Magnetic Field Amplification Research Articles

Electric field assisted amplification of magnetic fields in tilted Dirac cone systems

We show that the continuum limit of the tilted Dirac cone in materials such as $8Pmmn$-borophene and layered organic conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ deformation of the Minkowski spacetime of Dirac materials. From its Killing vectors we construct an emergent tilted-Lorentz (t-Lorentz) symmetry group for such systems. With t-Lorentz transformations we are able to obtain the exact solution of the Landau bands for a crossed configuration of electric and magnetic fields. For any given tilt parameter $0\le\zeta<1$ if the ratio $\chi=v_FB_z/cE_y$ of the crossed magnetic and electric fields that satisfies $\chi\ge 1+\zeta$ one can always find appropriate t-boosts in both valleys labeled by $\pm$ in such a way the electric field can be t-boosted away, whereby the resulting pure effective magnetic field $B^\pm_z$ governs the Landau level spectrum around each valley. The effective magnetic field in one of the valleys is always larger than the applied perpendicular magnetic field. This amplification comes at the expense of of diminishing the effective field in the opposite valleyand can be detected in various quantum oscillation phenomena in tilted Dirac cone systems. Tuning the ratio of electric and magnetic fields to $\chi_{\rm min}=1+\zeta$ leads to valley selective collapse of Landau levels. Our geometric description of the tilt in Dirac systems reveals an important connection between the tilt and an incipient "rotating source" when the tilt parameter can be made to depend on spacetime in certain way.

Cosmic ray acceleration by shocks: spectral steepening due to turbulent magnetic field amplification

Abstract We show that the energy required to turbulently amplify magnetic field during cosmic ray (CR) acceleration by shocks extracts energy from the CR and steepens the CR energy spectrum.

Hydrodynamical moving-mesh simulations of the tidal disruption of stars by supermassive black holes

When a star approaches a black hole closely, it may be pulled apart by gravitational forces in a tidal disruption event (TDE). The flares produced by TDEs are unique tracers of otherwise quiescent supermassive black holes (SMBHs) located at the centre of most galaxies. In particular, the appearance of such flares and the subsequent decay of the light curve are both sensitive to whether the star is partially or totally destroyed by the tidal field. However, the physics of the disruption and the fall-back of the debris are still poorly understood. We are here modelling the hydrodynamical evolution of realistic stars as they approach a SMBH on parabolic orbits, using for the first time the moving-mesh code AREPO, which is particularly well adapted to the problem through its combination of quasi-Lagrangian behaviour, low advection errors, and high accuracy typical of mesh-based techniques. We examine a suite of simulations with different impact parameters, allowing us to determine the critical distance at which the star is totally disrupted, the energy distribution and the fallback rate of the debris, as well as the hydrodynamical evolution of the stellar remnant in the case of a partial disruption. Interestingly, we find that the internal evolution of the remnant's core is strongly influenced by persistent vortices excited in the tidal interaction. These should be sites of strong magnetic field amplification, and the associated mixing may profoundly alter the subsequent evolution of the tidally pruned star.

Interactions between Radio Galaxies and Cluster Shocks. I. Jet Axes Aligned with Shock Normals

Abstract We report from a study utilizing 3D MHD simulations, including cosmic-ray electrons, of the interactions between radio galaxies (RGs) and dynamically active intracluster media (ICMs). Here we consider interactions involving plane ICM shocks having Mach numbers 2–4 and their normals aligned with steady, active bipolar RG jets penetrating uniform, stationary ICMs. The shock impact disrupts the preformed RG jet cocoons into ring vortex structures. Sufficiently strong post-shock winds can stop and even reverse the upwind jet and strip jets to virtually naked states, leaving them without a surrounding cocoon. Strong shock-induced vorticity can also disrupt the downwind jet, so that the ring vortex remnant of the cocoons appears ahead of that jet’s visible terminus. Magnetic field amplification in the ring vortex can significantly enhance its synchrotron emissions well after the vortex becomes isolated from the RG and its fresh cosmic-ray electron supply. We examine these dynamics and their observable consequences in detail.

Active Experiments in Space: The Future

Planned active space experiments and ideas for future active space experiments are reviewed. Three active experiments being readied are DSX (Demonstration and Space eXperiments), SMART (Space Measurement of Rocket-released Turbulence), and BeamPIE (Beam Plasma Interaction Experiment). Ideas for future experiments include relativistic-electron-beam experiments for magnetic-field-line tracing, relativistic-electron-beam experiments to probe the middle atmosphere, plasma-wave launching using superparamagnetic-nanoparticle amplification of magnetic fields, the heavy-ion mass loading of collisionless magnetic-field-line reconnection, the use of electrostatically charged tethers to pitch-angle scatter radiation-belt particles, cold plasma releases to modify magnetospheric plasma physics, and neutral-gas releases to enhance neutral-particle imaging of the magnetosphere. Technologies that are being developed to enable future space active experiments are reviewed: this includes the development of compact relativistic accelerators, superparamagnetic particle amplified antennae, CubeSats, and a new understanding of how to control dynamic spacecraft charging. New capabilities to use laboratory facilities to design space active experiments as well as new computer-simulation capabilities to design and understand space active experiments are reviewed.

Preliminary study for the laboratory experiment of cosmic-rays driven magnetic field amplification

To understand astrophysical magnetic-field amplification, we conducted a feasibility study for a laboratory experiment of a non-resonant streaming instability at the Photo Injector Test Facility at DESY, Zeuthen site (PITZ). This non-resonant streaming instability, also known as Bell’s instability, is generally regarded as a candidate for the amplification of interstellar magnetic field in the upstream region of supernova-remnant shocks, which is crucial for the efficiency of diffusive shock acceleration. In the beam-plasma system composed of a radio-frequency electron gun and a gas-discharge plasma cell, the goal of our experiment is to demonstrate the development of the non-resonant streaming instability and to find its saturation level in the laboratory environment. Since we find that the electron beam will be significantly decelerated on account of an electrostatic streaming instability, which will decrease the growth rate of desired non-resonant streaming instability, we discuss possible ways to suppress the electrostatic streaming instability by considering the characteristics of a field-emission-based quasi continuous-wave electron beam.

Derivation of Hemispheric Ionospheric Current Functions From Ground‐Level Magnetic Fields

AbstractThe horizontal currents in the high‐latitude ionosphere are the primary driver of the magnetic field perturbations that are observed at the surface of the Earth. These currents and their ground effects are an important aspect of the magnetosphere‐ionosphere coupling process. This paper discusses the method of inversion that uses spherical harmonic potential function, in which magnetic field measurements on the ground can be used to derive maps of the “ionospheric equivalent currents,” a mathematical representation of the horizontal currents flowing on a thin shell. It is shown that the use of both internal telluric and external current sources is required when fitting the spherical harmonic series; otherwise, the ionospheric currents will be overestimated. Furthermore, the inversion needs to compensate for magnetic effects of the magnetospheric ring current; otherwise, this current is projected onto the ionosphere. The amplification of the surface horizontal magnetic field and the suppression of the vertical magnetic field are demonstrated. The equivalent currents may be useful for estimating the ionospheric conductivity values. Additionally, these currents can be compared with the results from simulation models as a means of validation.

WENO–WOMBAT: Scalable Fifth-order Constrained-transport Magnetohydrodynamics for Astrophysical Applications

Due to increase in computing power, high-order Eulerian schemes will likely become instrumental for the simulations of turbulence and magnetic field amplification in astrophysical fluids in the next years. We present the implementation of a fifth order weighted essentially non-oscillatory scheme for constrained-transport magnetohydrodynamics into the code WOMBAT. We establish the correctness of our implementation with an extensive number tests. We find that the fifth order scheme performs as accurately as a common second order scheme at half the resolution. We argue that for a given solution quality the new scheme is more computationally efficient than lower order schemes in three dimensions. We also establish the performance characteristics of the solver in the WOMBAT framework. Our implementation fully vectorizes using flattened arrays in thread-local memory. It performs at about 0.6 Million zones per second per node on Intel Broadwell. We present scaling tests of the code up to 98 thousand cores on the Cray XC40 machine "Hazel Hen", with a sustained performance of about 5 percent of peak at scale.

Biermann battery effects on the turbulent dynamo in a colliding plasma jets produced by high-power lasers

The implication of the Biermann battery (BB) on turbulent magnetic field amplification in colliding plasma jets produced by high-power lasers is studied by using the FLASH code. It is found that the BB can play a significant role in turbulent field amplification. The small scale fluid structures introduced by turbulence can allow the BB to effectively amplify the magnetic field. When the flow is perpendicular to the magnetic field, the magnetic field amplification is shown to be greater than the case where the flow is parallel.

ALMA CO Observations of Supernova Remnant N63A in the Large Magellanic Cloud: Discovery of Dense Molecular Clouds Embedded within Shock-ionized and Photoionized Nebulae

Abstract We carried out new 12CO(J = 1–0, 3–2) observations of a N63A supernova remnant (SNR) from the LMC using the Atacama Large Millimeter/submillimeter Array (ALMA) and Atacama Submillimeter Telescope Experiment. We find three giant molecular clouds toward the northeast, east, and near the center of the SNR. Using the ALMA data, we spatially resolved clumpy molecular clouds embedded within the optical nebulae in both the shock-ionized and photoionized lobes discovered by previous Hα and [S ii] observations. The total mass of the molecular clouds is ∼800 M ☉ for the shock-ionized region and ∼1700 M ☉ for the photoionized region. Spatially resolved X-ray spectroscopy reveals that the absorbing column densities toward the molecular clouds are ∼(1.5–6.0) × 1021 cm−2, which are ∼1.5–15 times less than the averaged interstellar proton column densities for each region. This means that the X-rays are produced not only behind the molecular clouds, but also in front of them. We conclude that the dense molecular clouds have been completely engulfed by the shock waves, but have still survived erosion owing to their high density and short interacting time. The X-ray spectrum toward the gas clumps is well explained by an absorbed power-law model or a high-temperature plasma model, in addition to thermal plasma components, implying that the shock-cloud interaction is efficiently working for both cases through the shock ionization and magnetic field amplification. If the hadronic gamma-ray is dominant in the GeV band, the total energy of the cosmic-ray protons is calculated to be ∼(0.3–1.4) × 1049 erg, with an estimated interstellar proton density of ∼190 ± 90 cm−3, containing both the shock-ionized gas and neutral atomic hydrogen.

Bell-instability-mediated Spectral Modulation of Hadronic Gamma-Rays from a Supernova Remnant Interacting with a Molecular Cloud

Abstract Supernova remnants (SNRs) are believed to be the site of galactic cosmic-ray acceleration. However, the details of the cosmic-ray acceleration are still not well understood. Gamma-ray observation is a promising method to study cosmic-ray acceleration in the SNRs, because a hadronic gamma-ray can trace high-energy cosmic-rays above ∼GeV energy. Conventional theory predicts that the hadronic gamma-ray shows a flat νF ν spectrum from the pion-creation threshold energy to the maximum energy of diffusive shock acceleration. In this paper, by employing numerical simulations that solve a hybrid system of the magnetohydrodynamics of a molecular cloud and diffusive propagation of cosmic-rays, we demonstrate that the hadronic gamma-ray spectrum can be harder than the conventional one and that the modulated spectrum becomes consistent with observations. The modification mechanism is explained as follows: The cosmic-rays accelerated at the supernova blast wave shock propagate into a clump of a molecular cloud. The cosmic-ray streaming in the cloud induces the so-called Bell instability that induces Alfvén waves in the cloud. The induced magnetic field fluctuations prevent further cosmic-ray incursion by diminishing the diffusion coefficient for the cosmic-rays below ∼1 TeV energy. This mechanism reinforces recent claims of a similar spectral modification by magnetic field amplification around a molecular cloud by Inoue et al. and Celli et al.

On the Background-gyroresonant Character of Bell’s Instability in the Large-current Regime

Abstract We show that the Bell instability, which is widely considered potentially important for cosmic-ray (CR) acceleration, is the low-frequency limit of a gyroresonant interaction between the protons of the interstellar medium and shear-Alfvén waves. At large CR current densities, its growth rate is therefore limited by the proton gyrofrequency, and two modes emerge from the cold-beam dispersion relation. A third mode driven by electron gyroresonance is only weakly unstable at low current densities. We discuss implications for magnetic field amplification and its saturation in the vicinity of supernova remnants.

Nonlinear interfacial motion in magnetohydrodynamic flows

Nonlinear motion of vortex sheets with non-uniform current is investigated taking the magnetohydrodynamic Richtmyer–Meshkov instability (MHD RMI) and the magnetohydrodynamic Kelvin–Helmholtz instability (MHD KHI) as the examples. As the ratio of the magnetic force to the convective force increases, Alfvén oscillations appear and the nonlinear growth of the interface as a vortex sheet is suppressed. We show that the turbulent energy possessing the interface flows into the magnetic energy, which causes the strong magnetic field amplification for both instabilities. We also discuss the difference of the temporal evolution of the interface between MHD RMI and MHD KHI.

Shocks and Non-thermal Particles in Clusters of Galaxies

Galaxy clusters grow by gas accretion, mostly from mergers of substructures, which release powerful shock waves into cosmic plasmas and convert a fraction of kinetic energy into thermal energy, amplification of magnetic fields and into the acceleration of energetic particles. The modeling of the radio signature of cosmic shocks, combined with the lack of detected gamma-rays from cosmic ray (CR) protons, poses challenges to our understanding of how cosmic rays get accelerated and stored in the intracluster medium. Here we review the injection of CRs by cosmic shocks of different strengths, combining the detailed "microscopic" view of collisionless processes governing the creation of non-thermal distributions of electrons and protons in cluster shocks (based on analytic theory and particle-in-cell simulations), with the "macroscopic" view of the large-scale distribution of cosmic rays, suggested by modern cosmological simulations. We discuss time dependent non-linear kinetic models of particle acceleration by multiple internal shocks with large scale compressible motions of plasma. The models produce soft CR spectra containing a noticeable energy density in the super-thermal protons of energies below a few GeV which are difficult to constrain by Fermi observations. We consider the effect of plasma composition on CR injection and super-thermal particle population in the hot intracluster matter which can be constrained by fine high resolution X-ray spectroscopy of Fe ions.

The Origin and Early Evolution of a Bipolar Magnetic Region in the Solar Photosphere

Abstract Finding the formation mechanisms for bipolar configurations of a strong local magnetic field under control of the relatively weak global magnetic field of the Sun is a key problem of the physics of solar activity. This study is aimed at discriminating whether the magnetic field or fluid motion plays a primary, active role in this process. The very origin and early development stage of Active Region 12548 are investigated based on Solar Dynamics Observatory/Helioseismic and Magnetic Imager observations of 2016 May 20–25. Full-vector magnetic and velocity fields are analyzed in parallel. The leading and trailing magnetic polarities are found to grow asymmetrically in terms of their amplitude, magnetic flux, and the time variation of these quantities. The leading-polarity magnetic element originates as a compact feature against the background of a distributed trailing-polarity field, with an already existing trailing-polarity magnetic element. No signs of strong horizontal magnetic fields are detected between the two magnetic poles. No predominant upflow between their future locations precedes the origin of this bipolar magnetic region (BMR); instead, upflows and downflows are mixed, with some prevalence of downflows. Any signs of a large-scale horizontal divergent flow from the area where the BMR develops are missing; in contrast, a normal supergranulation and mesogranulation pattern is preserved. This scenario of early BMR evolution is in strong contradiction with the expectations based on the model of a rising Ω-shaped loop of a flux tube of strong magnetic field, and an in situ mechanism of magnetic-field amplification and structuring should operate in this case.

Proton Acceleration in Colliding Stellar Wind Binaries

Abstract The interaction between the strong winds in stellar colliding-wind binary (CWB) systems produces two shock fronts, delimiting the wind-collision region (WCR). There, particles are expected to be accelerated mainly via diffusive shock acceleration. We investigate the injection and acceleration of protons in typical CWB systems by means of Monte Carlo simulations, with both a test-particle approach and a nonlinear method modeling a shock locally modified by the backreaction of the accelerated protons. We use magnetohydrodynamic simulations to determine the background plasma in the WCR and its vicinity. This allows us to consider particle acceleration at both shocks, on either side of the WCR, with a realistic large-scale magnetic field. We highlight the possible effects of particle acceleration on the local shock profiles at the WCR. We include the effect of magnetic field amplification, due to resonant-streaming instability, and compare results without and with the backreaction of the accelerated protons. In the latter case, we find a lower flux of the nonthermal proton population and a considerable magnetic field amplification. This would significantly increase the synchrotron losses of relativistic electrons accelerated in CWB systems, lowering the maximal energy they can reach and strongly reducing the inverse Compton fluxes. As a result, γ-rays from CWBs would be predominantly due to the decay of neutral pions produced in nucleon–nucleon collisions. This might provide a way to explain why, in the vast majority of cases, CWB systems have not been identified as γ-ray sources, although they emit synchrotron radiation.

The Short GRB 170817A: Modeling the Off-axis Emission and Implications on the Ejecta Magnetization

Abstract The short GRB 170817A, detected by the Fermi Gamma-ray Burst Monitor, orbiting satellites and ground-based telescopes, was the electromagnetic counterpart of a gravitational-wave transient (GW170817) from a binary neutron star merger. After this merger, the γ-ray light curve exhibited a faint peak at ∼1.7 s and the X-ray, optical, and radio light curves displayed extended emission that increased in brightness up to ∼160 days. In this paper, we show that the X-ray, optical and radio fluxes are consistent with the synchrotron forward-shock model viewed off-axis when the matter in the outflow is parameterized through a power-law velocity distribution. We discuss the origin of the γ-ray peak in terms of internal and external shocks. We show that the γ-ray flux might be consistent with a synchrotron self-Compton reverse-shock model observed at high latitudes. Comparing the best-fit values obtained after describing the γ-ray, X-ray, optical, and radio fluxes with our model, we find that the afterglow and γ-ray emission occurred in different regions and also find evidence to propose that the progenitor environment was entrained with magnetic fields; therefore we argue for the presence of magnetic field amplification in the binary neutron star merger.

On the maximum energy of protons in the hotspots of AGN jets

We study particle acceleration and magnetic field amplification in the termination shocks (hotspots) of radiogalaxy jets. The cut-off of the synchrotron spectrum in the hotspots of powerful radiogalaxies is typically observed between infrared and optical frequencies, indicating that the maximum energy of non-thermal electrons accelerated at the jet termination shock is about 1 TeV for a canonical magnetic field of 100 μG. Based on theoretical considerations and observational data we show that the maximum energy of electrons cannot be constrained by synchrotron losses as usually assumed, unless the jet density is unreasonable large and most of the jet kinetic energy goes to non-thermal electrons. The maximum energy is ultimately determined by the ability to scatter particles back and forth the shock, and this limit applies to both electrons and protons. Therefore, the maximum energy of protons is also about 1 TeV when radiative cooling is not efficient. We show that non-resonant hybrid (Bell) instabilities generated by the streaming of cosmic rays can grow fast enough to amplify the jet magnetic field up to 100 μG and accelerate particles up to the maximum energies observed in the hotspots of radiogalaxies.

Self-Organized Kilotesla Magnetic-Tube Array in an Expanding Spherical Plasma Irradiated by kHz Femtosecond Laser Pulses.

By using a millijoule kHz femtosecond laser pulse to irradiate a preformed expanding spherical plasma, which is driven by a prepulse with intensity of 1×10^{14} W/cm^{2}, we observe fast-electron-mediated filamentary structures and an accompanying self-organized magnetic-tube array with 2000 T via time-resolved magneto-optical polarization rotation measurements. We reveal that these periodical filamentary structures predominantly originate from ejected energetic electron flows from the inner denser region of the spherical plasma, which will induce the electron Weibel instability and magnetic field organization and amplification in the expanding plasma in 2ps. These results open new paths to investigate amplification of intense magnetic fields and the radiation signature from gamma-ray bursts just by means of a much smaller and robust experimental platform.

ON THE INFLUENCE OF MHD TURBULENCE ON THE STRUCTURE OF THE RADIOGALAXY LOBES

There is considered the evolution of the shape for the radio galaxy lobes of FRI and FRII types from the point of view on changing the configuration of large-scale structure of magnetic field, and energy transport in the turbulent MHD waves. There have been studied the interaction and transformation of waves in the active regions of the lobes, and so studied the role of MHD waves and vortexes in media-mixing processes and in the amplification of the average magnetic field. The transport of low-energy e-CRs responsible for the radio emission in the MHz band (recorded at the UTR-2 and GURT telescopes) is analyzed for the sources like to the Cygnus A and M87. It is shown that the transport of e-CRs mainly corresponds to the diffusion of CRs on MHD and turbulence scatter, and the entrainment of CRs by quasi-regular post-jet flows inside to the lobe. So, the MHz radio emission that observed emphasizes the peculiarities in the lobe which arising when the magnetic field is in reorganization.