Magnetic Field Component Parallel Research Articles

On the Origin of the Photospheric Magnetic Field

This article presents results that challenge the paradigms that (1) the convection zone is the source of the radial magnetic field in the photosphere and (2) that coronal currents are neutralized from the perspective of the photosphere. We demonstrate, using a new analysis tool applied to simulations and observations, that bare or partially dressed current channels are supported by the solar corona and that fingerprints of these coronal current systems can be detected in the photosphere. These coronal current channels can be a significant source of the radial component of the magnetic field in the photosphere. The roots of these coronal current channels in the photosphere are the source of the magnetic field component parallel to the polarity inversion line in active region NOAA 12673. These analyses and observations transform our theoretical understanding of coronal evolution and argue for a reexamination of the present paradigm in which the convection zone is the sole source of the photospheric magnetic field.

A method for reconstructing the Galactic magnetic field using dispersion of fast radio bursts and Faraday rotation of radio galaxies

ABSTRACT With the rapid increase of fast radio burst (FRB) detections within the past few years, there is now a catalogue being developed for all-sky extragalactic dispersion measure (DM) observations in addition to the existing collection of all-sky extragalactic Faraday rotation measurements (RMs) of radio galaxies. We present a method of reconstructing all-sky information of the Galactic magnetic field component parallel to the line of sight, B∥, using simulated observations of the RM and DM along lines of sight to radio galaxies and FRB populations, respectively. This technique is capable of distinguishing between different input Galactic magnetic fields and thermal electron density models. Significant extragalactic contributions to the DM are the predominant impediment in accurately reconstructing the Galactic DM and 〈B∥〉 skies. We look at ways to improve the reconstruction by applying a filtering algorithm on the simulated DM lines of sight and we derive generalized corrections for DM observations at |b| > 10° that help to disentangle Galactic and extragalactic DM contributions. Overall, we are able to reconstruct both large-scale Galactic structure and local features in the Milky Way’s magnetic field from the assumed models. We discuss the application of this technique to future FRB observations and address possible differences between our simulated model and observed data, namely: adjusting the priors of the inference model, an unevenly distributed population of FRBs on the sky, and localized extragalactic DM structures.

Turbulent magnetic field in the H II region Sh 2–27

Context. Magnetic fields in the turbulent interstellar medium (ISM) are a key element in understanding Galactic dynamics, but there are many observational challenges. One useful probe for studying the magnetic field component parallel to the line of sight (LoS) is Faraday rotation of linearly polarized radio synchrotron emission, combined with Hα observations. H ii regions are the perfect laboratories to probe such magnetic fields as they are localized in space, and are well-defined sources often with known distances and measurable electron densities. We chose the H ii region Sharpless 2–27 (Sh 2–27) as it is located at intermediate latitudes (b ~ 23°), meaning that it suffers from little LoS confusion from other sources. In addition, it has a large angular diameter (~10°), enabling us to study the properties of its magnetic field over a wide range of angular scales. Aims. By using a map of the magnetic field strength along the LoS (B‖)for the first time, we investigate the basic statistical properties of the turbulent magnetic field inside Sh 2–27. We study the scaling of the magnetic field fluctuations, compare it to the Kolmogorov scaling, and attempt to find an outer scale of the turbulent magnetic field fluctuations. Methods. We used the polarized radio synchrotron emission data from the S-band Polarization All-Sky Survey (S-PASS) at 2.3 GHz, which allowed us to test the impact of Sh 2–27 on diffuse Galactic synchrotron polarization. We estimated the rotation measure (RM) caused by the H ii region, using the synchrotron polarization angle. We used the Hα data from the Southern Hα Sky Survey Atlas to estimate the free electron density (ne) in the H ii region. Using an ellipsoid model for the shape of Sh 2–27, and with the observed RM and emission measure (EM), we estimated the LoS averaged B‖for each LoS within the ellipsoid. To characterize the turbulent magnetic field fluctuations, we computed a second-order structure function of B‖ We compared the structure function to Kolmogorov turbulence, and to simulations of Gaussian random fields processed in the same way as the observations. Results. We present the first continuous map of B‖ computed using the diffuse polarized radio emission in Sh 2–27. We estimate the median value of ne as 7.3 ± 0.1 cm−3, and the median value of B‖ as −4.5 ± 0.1 µG, which is comparable to the magnetic field strength in diffuse ISM. The slope of the structure function of the estimated B‖-map is found to be slightly steeper than Kolmogorov, consistent with our Gaussian-random-field B‖simulations revealing that an input Kolmogorov slope in the magnetic field results in a somewhat steeper slope in B‖.These results suggest that the lower limit to the outer scale of turbulence is 10 pc in the H ii region, which is comparable to the size of the computation domain. Conclusions. The structure functions of B‖ fluctuations in Sh 2–27 show that the magnetic field fluctuations in this H ii region are consistent with a Kolmogorov-like turbulence. Comparing the observed and simulated B‖ structure functions results in the estimation of a lower limit to the outer scale of the turbulent magnetic field fluctuations of 10 pc, which is limited by the size of the field of view under study. This may indicate that the turbulence probed here could actually be cascading from the larger scales in the ambient medium, associated with the interstellar turbulence in the general ISM, which is illuminated by the presence of Sh 2–27.

Magnetic fields in the Milky Way from pulsar observations: effect of the correlation between thermal electrons and magnetic fields

ABSTRACTPulsars can act as an excellent probe of the Milky Way magnetic field. The average strength of the Galactic magnetic field component parallel to the line of sight can be estimated as $\langle B_\parallel \rangle = 1.232 \, \text{RM}/\text{DM}$, where RM and DM are the rotation and dispersion measure of the pulsar. However, this assumes that the thermal electron density and magnetic field of the interstellar medium are uncorrelated. Using numerical simulations and observations, we test the validity of this assumption. Based on magnetohydrodynamical simulations of driven turbulence, we show that the correlation between the thermal electron density and the small-scale magnetic field increases with increasing Mach number of the turbulence. We find that the assumption of uncorrelated thermal electron density and magnetic fields is valid only for subsonic and trans-sonic flows, but for supersonic turbulence, the field strength can be severely overestimated by using $1.232 \, \text{RM}/\text{DM}$. We then correlate existing pulsar observations from the Australia Telescope National Facility with regions of enhanced thermal electron density and magnetic fields probed by 12CO data of molecular clouds, magnetic fields from the Zeeman splitting of the 21 cm line, neutral hydrogen column density, and H α observations. Using these observational data, we show that the thermal electron density and magnetic fields are largely uncorrelated over kpc scales. Thus, we conclude that the relation $\langle B_\parallel \rangle = 1.232 \, \text{RM}/\text{DM}$ provides a good estimate of the magnetic field on Galactic scales, but might break down on sub-kpc scales.

Majorana-like localized spin density without bound states in topologically trivial spin-orbit coupled nanowires

In the topological phase of spin-orbit coupled nanowires Majorana bound states are known to localize at the nanowire edges and to exhibit a spin density orthogonal to both the magnetic field and the spin-orbit field. By investigating a nanowire exposed to a uniform magnetic field with an interface between regions with different spin-orbit couplings, we find that the orthogonal spin density is pinned at the interface even when both interface sides are in the topologically trivial phase, and even when no bound state is present at all. A trivial bound state may additionally appear at the interface, especially if the spin-orbit coupling takes opposite signs across the interface. However, it can be destroyed by a smoothening of the spin-orbit profile or by a magnetic field component parallel to the spin-orbit field. In contrast, the orthogonal spin density persists in various and realistic parameter ranges. We also show that, while the measurement of bulk equilibrium spin currents has been elusive so far, such robust orthogonal spin density peak may provide a way to detect spin current variations across interfaces.

Shubnikov—de Haas Oscillations in a Three-Dimensional Topological Insulator Based on a Strained HgTe Film in an Inclined Magnetic Field

Specific features in the formation of Landau levels in a three-dimensional topological insulator based on an 80-nm-thick strained mercury telluride film in an inclined magnetic field are studied. The magnetoresistance in the Hall bar geometry with a gate is measured at a temperature 1.9 K and in an applied magnetic field up to 10 T. The gate allows varying the Fermi level position from the valence to conduction band, passing through the bulk band gap. The samples are mounted on a rotating platform that makes it possible to change arbitrarily the angle between the magnetic field direction and the normal to the sample plane within the range of 0°–90°. It is found that the Shubnikov–de Haas oscillations are formed if the perpendicular magnetic field component exceeds 0.4 T, independent of the applied gate voltage. However, the sensitivity of the system to the parallel magnetic field component exhibits a pronounced dependence on the applied gate voltage. Namely, if the Fermi level is in the bulk band gap and the conductivity is determined by the surface states, the amplitude and position of Shubnikov–de Haas oscillations remain independent of the perpendicular magnetic field component even in the situation where the parallel component is twice as large as the perpendicular one. At high magnetic fields, the amplitude of oscillations is suppressed by the parallel field component. On the contrary, if the Fermi level is in the valence or conduction band, the parallel field component not only affects the amplitude of oscillations but also results even in their qualitative changes, e.g., giving rise to new minima related to Zeeman splitting. The observed behavior of the system is in agreement with the modern concepts concerning the spin polarization of the surface state in three-dimensional topological insulators and the spin degeneracy of charge carriers in the bulk.

Spectral Properties of Magnetohydrodynamic Turbulence Revealed by Polarization Synchrotron Emission with Faraday Rotation

Abstract We investigate how to recover the spectral properties of underlying magnetohydrodynamic (MHD) turbulence using fluctuation statistics of synchrotron polarization radiation, based on the synthetic observations. Taking spatially coincident, separated, and compounded synchrotron emission and Faraday rotation regions into account, we extract the power spectrum of synchrotron polarization intensities integrated along the line of sight. Our results demonstrate that in the short wavelength range, the power spectra reflect fluctuation statistics of the perpendicular component of turbulent magnetic fields, and the spectra at long wavelengths reveal the fluctuation of the Faraday rotation density, which is a product of the parallel component of magnetic field and thermal electron density. We find that our numerical results (in the case of spatially coincident regions) are in agreement with the analytical prediction in Lazarian & Pogosyan, and this theoretical prediction is applicable to more complicated settings, i.e., the spatially separated and compounded regions. We simulate telescopic observations that incorporate the effects of telescope angular resolution and noise, and find that statistics of underlying MHD turbulence can be recovered successfully. We expect that the technique can be applied to a variety of astrophysical environments, with existing synchrotron data cubes and a large number of forthcoming data sets from such as the LOw Frequency Array for Radio astronomy, the Square Kilometer Array, and the Five-hundred-meter Aperture Spherical radio Telescope.

New perspectives on the supernova remnant Puppis A based on a radio polarization study

We present a polarization study towards the supernova remnant (SNR) Puppis A based on original observations performed with the Australia Telescope Compact Array (ATCA). Based on the analysis of a feature detected outside the SNR shell (called `the tail' throughout the paper), it was possible to disentangle the emission with origin in Puppis A itself from that coming from the foreground Vela SNR. We found a very low polarization fraction, of about 3 percent on average. The upper limit of the magnetic field component parallel to the line of sight is estimated to be B$_\parallel \sim$ 20 $\mu$G. The statistical behavior of the magnetic vectors shows two preferential directions, almost perpendicular to each other, which are approximately aligned with the flat edges of Puppis A. A third, narrow peak oriented perpendicular to the Galactic plane suggests the existence of an interstellar magnetic field locally aligned in this direction. There is evidence that the magnetic vectors along the shell are aligned with the shock front direction. The low polarization fraction and the statistical behavior of the magnetic vectors are compatible with a scenario where the SNR evolves inside a stellar wind bubble with a box-like morphology, produced by the interaction of the different stellar winds, one of them magnetized, launched by the SN progenitor. This scenario can furthermore explain the morphology of Puppis A, rendering little support to the previously accepted picture which involved strong density gradients to explain the flat, eastern edge of the shell.

Control of vacuum arc source cathode spots contraction motion by changing electromagnetic field

This paper investigates the magnetic field component impact on cathode spots motion trajectory and the mechanism of periodic contraction. Electromagnetic coils and permanent magnets were installed at the different sides of cathode surface, the photographs of cathode spots motion trajectory were captured by a camera. Increasing the number of magnets and decreasing the distance between magnets and cathode both lead to enhancing cathode spots motion velocity. Radii of cathode spots trajectory decrease gradually with the increasing of electromagnetic coil’s current, from 40 mm at 0 A to 10 mm at 2.7 A. Parallel magnetic field component intensity influence the speed of cathode spots rotate motion, and perpendicular magnetic field component drives spots drift in the radial direction. Cathode spot’s radial drift is controlled by changing the location of the ‘zero line’ where perpendicular magnetic component shifts direction and the radius of cathode spots trajectory almost equal to ‘zero line’.

Identification of waves in the lower-hybrid frequency range in the scrape-off layer plasma of Alcator C-Mod

Polarization resolved measurements of the parallel refractive index of the driven RF waves in the lower hybrid (LH) range of frequencies are performed using arrays of RF magnetic probes in the scrape-off layer plasma of Alcator C-Mod. The measured of the RF magnetic field component parallel to the background magnetic field is about −1.6, which corresponds to the peak of the launched LH spectrum. Based on the wave dispersion relationship, this wave is identified as the LH slow wave. On the other hand, the RF magnetic field component perpendicular to the magnetic field is found to have a lower of −1.2, and is detected only near the last closed flux surface. This wave is identified as the LH fast wave generated by slow-fast wave mode conversion.

Estimate of an environmental magnetic field of fast radio bursts

Fast radio bursts (FRBs) are a type of newly-discovered transient astronomical phenomenon. They have short durations, high dispersion measures and a high event rate. However, due to unknown distances and undetected electromagnetic counterparts at other wavebands, it is difficult to further investigate FRBs. Here we propose a method to study their environmental magnetic field using an indirect method. Starting with dispersion measures and rotation measures (RMs), we try to obtain the parallel magnetic field component B̅‖ which is the average value along the line of sight in the host galaxy. Because both RMs and redshifts are now unavailable, we demonstrate the dependence of B̅‖ on these two separate quantities. This result, if the RM and redshift of an FRB are measured, would be expected to provide a clue towards understanding an environmental magnetic field of an FRB.

Quantum oscillations in Weyl and Dirac semimetal ultrathin films

We show that a thin film of Weyl or Dirac semimetal with a strong in-plane magnetic field becomes a novel two-dimensional Fermi liquid with interesting properties. The Fermi surface in this system is strongly anisotropic, which originates from a combination of chiral bulk channels and the Fermi arcs. The area enclosed by the Fermi surface depends strongly on the in-plane magnetic field component parallel to the Weyl/Dirac node splitting, which leads to unusual behavior in quantum oscillations when the magnetic field is tilted out of the plane. We estimate the oscillation frequencies and the regimes where such effects could be seen in Cd$_3$As$_2$, Na$_3$Bi, and TaAs.

Correlations of global sea surface temperatures with the solar wind speed

A significant correlation between the solar wind speed (SWS) and sea surface temperature (SST) in the region of the North Atlantic Ocean has been found for the Northern Hemisphere winter from 1963 to 2010, based on 3-month seasonal averages. The correlation is dependent on Bz (the interplanetary magnetic field component parallel to the Earth's magnetic dipole) as well as the SWS, and somewhat stronger in the stratospheric quasi-biennial oscillation (QBO) west phase than in the east phase. The correlations with the SWS are stronger than those with the F10.7 parameter representing solar UV inputs to the stratosphere. SST responds to changes in tropospheric dynamics via wind stress, and to changes in cloud cover affecting the radiative balance. Suggested mechanisms for the solar influence on SST include changes in atmospheric ionization and cloud microphysics affecting cloud cover, storm invigoration, and tropospheric dynamics. Such changes modify upward wave propagation to the stratosphere, affecting the dynamics of the polar vortex. Also, direct solar inputs, including energetic particles and solar UV, produce stratospheric dynamical changes. Downward propagation of stratospheric dynamical changes eventually further perturbs tropospheric dynamics and SST.

Investigation of the magnetic dipole field at the atomic scale in quasi-one-dimensional paramagnetic conductor Li0.9Mo6O17

We report magnetic dipole field investigation at the atomic scale in a single crystal of quasi-one-dimensional (Q1D) paramagnetic conductor Li0.9Mo6O17, using a paramagnetic electron model and 7Li-NMR spectroscopy measurements with an externally applied magnetic field B0 = 9 T. We find that the magnetic dipole field component () parallel to B0 at the Li site from the Mo electrons has no lattice axial symmetry; it is small around the middle between the lattice a and c axes in the ac-plane with the minimum at the field orientation angle , while the maximum is at when B0 is applied perpendicular to b (), where represents the direction of . Further estimation indicates that has a maximum value of 0.35 G at B0 = 9 T. By minimizing the potential magnetic contributions to the NMR spectra satellites with the NMR spectroscopy measurements at the direction where the value of the magnetic dipole field component is ∼0, the behavior of the electron charge statics is exhibited. This work demonstrates that the magnetic dipole field of the Mo electrons is the dominant source of the local magnetic fields at the Li site, and suggests that the unknown metal-‘insulator’ crossover at low temperatures is not a charge effect. The work also reveals valuable local electric and magnetic field information for further NMR investigation as recently suggested (2012 Phys. Rev. B 85 235128) regarding the unusual properties of the material.

Evolution of Kelvin-Helmholtz instability at Venus in the presence of the parallel magnetic field

Two-dimensional MHD simulations were performed to study the evolution of the Kelvin-Helmholtz (KH) instability at the Venusian ionopause in response to the strong flow shear in presence of the in-plane magnetic field parallel to the flow direction. The physical behavior of the KH instability as well as the triggering and occurrence conditions for highly rolled-up vortices are characterized through several physical parameters, including Alfvén Mach number on the upper side of the layer, the density ratio, and the ratio of parallel magnetic fields between two sides of the layer. Using these parameters, the simulations show that both the high density ratio and the parallel magnetic field component across the boundary layer play a role of stabilizing the instability. In the high density ratio case, the amount of total magnetic energy in the final quasi-steady status is much more than that in the initial status, which is clearly different from the case with low density ratio. We particularly investigate the nonlinear development of the case that has a high density ratio and uniform magnetic field. Before the instability saturation, a single magnetic island is formed and evolves into two quasi-steady islands in the non-linear phase. A quasi-steady pattern eventually forms and is embedded within a uniform magnetic field and a broadened boundary layer. The estimation of loss rates of ions from Venus indicates that the stabilizing effect of the parallel magnetic field component on the KH instability becomes strong in the case of high density ratio.

The Alfvén mode gyrokinetic equation in finite‐pressure magnetospheric plasma

AbstractThe paper is concerned with the derivation of the Alfvén mode equation in finite‐pressure space plasma in gyrokinetic approach. The long plasma approximation is used, where the bounce frequency is much lower than both wave and drift frequencies. The only ultralow frequency mode in the long plasma approximation is the Alfvén‐ballooning compressional mode, which is described by the Alfvénic dispersion relation with some additional (ballooning) terms caused by the field line curvature and plasma pressure effects. Due to these effects the Alfvén mode acquires also considerable parallel magnetic field component. The long plasma approximation allowed us to consider the correspondence between the gyrokinetic and MHD approaches for the Alfvén mode equation. It is shown that in 0 < β ≪ 1 case, MHD approach gives correct Alfvén wave description, where β is plasma to magnetic pressure ratio.

Using Excitation Invariance in the Characterization of Defects by Eddy Current Image Constructions

In this article the eddy current method is used to characterize defects in conductive non-ferromagnetic plates. A special attention is paid to the presentation of results in the form of images. An inverse problem with Tikhonov regularization is solved to obtain the two-dimensional distribution of currents inside the material under test. With this purpose a uniform field probe is used to guarantee a spatially invariant excitation field for probe scanning translations. The probe contains a magnetic field detector based on high sensitivity giant magneto-resistors which reads the magnetic field component parallel to the excitation and induction currents.

2D and 3D homogenization of laminated cores in the frequency domain

This paper presents 2D and 3D homogenization techniques for laminated iron cores in the frequency domain. In 2D, two approaches (analytical and numerical) are compared in terms of complex reluctivity considering tangential magnetic field to laminations. The analytical approach is generalized to 3D problems. The validation performed on homogenized block gives satisfactory results only in case of closed-core or thin air gap open-core. Thus, we have a correct approximation only when the magnetic field is essentially parallel to laminations. In order to remediate the poor approximation of losses due to non parallel magnetic field component and model large air gap devices, we propose an alternative to block homogenization. A validation is done on a laminated open-core inductance.

A study of the starless dark cloud LDN 1570: Distance, dust properties, and magnetic field geometry

We wish to map the magnetic field geometry and to study the dust properties of the starless cloud, L1570, using multi-wavelength optical polarimetry and photometry of the stars projected on the cloud. We made R-band imaging polarimetry of the stars projected on a cloud, L1570, to trace the magnetic field orientation. We also made multi-wavelength polarimetric and photometric observations to constrain the properties of dust in L1570. We estimated a distance of 394 +/- 70 pc to the cloud using 2MASS JHKs colours. Using the values of the Serkowski parameters namely $\sigma_{1}$, $\bar \epsilon$, {\lambda}max and the position of the stars on near infrared color-color diagram, we identified 13 stars that could possibly have intrinsic polarization and/or rotation in their polarization angles. One star, 2MASS J06075075+1934177, which is a B4Ve spectral type, show the presence of diffuse interstellar bands in the spectrum apart from showing H{\alpha} line in emission. There is an indication for the presence of slightly bigger dust grains towards L1570 on the basis of the dust grain size-indicators such as {\lambda}max and Rv values. The magnetic field lines are found to be parallel to the cloud structures seen in the 250{\mu}m images (also in 8{\mu}m and 12{\mu}m shadow images) of L1570. Based on the magnetic field geometry, the cloud structure and the complex velocity structure, we believe that L1570 is in the process of formation due to the converging flow material mediated by the magnetic field lines. Structure function analysis showed that in the L1570 cloud region the large scale magnetic fields are stronger when compared with the turbulent component of magnetic fields. The estimated magnetic field strengths suggest that the L1570 cloud region is sub-critical and hence could be strongly supported by the magnetic field lines.

Formation of a transient front structure near reconnection point in 3‐D PIC simulations

Massively parallel numerical simulations of magnetic reconnection are presented in this study. Electromagnetic full‐particle implicit code iPIC3D is used to study the dynamics and 3‐D evolution of reconnection outflows. Such features as Hall magnetic field, inflow and outflow, and diffusion region formation are very similar to 2‐D particle‐in‐cell (PIC) simulations. In addition, it is well known that instabilities develop in the current flow direction or oblique directions. These modes could provide for anomalous resistivity and diffusive drag and can serve as additional proxies for magnetic reconnection. In our work, the unstable evolution of reconnection transient front structures is studied. Reconnection configuration in the absence of guide field is considered, and it is initialized with a localized perturbation aligned in the cross‐tail direction. Our study suggests that the instabilities lead to the development of finger‐like density structures on ion‐electron hybrid scales. These structures are characterized by a rapid increase of the magnetic field, normal to the current sheet (Bz). A small decrease in the magnetic field component parallel to the reconnection X line and the component perpendicular to the current sheet is observed in the region ahead of the front. The instabilities form due to fact that the density gradient inside the front region is opposite to the direction of the acceleration Lorentz force. Such density structures may possibly further develop into larger‐scale earthward flux transfer events during magnetotail reconnection. In addition, oscillations mainly in the magnetic and electric fields and the electron density are observed shortly before the arrival of the main front structure which is consistent with recent THEMIS observations.