Magnetic Field Configuration Research Articles

Observations of Small‐Scale Magnetic Flux Ropes Associated With Significant Values of Partial Variance Increment Indices

AbstractWe report recent findings for the magnetic field configurations of small‐scale magnetic flux ropes (SFRs) broadly defined and identified by using the Grad‐Shafranov‐based techniques for in situ measurements via the Parker Solar Probe (PSP), Solar Orbiter (SolO), and two Helios spacecraft. Since the current sheets were found to occur at boundaries of SFRs and/or inside SFRs at 1 AU via the partial variance increment (PVI) and the Grad‐Shafranov (GS) reconstruction technique by Pecora et al. (2019), https://doi.org/10.3847/2041‐8213/ab32d9, we further examine such a co‐existence in this study by assessing the maximum PVI indices within SFR intervals using the above four spacecraft observations throughout the inner heliosphere (1 AU). Less than 15% of SFRs have maximum PVI indices exceeding a threshold value of 6 that is used to indicate a current sheet structure. Three representative events are selected to explain the most common situations. (a) Current sheets occur at SFR boundaries and near the center. Each could be a weak switchback feature in the time‐series profile of the gradually bipolar magnetic field rotations. (b) An SFR configuration is confirmed by both the measurement of counterstreaming electrons and the GS reconstruction result, despite that a large PVI value occurs near the SFR center which is due to an arbitrary kink instead of a current sheet. (c) A current sheet is falsely identified as an SFR where a significant PVI value ( 7) occurs near the center. In the end, we discuss the necessity of using multi‐point spacecraft measurements to discern the structures associated with SFRs.

Constraints on axion-like particles from the gamma-ray observation of the Galactic Center

High energy photons originating from the Galactic Center (GC) region have the potential to undergo significant photon-axion-like particle (ALP) oscillation effects, primarily induced by the presence of intense magnetic fields in this region. Observations conducted by imaging atmospheric Cherenkov telescopes have detected very high energy gamma-rays originating from a point source known as HESS J1745-290, situated in close proximity to the GC. This source is conjectured to be associated with the supermassive black hole Sagittarius A*. The GC region contains diverse structures, including molecular clouds and non-thermal filaments, which collectively contribute to the intricate magnetic field configurations in this region. By utilizing a magnetic field model specific in the GC region, we explore the phenomenon of photon-ALP oscillations in the gamma-ray spectrum of HESS J1745-290. Our analysis does not reveal any discernible signature of photon-ALP oscillations, yielding significant constraints that serve as a complement to gamma-ray observations of extragalactic sources across a broad parameter region. The uncertainties arising from the outer Galactic magnetic field models have minor impacts on our results, except for ALP masses around 10-7 eV, as the dominant influence originates from the intense magnetic field strength in the inner GC region.

INFLUENCE OF APPLYING MAGNETIC FIELD ON THERMAL AND HYDRAULIC CHARACTERISTICS OF FE3O4/WATER MAGNETIC NANOFLUID MOVING IN A TWISTED DUCT

This study introduces an investigation of the effect of combining passive and active techniques on enhancing heat transfer by using a ferrofluid passing through a twisted duct subjected to an external magnetic field. The effect of changing the number of magnets, magnetic flux density, nanoparticle volume fraction, and twist ratio on the heat transfer enhancement is studied. The optimum magnetic field configuration was achieved by adjusting the rate of increase of magnetic flux density to 80% between each two successive magnets. Results show that the proposed hybrid technique is promising in providing significant heat transfer enhancement compared to the traditional techniques, but the pressure drop values become relatively higher due to the increased friction levels. The twist ratio value that achieves the optimum thermo-hydraulic performance is found to be 3.7. The limit of magnetic flux density, which, if exceeded, causes the thermo-hydraulic performance to decline, is 800 Gauss. It is found that applying a magnetic field with 1,000 Gauss flux density with a nanoparticle volume concentration of 5% and a twist ratio of 3.7 at Reynolds number of 8,400, enhances the Nusselt number by 25.78% compared to the case using water without twisting in the absence of a magnetic field.

Depolarization by Jet Precession in Early Optical Afterglows of Gamma-Ray Bursts

Abstract Polarization observations provide a unique way to probe the nature of jet magnetic fields in gamma-ray bursts (GRBs). Recently, some GRBs have been detected to be polarized in their early optical afterglows. However, the measured polarization degrees (PDs) of these GRBs are much lower than those predicted by theoretical models. In this work, we investigate the depolarization induced by jet precession in combination with the measured PDs of the GRB early optical afterglows in the reverse shock (RS) dominated phase (∼102–103 s). We calculate the PDs of RS emissions with and without jet precession in both magnetic field configurations, i.e., aligned and toroidal magnetic fields, and meanwhile explore the effects of different parameters on the PDs. We find that the PDs are slightly affected by the configurations of the ordered magnetic fields and are positively related to the precession period. Moreover, the PDs are sensitive to the observed angle, and the measured low PDs favor a small one. Thus, as one of the plausible origins of the structured jets, jet precession could be considered as an alternative mechanism for the low PDs observed in GRB early optical afterglows.

Combined Fit of Spectrum and Composition for FR0 Radio-galaxy-emitted Ultra–high energy Cosmic Rays with Resulting Secondary Photons and Neutrinos

This study comprehensively investigates the gamma-ray dim population of Fanaroff–Riley Type 0 (FR0) radio galaxies as potentially significant sources of ultra–high energy cosmic rays (UHECRs, E > 1018 eV) detected on Earth. While individual FR0 luminosities are relatively low compared to the more powerful Fanaroff–Riley Type 1 and Type 2 galaxies, FR0s are substantially more prevalent in the local universe, outnumbering the more energetic galaxies by a factor of ∼5 within a redshift of z ≤ 0.05. Employing CRPropa3 simulations, we estimate the mass composition and energy spectra of UHECRs originating from FR0 galaxies for energies above 1018.6 eV. This estimation fits data from the Pierre Auger Observatory (Auger) using three extensive air shower models; both constant and energy-dependent observed elemental fractions are considered. The simulation integrates an approximately isotropic distribution of FR0 galaxies, extrapolated from observed characteristics, with UHECR propagation in the intergalactic medium, incorporating various plausible configurations of extragalactic magnetic fields, both random and structured. We then compare the resulting emission spectral indices, rigidity cutoffs, and elemental fractions with recent Auger results. In total, 25 combined energy-spectrum and mass-composition fits are considered. Beyond the cosmic-ray fluxes emitted by FR0 galaxies, this study predicts the secondary photon and neutrino fluxes from UHECR interactions with intergalactic cosmic photon backgrounds. The multimessenger approach, encompassing observational data and theoretical models, helps elucidate the contribution of low-luminosity FR0 radio galaxies to the total cosmic-ray energy density.

Generation of shear flows induced by AE / EPM in LHD plasma

Abstract The generation of shear flows (SFs) by Alfven Eigenmodes (AEs) and energetic particle modes (EPMs) have important effects on the operation of future nuclear fusion reactors, because SFs regulate the saturation of the AEs/EPMs, the transport of EPs and thermal plasma, as well as the formation of transport barriers among other consequences. The aim of this study is the analysis of SFs generation during the saturation phase of AEs and EPMs in LHD plasma. Experiments performed in the 23rd and 24th LHD experimental campaigns are dedicated to explore the destabilization of AEs/EPMs in discharges with different heating patterns, thermal plasma and magnetic field configurations. In particular, the shots 176490 and 179697 show the destabilization of MHD bursts and energetic-ion-driven resistive interchange modes (EIC), respectively. Charge exchange spectroscopy measurements in both discharges indicate that the generation of SFs by AE/EPM is uncorrelated with the perturbation induced by the neutral beam injector (NBI). Nonlinear simulations performed using the gyro-fluid code FAR3d show the generation of zonal structures, especially SFs, induced during the saturation phase of Toroidal Alfven Eigenmodes (TAEs) triggered in the MHD burst as well as by the 1 / 1 EIC in the bursting phase. The simulations indicate that SFs are caused by the radial electric fields powered by energy transfers from the unstable AE/EPM towards the thermal plasma. The strongest SFs are measured during the EIC bursting phase once the 1 / 1 EPM overlaps with nearby resonances at the plasma periphery. Likewise, the largest SFs during the MHD burst are observed once TAEs radially overlap in the inner-middle plasma region.

On the Geometry of the Near-core Magnetic Field in Massive Stars

Abstract It is well-known that the cores of massive stars sustain a stellar dynamo with a complex magnetic field configuration. However, the same cannot be said for the field's strength and geometry at the convective–radiative boundary, which are crucial when performing asteroseismic inference. In this Letter, we present 3D magnetohydrodynamic (MHD) simulations of a 7 M ⊙ mid-main-sequence star, with particular attention given to the convective–radiative boundary in the near-core region. Our simulations reveal that the toroidal magnetic field is significantly stronger than the poloidal field in this region, contrary to recent assumptions. Moreover, the rotational shear layer, also important for asteroseismic inference, is specifically confined within the extent of the Brunt–Väisälä frequency peak. These results, which are based on the inferred properties of HD 43317, have widespread implications for asteroseismic studies of rotation, mixing, and magnetism in stars. While we expect our results to be broadly applicable across stars with similar Brunt–Väisälä frequency profiles and stellar masses, we also expect the MHD parameters (e.g., Rem) and the initial stellar rotation rate to impact the geometry of the field and differential rotation at the convective–radiative interface.

SPATIAL DISTRIBUTION OF ELECTRIC FIELD IN A TWO STAGE HALL TYPE PLASMA SOURCE

The two-stage plasma source of Hall type having common anode and two cathodes residing at locations with essentially different magnetic field configuration is experimentally studied. Considered are peculiarities of spatial distribution of floating potential measured by moved Langmuir probes. In particular, it was established that in the middle plane of the anode on the axis of the device, the value of the floating potential of the plasma is up to 95% of the anode voltage.

The impact of resistivity on the variability of black hole accretion flows

The accretion of magnetized plasma onto black holes is a complex and dynamic process in which the magnetic field plays a crucial role. The amount of magnetic flux that is accumulated near the event horizon significantly impacts the accretion flow behavior. Resistivity, which is a measure of how easily magnetic fields can dissipate, is thought to be a key factor influencing this process. This work explores the influence of resistivity on the accretion flow variability. We investigated simulations that reached the limit of the magnetically arrested disk (MAD) and simulations with an initial multi-loop magnetic field configuration. We employed 3D resistive general relativistic magnetohydrodynamic (MHD) simulations to model the accretion process under various regimes, where resistivity is globally constant (uniform resistivity). Our findings reveal distinct flow behaviors depending on resistivity. High-resistivity simulations never achieved the MAD state, which indicates a disturbed magnetic-flux accumulation process. Conversely, low-resistivity simulations converged toward the ideal MHD limit. The key results are that i) for the standard MAD model, resistivity plays a minimum role in flow variability, suggesting that flux eruption events dominate the dynamics. ii) High-resistivity simulations exhibit strong magnetic field diffusion into the disk that rearranges the efficient magnetic flux accumulation from the accretion flow. iii) In multi-loop simulations, resistivity significantly reduces the flow variability, which was not expected. However, magnetic flux accumulation becomes more variable as a result of frequent reconnection events at very low resistivity values. This study shows that resistivity affects how much the flow is distorted as a result of the magnetic field dissipation. Our findings provide new insights into the interplay between magnetic field accumulation, resistivity, variability, and the dynamics of black hole accretion.

Implementation of an Asymmetric Internal Field in the Comprehensive Inner Magnetosphere‐Ionosphere (CIMI) Model

AbstractA Comprehensive Inner Magnetosphere‐Ionosphere (CIMI) model has been developed to study the dynamics of the cold plasmasphere and the energetic plasmas in the inner magnetosphere, as well as their couplings with each other and with the ionosphere. The CIMI model is able to predict the cold plasma density and energetic electron and ion fluxes in geospace. Furthermore, CIMI is capable of predicting the Region 2 currents, penetration electric field, electron and ion precipitation and magnetospheric heat flux into the ionosphere. The CIMI model includes a realistic magnetic field configuration with a combination of an internal field and an external field imposed by the interaction of the solar wind with the magnetosphere. The internal field has previously been assumed to be a dipole. Recently, the International Geomagnetic Reference Field (IGRF) has been implemented. This new capability enables studies of north‐south and longitudinal dependences in particle precipitation and heat flux, as well as the corresponding asymmetries in ionospheric and thermospheric responses. In this paper, we will briefly review the CIMI equations and model output. Then we will describe the new implementation of the IGRF model into CIMI and how to estimate the north‐south asymmetry in precipitating fluxes from the differences in field strength between magnetic conjugate points. The inclusion of a realistic internal field leads CIMI into a better position to couple with sophisticated ionosphere‐thermosphere models, most of which are using the IGRF model.

Suppression of Collisionless Magnetic Reconnection in the High Ion β, Strong Guide Field Limit

In magnetic reconnection, the ion bulk outflow speed and ion heating have been shown to be set by the available reconnecting magnetic energy, i.e., the energy stored in the reconnecting magnetic field (B r ). However, recent simulations, observations, and theoretical works have shown that the released magnetic energy is inhibited by upstream ion plasma beta β i —the relative ion thermal pressure normalized to magnetic pressure based on the reconnecting field—for antiparallel magnetic field configurations. Using kinetic theory and hybrid particle-in-cell simulations, we investigate the effects of β i on guide field reconnection. While previous works have suggested that guide field reconnection is uninfluenced by β i , we demonstrate that the reconnection process is modified and the outflow is reduced for sufficiently large βi>(Br2+Bg2)/Br2 . We develop a theoretical framework that shows that this reduction is consistent with an enhanced exhaust pressure gradient, which reduces the outflow speed as vout∝1/βi . These results apply to systems in which guide field reconnection is embedded in hot plasmas, such as reconnection at the boundary of eddies in fully developed turbulence like the solar wind or the magnetosheath as well as downstream of shocks such as the heliosheath or the mergers of galaxy clusters.

Radio-frequency ion thruster with magnetic shielding of the discharge chamber walls

We presented the results of a computational study on optimizing the shape of the main elements of a radio-frequency ion thruster – the discharge chamber and the ion-extraction system grids. The possibility of improving the integral characteristics of thrusters and ion sources due to the use of an additional magnetostatic field in the RF discharge region was considered. The performed series of calculations made it possible to determine the optimal geometry of the discharge chamber and of the RIT ion-extraction system grids, as well as the configuration of the additional magnetic field, at which the best values of the integral characteristics were achieved.

Quantum information for graphene wormholes

Abstract This paper explores the interplay between quantum information theory and the stabilization of graphene wormholes using external magnetic fields. Utilizing Shannon entropy, we analyze how quantum information can be applied to control and stabilize these structures. By studying graphene’s quantum states under different magnetic field strengths and configurations, we gain insights into the entanglement and coherence properties governing their behavior. The findings demonstrate the potential of quantum information metrics to enhance the stability and control of graphene wormholes, with implications for quantum computing and material science innovations.

The magnetic maze: a system with tunable scale invariance

Random magnetic field configurations are ubiquitous in nature. Such fields lead to a variety of dynamical phenomena, including localization and glassy physics in some condensed matter systems and novel transport processes in astrophysical systems. Here we consider the physics of a charged quantum particle moving in a “magnetic maze”: a high-dimensional space filled with a randomly chosen vector potential and a corresponding magnetic field. We derive a path integral description of the model by introducing appropriate collective variables and integrating out the random vector potential, and we solve for the dynamics in the limit of large dimensionality. We derive and analyze the equations of motion for Euclidean and real-time dynamics, and we calculate out-of-time-order correlators. We show that a special choice of vector potential correlations gives rise, in the low temperature limit, to a novel scale-invariant quantum theory with a tunable dynamical exponent. Moreover, we show that the theory is chaotic with a tunable chaos exponent which approaches the chaos bound at low temperature and strong coupling.

Magnetic interaction of stellar coronal mass ejections with close-in exoplanets: implication on planetary mass-loss and Ly α transits

ABSTRACT Coronal mass ejections (CMEs) erupting from the host star are expected to affect the atmospheric erosion processes of planets. For planets with a magnetosphere, the embedded magnetic field in the CMEs is thought to be the most important parameter to affect planetary mass-loss. In this work, we investigate the effect of different magnetic field structures of stellar CMEs on the atmosphere of a hot Jupiter with a dipolar magnetosphere. We use a time-dependent 3D radiative magnetohydrodynamic (MHD) atmospheric escape model that self-consistently models the outflow from hot Jupiter’s magnetosphere and its interaction with stellar CMEs. For our study, we consider three configurations of magnetic field embedded in CMEs – (a) northward $B_z$ component, (b) southward $B_z$ component, and (c) radial component. We find that both the CMEs with northward $B_z$ and southward $B_z$ increase the planetary mass-loss rate when the CME arrives from the stellar side, with the mass-loss rate remaining higher for the CME with northward $B_z$ until it arrives on the opposite side. The largest magnetopause is found for the CME with a southward $B_z$ component. During the passage of a CME, the planetary magnetosphere goes through three distinct changes – (1) compressed magnetosphere, (2) enlarged magnetosphere, and (3) relaxed magnetosphere for all three CME configurations. The computed synthetic Ly $\alpha$ transit absorption generally increases when the CME is in interaction with the planet for all magnetic configurations but the maximum Ly $\alpha$ absorption is found for the case of radial CME with the most compressed magnetosphere.

Combined response of polar magnetotaxis to oxygen and pH: Insights from hanging drop assays and microcosm experiments

Magnetotactic bacteria (MTB) combine passive alignment with the Earth magnetic field with a chemotactic response (magneto-chemotaxis) to reach their optimal living depth in chemically stratified environments. Current magneto-aerotaxis models fail to explain the occurrence of MTB far below the oxic-anoxic interface and the coexistence of MTB cells with opposite magnetotactic polarity at depths that are unrelated with the redox gradient. Here we propose a modified model of polar magnetotaxis which explains these observations, as well as the distinct concentration profiles and magnetotactic advantages of two types of MTB inhabiting a freshwater sediment: a group of unidentified cocci (MC), and a giant rod-shaped bacterium (MB) apparently identical to M. bavaricum (MB). This model assumed that magnetotactic polarity is set by a threshold mechanism in counter gradients of oxygen and a second group of repellents, with, in case of MB, includes H+ ions. MTB possessing this type of polar magnetotaxis can shuttle between two limit depths across the redox gradient (redox taxis), as previously postulated for M. bavaricum and other members of the Nitrospirota group. The magnetotaxis of MB and MC is predominantly dipolar whenever the presence of a magnetic field ensures a magnetotactic advantage. In addition, MB can overcome unfavorable magnetic field configurations through a temporal sensing mechanism. The availability of threshold and temporal sensing mechanisms of different substances can generate a rich variety of responses by different types of MTB, enabling them to exploit multiple ecological niches.

Generation of 10 kT axial magnetic fields using multiple conventional laser beams: A sensitivity study for kJ PW-class laser facilities

Strong multi-kilotesla magnetic fields have various applications in high-energy density science and laboratory astrophysics, but they are not readily available. In our previous work [Y. Shi et al., Phys. Rev. Lett. 130, 155101 (2023)], we developed a novel approach for generating such fields using multiple conventional laser beams with a twist in the pointing direction. This method is particularly well-suited for multi-kilojoule petawatt-class laser systems like SG-II UP, which are designed with multiple linearly polarized beamlets. Utilizing three-dimensional kinetic particle-in-cell simulations, we examine critical factors for a proof-of-principle experiment, such as laser polarization, relative pulse delay, phase offset, pointing stability, and target configuration, and their impact on magnetic field generation. Our general conclusion is that the approach is very robust and can be realized under a wide range of laser parameters and plasma conditions. We also provide an in-depth analysis of the axial magnetic field configuration, azimuthal electron current, and electron and ion orbital angular momentum densities. Supported by a simple model, our analysis shows that the axial magnetic field decays owing to the expansion of hot electrons.

Seismic differences between solar magnetic cycles 23 and 24 for low-degree modes

Solar magnetic activity follows regular cycles of about 11 years with an inversion of polarity in the poles every sim 22 years. This changing surface magnetism impacts the properties of the acoustic modes. The acoustic mode frequency shifts are a good proxy of the magnetic cycle. In this Letter we investigate solar magnetic activity cycles 23 and 24 through the evolution of the frequency shifts of low-degree modes (ell = 0, 1, and 2) in three frequency bands. These bands probe properties between 74 and 1575 km beneath the surface. The analysis was carried out using observations from the space instrument Global Oscillations at Low Frequency and the ground-based Birmingham Solar Oscillations Network and Global Oscillation Network Group. The frequency shifts of radial modes suggest that changes in the magnetic field amplitude and configuration likely occur near the Sun's surface rather than near its core. The maximum shifts of solar cycle 24 occurred earlier at mid and high latitudes (relative to the equator) and about 1550 km beneath the photosphere. At this depth but near the equator, this maximum aligns with the surface activity but has a stronger magnitude. At around 74 km deep, the behaviour near the equator mirrors the behaviour at the surface, while at higher latitudes, it matches the strength of cycle 23.

Droplet generation in a co-flowing microchannel under the non-uniform magnetic fields produced by an electric current loop

Micro-magnetofluidics, the study of fluid behavior under magnetic fields in microscale systems, is vital for applications like drug delivery, chemical synthesis, and lab-on-a-chip technologies. Controlling droplet size and formation frequency in these systems is challenging due to the complex interplay of magnetic forces and fluid dynamics. This study introduces a novel approach to control droplet generation in a co-flowing microchannel under a non-uniform magnetic field generated by an electric current loop. Critical parameters such as electric current intensity, continuous phase flow rate, and current loop position are systematically examined for their impact on droplet behavior. The results highlight the unique influence of the magnetic field configuration, specifically the electric current loop, in inducing a transition from dripping to jetting flow patterns with increasing current intensity, leading to larger droplets and reduced generation frequency. Additionally, a distinct behavior of droplet coalescence near the current loop, followed by re-separation, is observed when the loop is positioned downstream of the inlet. Moreover, increasing the continuous phase flow rate consistently reduced droplet size and increased generation frequency, regardless of the current loop’s position.

Electron Scattering Due To Asymmetric Drift‐Orbit Bifurcation: Geometric Jumps of Adiabatic Invariant

AbstractRadial transport of energetic electrons is one of the key processes responsible for the variability of the outer radiation belt. This transport amounts to a violation of the drift motion. One of the mechanisms that can lead to such violation and associated radial transport is drift‐orbit bifurcation. This arises naturally from solar wind compression of the dayside magnetosphere, which results in a local maximum of the field strength at the equator and two off‐equatorial minima at the north and south segments of the field line. Azimuthally drifting, near‐equatorially mirroring electrons can be trapped, bouncing along the field line in either of those minima for a portion of their drift orbit around Earth. Trapping and the ensuing de‐trapping are associated with jumps of the second adiabatic invariant, making the third adiabatic invariant undefined and the drift orbit open. Drift‐orbit bifurcation has been previously investigated for north‐south and dawn‐dusk symmetric configurations of the magnetospheric magnetic field. Here we study the implications of an asymmetry in the drift‐orbit bifurcation due to a large IMF field. Using the theory of separatrix crossings in Hamiltonian systems with a slow and a fast variable, we demonstrate that there are geometric (in phase space) jumps of adiabatic invariants due to the asymmetry of the magnetic field configuration. These jumps have magnitudes comparable to the initial invariant magnitudes and are dictated by the topology of the magnetic field. We develop a technique that allows estimation of the jumps in a given magnetic field configuration. We also assess the radial transport expected from asymmetric drift‐orbit bifurcation. We find that such transport can reach (Earth's radius) per drift period, depending on the magnitude of the IMF .