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The three Swarm satellites provide an optimum, low Earth orbit (LEO) and multi‐spacecraft platform, to explore for the first time the local correlation between field‐aligned currents (FACs), auroral electrojets, and magnetic perturbations at the Earth's surface. By combining Swarm and ground magnetic field data, one can investigate systematically the full correlation chain, whose final link controls the ground induced currents and related space weather effects. We introduce an integrated FAC product, the Sheet FAC (SFAC) index, as a convenient measure of the in‐situ FAC data, and explore the correlations SFAC‐AE, SFAC‐PEJ and SFAC‐dH, with AE the standard auroral electrojet index, PEJ the local, Swarm based, polar electrojet index, and dH the horizontal magnetic field perturbation at the Earth's surface. Given the good SFAC‐dH correlation, we also suggest an extension of SFAC to higher LEO satellites, which cannot observe any more the electrojet currents, but are fully capable to monitor SFAC. Plain Language Summary ‘Space weather’ resembles, to some extent, ordinary weather. Likewise, storms in space, termed ‘magnetic storms’, have common features with ordinary storms. Just like ordinary storms, magnetic storms can cause damage, and similar to ordinary weather, space weather needs to be monitored and, ideally, predicted. The SFAC index, introduced in the paper, is shown to be a potentially useful tool for such goals, able to capture local effects. This is essential for efficient monitoring. Moreover, the SFAC index can be extended to many satellites, which is important too. Just like for ordinary weather, space weather prediction requires measurements of key parameters that are used as input by specific models. The more and denser the measurements, the better the output, namely the prediction. Key Points The newly introduced SFAC index is shown to be robust and appropriate for space weather monitoring by low Earth orbit satellites SFAC appears to be able to capture local features related to magnetospheric dynamics, as driven, in particular, by bursty bulk flows While the introduction of SFAC takes advantage of Swarm features, the index can be extended to other low Earth orbit satellites

Many high‐latitude modeling studies utilize the horizontal ionospheric Hall current in calculating ground‐based magnetic perturbations, but low‐latitude and midlatitude studies should include current systems such as the magnetospheric, field‐aligned, and Pedersen currents. Recently, by including all these current systems, a more precise ground‐based perturbation calculator has been implemented in the Space Weather Modeling Framework. Using this new method, ground‐based perturbations generated by different current systems are analyzed at low, middle, and high latitudes. As a result of the current systems, MLT‐UT maps of ground‐based perturbations are studied. Furthermore, nine storms events are simulated at more than 20 low‐latitude and midlatitude magnetometer locations and compared with observational ground‐based perturbations. These studies show that for specifying the northward component of the ground magnetic perturbations, the inclusion of magnetospheric, field‐aligned, and Pedersen current is important and improves the prediction significantly over the prediction only considering the Hall current in the calculation. The improvement is the most during the storm main phase. However, for the vertical and eastward components of the perturbations, which were typically smaller than the northward component, the inclusion of these current systems actually made the specifications worse because the ring current in the model rotates more toward the dayside than in reality.

In this study we investigate the performance of the University of Michigan’s Space Weather Modeling Framework (SWMF) in prediction of ground magnetic perturbations (Δ B ) and their rate of change with time (d B /d t ), which is directly connected to geomagnetically induced currents (GICs). We use the SWMF set-up where the global magnetosphere provided by the Block Adaptive Tree Solar-wind Roe-type Upwind Scheme (BATS-R-US) MHD code, is coupled to the inner magnetosphere and the ionospheric electrodynamics. The validation is done for Δ B and d B /d t separately. The performance is evaluated via data-model comparison through a metrics-based approach. For Δ B , the normalized root mean square error (nRMS) and the correlation coefficient are used. For d B /d t , the probability of detection, the probability of false detection, the Heidke skill score, and the frequency bias are used for different d B /d t thresholds. The performance is evaluated for eleven ground magnetometer stations located between 59° and 85° magnetic latitude and spanning about five magnetic local times. Eight geomagnetic storms are studied. Our results show that the SWMF predicts the northward component of the perturbations better at lower latitudes (59°–67°) than at higher latitudes (>67°), whereas for the eastward component, the model performs better at high latitudes. Generally, the SWMF performs well in the prediction of d B /d t for a 0.3 nT/s threshold, with a high probability of detection ≈0.8, low probability of false detection (<0.4), and Heidke skill score above zero. To a large extent the model tends to predict events as often as they are actually occurring in nature (frequency bias 1). With respect to the metrics measures, the d B /d t prediction performance generally decreases as the threshold is raised, except for the probability of false detection, which improves.

In this study we investigate the performance of the University of Michigan’s Space Weather Modeling Framework (SWMF) in prediction of ground magnetic perturbations (ΔB) and their rate of change with time (dB/dt), which is directly connected to geomagnetically induced currents (GICs). We use the SWMF set-up where the global magnetosphere provided by the Block Adaptive Tree Solar-wind Roe-type Upwind Scheme (BATS-R-US) MHD code, is coupled to the inner magnetosphere and the ionospheric electrodynamics. The validation is done for ΔB and dB/dt separately. The performance is evaluated via data-model comparison through a metrics-based approach. For ΔB, the normalized root mean square error (nRMS) and the correlation coefficient are used. For dB/dt, the probability of detection, the probability of false detection, the Heidke skill score, and the frequency bias are used for different dB/dt thresholds. The performance is evaluated for eleven ground magnetometer stations located between 59° and 85° magnetic latitude and spanning about five magnetic local times. Eight geomagnetic storms are studied. Our results show that the SWMF predicts the northward component of the perturbations better at lower latitudes (59°–67°) than at higher latitudes (>67°), whereas for the eastward component, the model performs better at high latitudes. Generally, the SWMF performs well in the prediction of dB/dt for a 0.3 nT/s threshold, with a high probability of detection ≈0.8, low probability of false detection (<0.4), and Heidke skill score above zero. To a large extent the model tends to predict events as often as they are actually occurring in nature (frequency bias 1). With respect to the metrics measures, the dB/dt prediction performance generally decreases as the threshold is raised, except for the probability of false detection, which improves.

A bstract Nuclear matter with a strong magnetic field is prevalent inside neutron stars and heavy-ion collisions. In a sufficiently large magnetic field, the ground state is either a chiral soliton lattice (CSL), an array of solitons of the neutral pion field, or a domain-wall Skyrmion phase in which Skyrmions emerge inside the chiral solitons. In the region of large chemical potential and a magnetic field lower than its critical value for CSL, a Skyrmion crystal is expected to take up the ground state based on the chiral perturbation theory at the next leading order. We determine the phase boundary between such a Skyrmion crystal and the QCD vacuum. We examine the previous conjecture that a Skyrmion in magnetic field could be in a form of a neutral pion domain wall bounded by a superconducting ring of charged pions with the radius determined by the quantization condition of the penetrating magnetic flux. We also validate that a Skyrmion would shrink to null without the Skyrme term, although Derrick’s scaling law is modified by a background magnetic field, and the stability at the leading order is not ruled out in theory.

Using a conjunction of Cluster in the mid‐altitude dayside magnetosphere and Swarm in the low‐altitude ionosphere, we show, by employing multi‐spacecraft analysis, that matched, strong magnetic perturbations and the corresponding mesoscale field‐aligned current (FAC) structures are measured in the high latitude polar cusp region during the 7 October 2015 storm. Two pairs of opposite (positive/negative) FACs are observed by both Cluster and Swarm, which may relate to pulsed magnetic reconnection at the dayside magnetopause. Furthermore, the current intensity of these matched FACs decreases from high to low latitude, consistent with the time elapsed since reconnection. Corresponding geomagnetic disturbances are also observed by ground stations. Our observations provide direct evidence for the coupling of mesoscale FACs between the magnetosphere, ionosphere and ground in the polar cusp region, where the signatures are driven in this case by conditions suitable for inducing reconnection. Plain Language Summary Field‐aligned currents (FACs) are the key medium for the interaction between the distant space magnetosphere (a region filled with Earth's magnetic field) and the near‐Earth space ionosphere. Magnetic reconnection is the most important process to transfer solar wind energy from dayside magnetosphere to nightside, which is accompanied by the generation of FACs extending from the high latitude region of the ionosphere to the magnetosphere. For such structures, extending across different space regions, joint observations by multiple spacecraft are necessary. Using simultaneous measurements of Cluster in the magnetosphere, Swarm in the ionosphere and geomagnetic stations on the ground, coordinated mesoscale FAC structures could show matched signatures in the magnetosphere and the ionosphere and the corresponding geomagnetic disturbances on the ground. Our observations provide direct evidence for the magnetosphere‐ionosphere‐ground coupling during the pulsed magnetic reconnection process. Key Points Multi‐spacecraft Cluster and Swarm reveal matched magnetic perturbation and corresponding mesoscale field‐aligned currents (FACs) at different altitudes in the cusp region Multiple pairs of opposite FACs associated with pulsed magnetic reconnection are dominant currents system at the dayside during storm time Direct evidence for detailed dayside mesoscale FACs coupling between magnetosphere, ionosphere and ground is provided

It has long been hoped that spin liquid states might be observed in materials that realize the triangular-lattice Hubbard model. However, weak spin–orbit coupling and other small perturbations often induce conventional spin freezing or magnetic ordering. Sufficiently strong spin–orbit coupling, however, can renormalize the electronic wavefunction and induce anisotropic exchange interactions that promote magnetic frustration. Here we show that the cooperative interplay of spin–orbit coupling and correlation effects in the triangular-lattice magnet NaRuO2 produces an inherently fluctuating magnetic ground state. Despite the presence of a charge gap, we find that low-temperature spin excitations generate a metal-like term in the specific heat and a continuum of excitations in neutron scattering, reminiscent of spin liquid states previously found in triangular-lattice organic magnets. Further cooling produces a crossover into a different, highly disordered spin state whose dynamic spin autocorrelation function reflects persistent fluctuations. These findings establish NaRuO2 as a cousin to organic, Heisenberg spin liquid compounds with a low-temperature crossover in quantum disorder.Spin liquids are predicted to emerge in materials that combine strong electronic correlations with geometric frustration. Evidence has now been found for a spin liquid state in the triangular-lattice material NaRuO2.

Using a dynamical systems approach, we examine the persistence and predictability of geomagnetic perturbations across a range of different latitudes and levels of geomagnetic activity. We look at the horizontal components of the magnetic field measured on the ground between 13 and 24 March 2015, at approximately 40 observatories in the Northern Hemisphere. We introduced two dynamical indicators: the extremal index θ, which quantifies the persistence of the system in a particular state and the instantaneous dimension d, which measures the active number of degrees of freedom of the system. The analysis revealed that during disturbed periods, the instantaneous dimension of the horizontal strength of the magnetic field, which depends on latitude, increases, indicating that the geomagnetic response is externally driven. Furthermore, during quiet times, the instantaneous dimension values fluctuate around the state-space dimension, indicating a more stochastic and thus less predictable nature system.

A bstract We study the ground state of the low energy dense QCD with the assumption of chiral condensates of quarks. Under an external magnetic field, mesons could form soliton lattices via the chiral anomaly. For such scenarios, we present a unified description of pions and η meson with a U(2) field in the framework of the chiral perturbation theory. Our result shows the ground state is a mixture of the magnetized domain walls formed by neutral pion π 0 and η meson when they coexist. The winding number of the ground state would alter according to the strength of the magnetic field. When the magnetic field is strong or the chemical potential is large, the proportion of the mixture is determined by the decay constants and the contributions to the anomalous action of π 0 and η meson. The resulting configuration is either a mixed soliton lattice or a quasicrystal which could be dubbed a “chiral soliton quasicrystal”.

Remarkably, complex assemblies of superconducting wires, electrodes, and Josephson junctions are compactly described by a handful of collective phase degrees of freedom that behave like quantum particles in a potential. Almost all these circuits operate in the regime where quantum phase fluctuations are small—the associated flux is smaller than the superconducting flux quantum—although entering the regime of large fluctuations would have profound implications for metrology and qubit protection. The difficulty arises from the apparent need for circuit impedances vastly exceeding the resistance quantum. Independently, exotic circuit elements that require Cooper pairs to form pairs in order to tunnel have been developed to encode and topologically protect quantum information. In this work, we demonstrate that pairing Cooper pairs magnifies the phase fluctuations of the circuit ground state. We measure a tenfold suppression of flux sensitivity of the first transition energy only, implying a twofold increase in the vacuum phase fluctuations and showing that the ground state is delocalized over several Josephson wells.