiver observed lower‐hybrid (LH) waves with amplitudes ranging from 1 to 10 mV/m and frequencies of 4.5–6 kHz. Slight variations of the spacings of the bands in the UH waves were closely correlated with variations in the LH peak frequencies. Two possible wave‐wave interactions are explored to explain this phenomenon: decay of an UH wave into a lower frequency UH wave and a LH wave, and coalescence of independent UH waves and LH waves that spawn UH waves. Using a dispersion relation calculator with electron and ion distribution functions based off those observed by the particle instruments suggests that UH waves, and to a lesser degree LH waves, can be excited by linear instabilities. Kinematic analysis of the waves dispersion relations and the wave matching conditions show that wave‐wave interactions linking UH and LH modes are possible through either decay or coalescence. This analysis along with comparisons of the energy densities of the waves, and the ratio of their occupation numbers suggest that the decay process is more likely than coalescence. Key Points: Modulated upper‐hybrid (UH) waves coincided with enhanced power near the local lower‐hybrid (LH) frequencyThe spacings of the banded UH waves are correlated with the frequency of the peak spectral density near the LH peaksKinematic constraints and energy densities of wave modes suggest wave‐wave process is plausible, with decay more likely than coalescence [ABSTRACT FROM AUTHOR]

Investigations of the evolution of subauroral polarization streams (SAPS) are important to understand the plasma transport processes in the polar ionosphere. In this article, three cases of SAPS evolution are analyzed using the observations made by the Super Dual Auroral Radar Network mid‐latitude radar chain, Special Sensor Ultraviolet Spectrographic Imagers instrument and Active Magnetosphere and Planetary Electrodynamics Response Experiment. The three cases occur in the main phase, beginning of recovery phase and end of recovery phase of three magnetic storms. The cases last from 1.5 to 2 h during 1800–0300 MLT (magnetic local time). The observations show that the SAPS velocity evolution is strongly controlled by the ring current injection during the storm main phase. The responses of SAPS velocity are prompt and appear first near the dusk side and then expand to the night side. Besides, the SAPS velocity has a good linear correlation (R = −0.85) with SYMH index during the evolution of the magnetic storm. The SAPS velocity also shows dependence on the AE index and interplanetary magnetic field (IMF) Bz. The correlations between SAPS velocity and AE/Bz increase with the decay of the magnetic storm. However, the effect of substorm/IMF on SAPS velocity is not strong compared to that of the magnetic storm. Key Points: Increasing subauroral polarization streams (SAPS) velocity expands longitudinally from post‐dusk side in main phase and shrinks back to post‐dusk side in recovery phaseIn the main phase, SAPS T‐Slope has similar time profile with SYMH index and ring current strongly controls the velocity evolutionGood linear correlations exist between SAPS velocity and SYMH/AE/Bz while the correlations are different in different phases [ABSTRACT FROM AUTHOR]

The influence of east‐west component of interplanetary magnetic field (IMF‐By) on ionospheric plasma convection over polar region is well established by both observation and modeling studies. However, its influence beyond polar latitudes has rarely been addressed. To understand the effects of IMF‐By at low‐latitudes during geomagnetic storm times, we have designed simulation experiments using Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model. This communication shows that strong IMF‐By and its polarity has effects on the storm‐time ionosphere‐thermosphere dynamics even near equator. It can change zonal electric field, magnitude of morning/evening time pre‐reversal‐enhancements (PRE) and meridional winds. At the storm onset when prompt penetration electric fields prevail, a strong IMF‐By, in general, reduces the vertical plasma drifts over the equator and the largest effects are observed near sunrise and sunset terminators. Later when disturbance dynamo is active, morning PRE appears, and a strongly positive IMF‐By strengthens morning PRE and weakens evening PRE, with respect to zero IMF‐By conditions. A strongly negative IMF‐By just shows the reverse, thus indicates dawn‐dusk asymmetry due to IMF‐By. It is also seen that the meridional winds are oppositely directed during positive and negative IMF‐By conditions. The effects of IMF‐By transition are observed on the evening PRE within 2 h from the transition. The new insights successfully explain the observations from a few earlier available works. However, the presented results should be treated with caution in view of model limitations associated with the empirical patterns of high latitude convection and preliminary scheme of merging high latitude‐low latitude potentials. Plain Language Summary: Storm time reconnection results in exchange of huge amount of momentum, energy and particles, which causes changes in the Earth's ionosphere‐thermosphere system. When the primary driver north‐south component of interplanetary magnetic field (IMF‐Bz), controls this flow of energy and particles, east‐west component of interplanetary magnetic field (IMF‐By) modulates this flow thereby altering the impacts on the Earth's ionosphere during geomagnetic storm evolution. Our knowledge about this modulation is limited to polar regions only, where it is quite pronounced. The impact of IMF‐By beyond polar ionosphere is sparse and has rarely been reported. This is because of the masking of its influence by the overwhelming effects of IMF‐Bz. In this study, special experiments are designed using Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model to understand the impact of IMF‐By on ionosphere near equator, during storm times. It is found that a strong IMF‐By during geomagnetic storm and its polarity can substantially influence the plasma convection up to equatorial latitudes and alter the neutral dynamics as well. A reliable space weather forecasting must incorporate the effects of IMF‐By. Key Points: East‐west component of interplanetary magnetic field (IMF‐By) affects the ionospheric electrodynamics beyond polar latitudes even near equator, during southward IMFA strong IMF‐By reduces prompt penetration electric field near equator, and the effects are stronger near sunset and sunrise terminatorsRole of IMF‐By polarity is seen during disturbance dynamo, giving dawn‐dusk asymmetry; meridional wind reverses on IMF‐By polarity reversal [ABSTRACT FROM AUTHOR]

We used reanalyzed Jicamarca radar measurements to study the response of equatorial ionospheric electrodynamics and spread F during the main phase of the large September 2017 geomagnetic storm. Our observations near dusk on 7 September show very large upward drifts followed by a large short‐lived downward drift perturbation that completely suppressed the lower F region plasma irregularities and severely decreased the backscattered power from the higher altitude spread F. We suggest that this large short‐lived westward electric field perturbation is most likely of magnetospheric origin and is due to a sudden and very strong magnetic field reconfiguration. Later in the early night period, data indicate large, mostly upward, drift perturbations generally consistent with standard undershielding and overshielding electric field effects, but with amplitudes significantly larger than expected. Our analysis suggests that occurrence of storm‐time substorms is one of the major factors causing the large nighttime westward and eastward electric field perturbations observed at Jicamarca near the storm main phase. Our analysis also suggests that magnetospheric substorms play far more important roles on the electrodynamics of the equatorial nighttime ionosphere than has generally been thought. Key Points: Very strong changes on the vertical drifts and equatorial spread F intensity after dusk on September 7, 2017Very large eastward and westward electric field perturbations from about 19 to 22 LT, likely enhanced by substormsStrong short‐lived response of equatorial spread F to a very large westward disturbance electric field at 19 LT [ABSTRACT FROM AUTHOR]

Many of the details of how terrestrial gamma-ray flashes (TGFs) are produced, including their association with upward-propagating in-cloud lightning leader channels, remain poorly understood. Measurements of the low-frequency radio emissions associated with TGF production continue to provide unique views and key insights into the electrodynamics of this process. Here we report further details on the connection between energetic in-cloud pulses (EIPs) and TGFs. With coordinated measurements from both ground-based radio sensors and space-based gamma-ray detectors on the Fermi and Reuven Ramaty High Energy Solar Spectroscopic Imager spacecraft, we find that all ten +EIPs that occurred within the searched space-and-time window are associated with simultaneous TGFs, including two new TGFs that were not previously identified by the gamma-ray measurements alone. The results in this study not only solidify the tight connection between +EIPs and TGFs, but also demonstrate the practicability of detecting a subpopulation of TGFs with ground-based radio sensors alone. Plain Language Summary Terrestrial gamma-ray flashes (TGFs) are kinds of high-energy emissions produced during thunderstorms and are almost exclusively detected with spacecraft-based gamma-ray detectors. Because of the strong atmospheric gamma-ray attenuation from the low-altitude atmosphere and the geometry-driven short horizontal detection range, the ground detection of TGFs with gamma-ray detectors is challenging. Recently a class of distinct radio emissions during thunderstorms, called energetic in-cloud pulses (EIPs), was found to be closely linked to the production of some TGFs. In this study, we present several additional lines of strong evidence that strengthen the connection between a subpopulation of TGFs and EIPs. Importantly, by identifying EIPs from radio signals, we found two new TGFs that were previously unreported from space-based detection. This demonstrates that some TGFs can be detected from the radio measurements alone when the spacecraft-based detectors are unavailable. [ABSTRACT FROM AUTHOR]

Auroral omega bands have been observed since the 1960s and their ionospheric electrodynamic properties are well established. At the same time magnetospheric source is poorly investigated leaving a room for few competing hypotheses on how the omega band forms. Here we present the statistical study of the projections of about 400 omega bands from the ionosphere to magnetosphere to investigate the location and properties of a possible omega source region in the magnetosphere. We used the Magnetometers--Ionospheric Radars--All-sky Cameras Large Experiment all-sky images on the list of individual omega structures (Partamies et al., 2017, http://doi.org/10.5194/angeo-35-1069-2017) which were observed in the Fennoscandian Lapland in the period of 1997-2007, and a new empirical magnetic field model (Tsyganenko & Andreeva, 2016, https://doi.org/10.1002/2016ja023217) to identify the magnetospheric equatorial projection of the observed omegas. We found that 90% of the auroral omega structures map to the radial distances of 6-14 RE from the Earth, coinciding with the bursty bulk flow braking region. An average magnetic field configuration in the vicinity of magnetospheric omega projections corresponds approximately to the transition region between tail- and dipole-like magnetic configuration. Velocity estimates for omega projections reveal the dawnward as well as the radial propagation in the magnetosphere with a typical speed of up to few tens of km/s. It is shown that the source of omega structures propagates earthward in most of the events. [ABSTRACT FROM AUTHOR]

Large thermospheric neutral density enhancements in the cusp region have been examined for many years. The Challenging Minisatellite Payload (CHAMP) satellite for example has enabled many observations of the perturbation, showing that it is mesoscale in size and exists statistically over solar cycle timescales. Further studies examining the relationship with magnetospheric energy input have shown that fine‐scale Poynting fluxes are associated with the density perturbations on a case‐by‐case basis, whilst others have found that mesoscale downward fluxes also exist in the cusp region statistically. In this study, we use nearly 8 years of the overlapping Super Dual Auroral Radar Network and Active Magnetosphere and Planetary Electrodynamics Response Experiment datasets to generate global‐scale patterns of the high‐latitude and height‐integrated Poynting flux into the ionosphere, with a time resolution of 2 min. From these, average patterns are generated based on the interplanetary magnetic field orientation. We show the cusp is indeed an important feature in the Poynting flux maps, but the magnitude does not correlate well with statistical neutral mass density perturbations observed by the CHAMP satellite on similar spatial scales. Importantly, the lack of correlation between mesoscale height‐integrated Poynting fluxes and the cusp neutral mass density enhancement gives possible insight into other processes that may account for the discrepancy, such as energy deposition at finer scale sizes or at higher altitudes than captured. Key Points: Statistical patterns of the total downward Poynting flux into the atmosphere have been derived using Super Dual Auroral Radar Network and Active Magnetosphere and Planetary Electrodynamics Response Experiment dataStatistical patterns of neutral mass density perturbations as a percentage of the background density have been derived using Challenging Minisatellite Payload dataMesoscale downward Poynting flux in the cusp region do not correlate very well with neutral mass density enhancements at a similar scale [ABSTRACT FROM AUTHOR]