Field Reversal Research Articles

Further evidence of unstable reverse polarity geomagnetic field across the Olduvai subchron: paleomagnetic and multispecimen paleointensity study on Khertvisi lava flows (Lesser Caucasus)

Goguitchaichvili et al. (Physics of the Earth and Planetary Interiors, doi.org/10.1016/j.pepi.2020.106641, 2021) reported a detailed rock-magnetic and paleointensity study of a Lesser Caucasus lava sequence comprised between 1.93 ± 0.09 and 1.78 ± 0.11 Ma. Reverse polarity magnetization determined for 20 consecutive lava flows permitted to suggest that the Olduvai subchron is probably disrupted by a short reverse-polarity episode. No paleointensity was obtained because of thermally unstable samples. Here, we report a detailed rock-magnetic, paleomagnetic, and multispecimen parallel differential pTRM absolute paleointensities performed on the nearby parallel Khertvisi lava succession. New isotopic age determinations indicate that the lavas of the Khertvisi section erupted in the Early Pleistocene, at the boundary of Gelasian and Calabrian ages in a time span of 1.88 ± 0.10 to 1.71 ± 0.12 Ma. All lava flows yielded well-defined reverse polarity magnetization with site mean directions Inc. = −56.1°, Dec = 189.5°, N = 20 (lava flows), α95 = 2.3°, k = 187. These directions are close to the Toloshi mean paleodirections, and both slightly deviate clockwise from the geocentric axial dipole and expected Plio-Quaternary directions. The values of the virtual geomagnetic pole (VGP) scatter parameters indicate that the studied sequence was formed during a very short time, most probably insufficient to average paleosecular variation. The multispecimen methodology provided a relatively low paleointensity for most analyzed lava flows, comparable to the transitional field intensity in Georgia during the Bruhnes and Matuyama chrons. These results definitively confirm a rather unstable, reverse polarity geomagnetic field regime across the Olduvai normal superchron. Thus, we propose to consider the Khertvisi section as the firm, volcanic evidence of Olduvai's short-lived geomagnetic event.

Effects of Oil Flow on Space Charge Behaviors in Oil–Paper Composite Insulation

Although the circulating flow of insulating oil can enhance heat dissipation in converter transformers, it can also influence the space charge distribution in oil–paper composite insulation. In this article, the oil–paper composite insulation between flat electrodes was used as the experimental model. Based on the pulsed electroacoustic (PEA) method, we investigated the effect of uncharged flowing oil on the space charge distribution in composite insulation under a dc electric field and polarity reversal. Under a dc electric field, as the oil flow velocity increased, the amount of interface charge between the oil and paper first increased and then decreased. During the polarity reversal process, the field distortion was aggravated at slower oil flows. These experimental results were explained in terms of the effect of oil flow on ionized charges in the oil.

A 4.4-GHz 193.2-dB FoM 8-Shaped-Inductor Based LC-VCO Using Orthogonal-Coupled Triple-Coil Transformer

A 4.2-to-4.6 GHz 8-shaped-inductor based LC-VCO with an orthogonal-coupled triple-coil transformer is presented in this brief. The orthogonal-coupled triple-coil transformer is designed to reduce magnetic coupling and improve phase noise. The mutual inductance between the 8-shaped inductor and the tail-filtering inductor is negligible due to the symmetrical and reversal magnetic field generated by the two loops of the 8-shaped inductor, which avoids the additional chip area occupation of the bulky tail-filtering inductor. In addition, an 8-shaped transformer with thin line width is proposed to bias the varactor instead of using resistors, which avoids the resistor thermal noise contribution. The VCO is implemented in a 180 nm CMOS process, which consumes 7.4 mA current from a 1 V power supply. By adopting the 8-shaped inductor with noise filtering and noise-free biasing technique from the orthogonal-coupled triple-coil transformer, the measured phase noise achieves −149.3 dBc/Hz at 10 MHz offset away from a 4.3 GHz carrier and the resulting figure-of-merit (FoM) is −193.2 dB. To the best knowledge of the authors, this brief presents the VCO with the best FoM among published 8-shaped-inductor based LC-VCOs.

Magnetic Switchbacks Heat the Solar Corona

Magnetic switchbacks are short magnetic field reversals ubiquitously observed in the solar wind. The origin of switchbacks remains an important open science question, because of switchbacks’ possible role in the heating and acceleration of the solar wind. Here, we report observations of 501 robust switchbacks, using magnetic and plasma measurements from the first eight encounters by the Parker Solar Probe. More than 46% (6%) of switchbacks are rotational (tangential; TD) discontinuities (RD), defined as magnetic discontinuities with large (small) relative normal components of magnetic field and proton velocity. Magnetic reconnection in the solar atmosphere can be a source of the observed RD-type switchbacks. It is discovered that: (1) the RD-to-TD ratio exponentially decays with increasing heliocentric distance at rate 0.06 [R S −1], and (2) TD-type switchbacks contain 64% less magnetic energy than RD-type switchbacks, suggesting that RD-type switchbacks may relax into TD-type switchbacks. It is estimated that relaxing switchbacks generated via magnetic reconnection in the solar atmosphere can transfer an additional 16% of the total reconnected magnetic energy into heating and/or accelerating the solar corona, within 11.6 [R S] of the reconnection site, below the critical Alfvén surface.

Geospace Concussion: Global Reversal of Ionospheric Vertical Plasma Drift in Response to a Sudden Commencement.

An interplanetary shock can abruptly compress the magnetosphere, excite magnetospheric waves and field-aligned currents, and cause a ground magnetic response known as a sudden commencement (SC). However, the transient (<∼1min) response of the ionosphere-thermosphere system during an SC has been little studied due to limited temporal resolution in previous investigations. Here, we report observations of a global reversal of ionospheric vertical plasma motion during an SC on 24 October 2011 using ∼6s resolution Super Dual Auroral Radar Network ground scatter data. The dayside ionosphere suddenly moved downward during the magnetospheric compression due to the SC, lasting for only ∼1min before moving upward. By contrast, the post-midnight ionosphere briefly moved upward then moved downward during the SC. Simulations with a coupled geospace model suggest that the reversed vertical drift is caused by a global reversal of ionospheric zonal electric field induced by magnetospheric compression during the SC.

Paleomagnetic study of impactites from the Karla impact structure suggests protracted postimpact hydrothermalism

AbstractWe present a paleomagnetic study of the ~10 km diameter Karla impact structure in Russia. We sampled the target carbonate rocks, and a yet undocumented fragmental melt‐bearing lithic breccia layer. This impact breccia, which contains carbonate melt, is enriched in stoichiometric magnetite by a factor of ~15 compared to the target lithologies, and carries a stable natural remanent magnetization. The weak remanent magnetization and the presence of both normal and reverse polarities down to the centimeter scale indicate that the breccia does not carry a thermoremanent magnetization (TRM), but rather a chemical remanent magnetization (CRM). The presence of stoichiometric magnetite and the absence of TRM suggest that the magnetite was formed during relatively low‐temperature postimpact hydrothermalism that affected the porous impact breccia layer. During this process, the breccia acquired a CRM. The paleomagnetic direction is compatible with a Cenozoic age for the impact event, but cannot bring more precise constraint on the age because of the stable position of the Eurasian plate over the last 60 Myr. However, the presence of both polarities indicates that mild hydrothermalism took place over a period of time long enough to span at least one reversal of the geomagnetic field, that is, over a time scale of the order of 100 kyr. This confirms that protracted hydrothermal systems associated with impact craters are long lived, even in relatively small craters such as Karla, and are key features of the geologic and environmental effects of impacts on Earth.

Repeating fast radio burst 20201124A originates from a magnetar/Be star binary

Fast radio bursts (FRBs) are cosmic sources emitting millisecond-duration radio bursts. Although several hundreds FRBs have been discovered, their physical nature and central engine remain unclear. The variations of Faraday rotation measure and dispersion measure, due to local environment, are crucial clues to understanding their physical nature. The recent observations on the rotation measure of FRB 20201124A show a significant variation on a day time scale. Intriguingly, the oscillation of rotation measure supports that the local contribution can change sign, which indicates the magnetic field reversal along the line of sight. Here we present a physical model that explains observed characteristics of FRB 20201124A and proposes that repeating signal comes from a binary system containing a magnetar and a Be star with a decretion disk. When the magnetar approaches the periastron, the propagation of radio waves through the disk of the Be star naturally leads to the observed varying rotation measure, depolarization, large scattering timescale, and Faraday conversion. This study will prompt to search for FRB signals from Be/X-ray binaries.

A comparative study of electrochemical and electrostatic doping modulation of magnetism in Fe3O4 via ultracapacitor structure

Electric field control of magnetism can boost energy efficiency and have brought revolutionary breakthroughs in the development of widespread applications in spintronics. Electrolyte gating plays an important role in magnetism modulation. In this work, reversible room-temperature electric field control of saturation magnetization in Fe3O4 via a supercapacitor structure is demonstrated with three types of traditional gate electrolytes for comparison. Different magnetization response and responsible mechanisms are revealed by Operando magnetometry PPMS/VSM and XPS characterization. The main mechanism in Na2SO4, KOH aqueous electrolytes is electrochemical effect, while both electrochemical and electrostatic effects were found in LiPF6 organic electrolyte. This work offers a kind of reference basis for selecting appropriate electrolyte in magnetism modulation by electrolyte-gating in the future, meanwhile, paves its way towards practical use in magneto-electric actuation, voltage-assisted magnetic storage, facilitating the development of high-performance spintronic devices.

Ion-Boosting the Charge Density and Piezoelectric Response of Ferroelectrets to Significantly High Levels.

In contrast to molecular-dipole polymers, such as PVDF, ferroelectrets are a new class of flexible spatially heterogeneous piezoelectric polymers with closed or open voids that act as deformable macro-dipoles after charging. With a spectrum of manufacturing processes being developed to engineer the heterogeneous structures, ferroelectrets are made with attractive piezoelectric properties well-suited for applications, such as pressure sensors, acoustic transducers, etc. However, the sources of the macro-dipole charges have usually been the same, microscopic dielectric barrier discharges within the voids, induced when the ferroelectrets are poled under a large electric field typically via a so-called corona poling, resulting in the separation and trapping of opposite charges into the interior walls of the voids. Such a process is inherently self-limiting, as the reverse internal field from the macro-dipoles eventually extinguishes the microdischarges, resulting in limited density of ions and not too high overall piezoelectric performance. Here, a new method to form ferroelectrets with gigantic electroactivity is proposed and demonstrated with the aid of an external ion booster. A laminate consisting of expanded polytetrafluoroethylene (ePTFE) and fluorinated-ethylene-propylene (FEP) was prefilled with bipolar ions produced externally by an ionizer and sequentially poled to force the separation of positive and negative ions into the open fibrous structure, rendering an impressive piezoelectric d33 coefficient of 1600 pC/N─an improvement by a factor of 4 in comparison with the d33 of a similar sandwich poled with nonenhanced corona poling. The (pre)filling clearly increases the ion density in the open voids significantly. The charges stored in the open-cell structure stays at a high level for at least 4 months. In addition, an all-organic nanogenerator was made from an ePTFE-based ferroelectret, with conducting poly(3,4-ethylene dioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) coated fabric electrodes. When poled with this ion-boosting process, it yielded an output power twice that of a similar sample poled in a conventional corona-only process. The doubling in output power is mainly brought about by the significantly higher charge density achieved with the aid of external booster. Furthermore, aside from the bipolar ions, extra monopolar ions can during the corona poling be blown into the open pores by using for instance a negative ionic hair dryer to produce a unipolar ePTFE-based ferroelectret with its d33 coefficient enhanced by a factor of 3. Ion-boosting poling thus unleashes a new route to produce bipolar or unipolar open-cell ferroelectrets with highly enhanced piezoelectric response.

Challenges and perspectives of the Industry 4.0 technologies within the last-mile and first-mile reverse logistics: A systematic literature review

The circular economy (CE) and supply chain management (SCM) paradigms were already synthetized within the concept of circular supply chain (CSC) but, recently, also Industry 4.0 has been identified as a disruptive factor in the field of CSC and as key for shifting toward sustainability. The aim of this paper is to evaluate the current contribution of Industry 4.0 technologies in logistics, with a focus in last and first mile activities.The study proposes a three-step methodological approach which includes bibliometric, network and content analyses has been adopted to systematically analyze the scientific literature and to investigate the evolving trends, challenges and perspectives within the last-mile and first-mile reverse logistics research field. From a managerial perspective, this study provides an overview of the potential associated with Industry 4.0 solutions adopted within the logistics management, focusing on interdependencies, possible trade-offs, and less understood pathways. The study has outlined that several gaps still remain present and future research developments should be addressed in this research field starting from in identifying critical points for the simultaneous management of last-mile and first-mile reverse logistics operations and evaluating economic feasibility of Industry 4.0 adoption. This study also provided feedbacks related to support define new business model in order to respond to new requirements deriving from the implementation of circular economy and Industry 4.0 paradigms.

Colloidal Suspensions Displaying Anomalous Phoretic Behavior: Field and Mobility Reversal.

We show that colloidal suspensions that acquire a surface charge by capturing ions from the surrounding solution display unexpected and remarkable phoretic behavior. Depending on suspension volume fraction, a critical zeta potential ζ exists where the effective electrophoretic mobility diverges, becoming virtually infinite. Beyond such critical value, a ζ-range is identified where mobility reversal occurs, i.e., the effective mobility becomes negative. This counterintuitive behavior is due to the salt gradient engendered by phoretic drift of this kind of particles, which capture and release ions (salt), respectively, at the start and the end of the phoretic path. This salt gradient deeply influences the electric field in the bulk electrolyte where the particles migrate: it can make the field vanish, hence the mobility divergence, or even entail inversion of the field, which is reflected in the mobility reversal. These findings should spur new concepts in a variety of traditional and emerging technologies involving, for example, the separation or targeting of colloids as well as in applications where the creation or manipulation of chemical gradients or electric fields in solution is critical.

MMS Observations of a Compressed Current Sheet: Importance of the Ambipolar Electric Field.

Spacecraft data reveal a nonuniform ambipolar electric field transverse to the magnetic field in a thin current sheet in Earth's magnetotail that leads to intense E×B velocity shear and nongyrotropic particle distributions. The E×B drift far exceeds the diamagnetic drift and thus drives observed lower hybrid waves. The shear-driven waves are localized to the magnetic field reversal region and are therefore ideally suited for the anomalous dissipation necessary for reconnection. It also reveals substructures embedded in the current density, indicating a compressed current sheet.

Thermo-magnetic convection regulating the solidification behavior and energy storage of Fe3O4 nanoparticles composited paraffin wax under the magnetic-field

Solidification is an essential process in phase-change energy storage and magnetic fields have great effects on it. While the majority of previous studies focused on non-uniform magnetic fields, this study investigated the differences using uniform magnetic fields. The thermo-magnetic convection model and the enthalpy porosity method describe the motion behavior of Fe3O4 nanoparticles under a uniform magnetic field and phase change process. The properties of Fe3O4/paraffin phase change nanocomposites are measured, and a visualization platform is built to verify the numerical model. The effect and mechanism of direction and intensity of magnetic induction, nanoparticle concentration on solidification performance, heat transfer, and energy release are investigated. Results showed that magnetic field could promote or inhibit solidification depending on its direction. The degree of global regulation increased with the concentration of nanoparticle and magnetic induction intensity, while the local nonuniformity increased or decreased under forward or reverse magnetic fields. The solid phase fraction and energy release of the cavity under positive or negative magnetic fields increased or decreased maximally by 29.2%, 19.23% or 4.6%, and 3.88% compared with paraffin. Essentially, the regulation was the result of different motion behaviors of nanoparticles in the liquid phase controlled by Kelvin force. In addition, methods and optimization suggestions for regulation were proposed, which were expected to provide a reference for the design of controllable energy storage devices.

Comparing Study on Formation of Large Discharge Currents in Atmospheric Pulse-Modulated Radio Frequency Discharges

In recent years, pulse-modulation is introduced in atmospheric radio frequency (RF) discharges to further control the discharge operation. In this article, a comparing study is performed by a 1-D fluid model to investigate the formation of large discharge currents (LDCs) in the first RF cycle during the power-on phase in an atmospheric pulse-modulated RF discharge controlled by dielectric barriers. The current–voltage characteristics and helium metastable (He*) distributions show the enhancement of a pulsed voltage on the ignition of RF discharge, which agrees well with the experimental observation. On the other hand, in a dielectric barrier discharge driven by a sinusoidal voltage of 500 MHz, double LDCs are observed in the positive and negative half-cycles, respectively, and the large pulse rise rate of applied voltage essentially influences the distribution of charged particles in the first cycle during the power-on phase. In this study, according to computational data the reverse electric field near the anode contributes greatly to the generation of LDCs in both types of discharge, and the appropriate discharge conditions are suggested to optimize the pulse-modulated RF discharges in applications.

Microscopic origin of coercivity enhancement by dysprosium substitution into neodymium permanent magnets

Neodymium (Nd) magnets (${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$), known as the strongest permanent magnets, are indispensable in realizing high efficiency in energy conversion devices. To enhance their coercivity at high temperatures, the Nd component is substituted with dysprosium (Dy) in the commercial products of the magnet. Surface Dy-rich shells are considered important for enhancing coercivity. However, the underlying microscopic mechanism has not been studied based on atomistic theories. In this study, first, the features and mechanisms of the enhancement were studied using an atomistic model of the Nd magnet. Subsequently, the threshold field (coercive force) for magnetization reversal and the dynamical features at absolute zero and finite temperatures were investigated. The results show that a change from surface to bulk nucleation occurs when the number of substituted layers increases and the anisotropy energy of Dy is resistant to temperature increase, which significantly enhances coercivity, especially at high temperatures.

Silver split nano-tube array as a meta-atomic monolayer for high-reflection band

In this work, an ultra-thin silver film-coated grating as a split silver nanotube array exhibits not only high TE polarized reflectance as a conventional subwavelength grating but also high TM polarized reflectance that is close to or higher than TE reflectance at certain wavelength range. The TM reflectance peak shifts with the morphology of the silver covering. The near-field analysis reveals that the silver nanotube array is an ultra-thin optical double negative metamaterial. The negative permeability associated magnetic field reversal is induced within the grating that is surrounded by a split current loop at the TM reflectance peak wavelength. The near field simulation is used to retrieve the equivalent electromagnetic parameters and optical constants that cause the anomalous TM high reflection. It is demonstrated that the TM impedances have a low magnitude and high magnitude with respect to unity for light incident onto the top and bottom of the grating at the peak wavelength, respectively.

Electronic excitation and electric field as switching mechanism for a single-molecule switch

DFT calculations show that a combination of an electric field and electronic excitation is a promising mechanism to force a molecular switch based on amino-imino tautomerisation into one of its two states. By calculating the effect of an electric field in the direction of the moving hydrogen on the shape of the barrier in the ground and low-lying excited states of previously proposed molecular switches consisting of 5- and 7-membered rings with adjacent amino and imino groups, we demonstrate that electric fields and photons in experimentally accessible ranges introduce sufficient asymmetry to push the switch into the desired configuration. Excitation to states with inverted order of the preferred geometry allows reversible switching without reversal of the electric field.

Jets from Accretion Disk Dynamos: Consistent Quenching Modes for Dynamo and Resistivity

Astrophysical jets are launched from strongly magnetized systems that host an accretion disk surrounding a central object. The origin of the magnetic field, which is a key component of the launching process, is still an open question. Here we address the question of how the magnetic field required for jet launching is generated and maintained by a dynamo process. By carrying out nonideal MHD simulations (PLUTO code), we investigate how the feedback of the generated magnetic field on the mean-field dynamo affects the disk and jet properties. We find that a stronger quenching of the dynamo leads to a saturation of the magnetic field at a lower disk magnetization. Nevertheless, we find that, while applying different dynamo feedback models, the overall jet properties remain unaffected. We then investigate a feedback model that encompasses a quenching of the magnetic diffusivity. Our modeling considers a more consistent approach for mean-field dynamo modeling simulations, as the magnetic quenching of turbulence should be considered for both a turbulent dynamo and turbulent magnetic diffusivity. We find that, after the magnetic field is saturated, the Blandford–Payne mechanism can work efficiently, leading to more collimated jets, which move, however, with slower speed. We find strong intermittent periods of flaring and knot ejection for low Coriolis numbers. In particular, flux ropes are built up and advected toward the inner disk thereby cutting off the inner disk wind, leading to magnetic field reversals, reconnection and, the emergence of intermittent flares.

Magnetic Field Dropouts and Associated Plasma Wave Emission near the Electron Plasma Frequency at Switchback Boundaries as Observed by the Parker Solar Probe

The first solar encounters by the Parker Solar Probe revealed the magnetic field to be dominated by short field reversals in the radial direction, referred to as “switchbacks.” While radial velocity and proton temperature were shown to increase inside the switchbacks, ∣B∣ exhibits very brief dropouts only at the switchback boundaries. Brief intensifications in spectral density measurements near the electron plasma frequency, f pe, were also observed at these boundaries, indicating the presence of plasma waves triggered by current systems in the form of electron beams. We perform a correlative study using observations from the Parker FIELDS Radio Frequency Spectrometer and Fluxgate Magnetometer to compare occurrences of spectral density intensifications at the electron plasma frequency (f pe emissions) and ∣B∣ dropouts at switchback boundaries during Parker’s first and second solar encounters. We find that only a small fraction of minor ∣B∣ dropouts are associated with f pe emissions. This fraction increases with ∣B∣ dropout size until all dropouts are associated with f pe emissions. Brief spikes in the differential electron flux measured by the SWEAP Solar Probe Analyzer for Electron sensors also occur in conjunction with nearly all f pe emissions. This suggests that in the presence of strong ∣B∣ dropouts, electron currents that create the perturbation in ∣B∣ along the boundaries are also stimulating plasma waves such as Langmuir waves.

Heteroepitaxial growth of InSb thin film on SrTiO3 (001) by pulsed laser deposition for magnetic Hall sensor application

Ⅲ–Ⅴ semiconductor compound InSb with a zinc blende structure and extremely high electron mobility was grown as a thin film on SrTiO3 (STO)(001) substrate by pulsed laser deposition. The growth of (111) oriented InSb was observed despite a large lattice mismatch of 1.6% ([1−10]InSb ∣∣ [010]STO) and 17.8% ([11−2]InSb ∣∣ [100]STO). Electron mobility of 94 cm2 V−1s−1 at room temperature was confirmed by Hall effect measurement and the magnetic field reversal down to 1 Oe was detected. Growth of InSb/STO (001) thin film could be a promising approach for the detection of magnetic signals from various perovskite type functional oxide materials and will find applications in future devices.