Multipolar Magnetic Field Research Articles

Possible Phase-dependent Absorption Feature in the X-Ray Spectrum of the Middle-aged PSR J0659+1414

Abstract We report on the energy-resolved timing and phase-resolved spectral analysis of X-ray emission from PSR J0659+1414 observed with XMM-Newton and NuSTAR. We find that the new data rule out the previously suggested model of the phase-dependent spectrum as a three-component (two blackbodies + power law) continuum, which shows large residuals between 0.3 and 0.7 keV. Fitting neutron star atmosphere models or several blackbodies to the spectrum does not provide a better description of the spectrum and requires spectral model components with unrealistically large emission region sizes. The fits improve significantly if we add a phase-dependent absorption feature with central energy 0.5–0.6 keV and equivalent width up to ≈50 eV. We detected the feature for about half of the pulse cycle. Energy-resolved pulse profiles support the description of the spectrum with a three-component continuum and an absorption component. The absorption feature could be interpreted as an electron cyclotron line originating in the pulsar magnetosphere and broadened by the nonuniformity of the magnetic field along the line of sight. The significant phase variability in the thermal emission from the entire stellar surface may indicate multipolar magnetic fields and a nonuniform temperature distribution. The strongly pulsed nonthermal spectral component detected with NuSTAR in the 3–20 keV range is well fit by a power-law model with a photon index Γ = 1.5 ± 0.2.

Experimental observation of drift wave turbulence in an inhomogeneous six-pole cusp magnetic field of MPD

This paper presents a detailed study on the controlled experimental observation of drift wave instabilities in an inhomogeneous six pole cusp magnetic field generated by an in-house developed multi-pole line cusp magnetic field device [Patel et al., Rev. Sci. Instrum. 89, 043510 (2018)]. The device is composed of six axially symmetric cusps and non-cusp (in between two consecutive magnets) regions. The observed instability has been investigated in one of these non-cusp regions by controlling the radial plasma density gradient with changing pole magnetic field which is a unique feature of this device. It has been observed that the frequency of the instability changes explicitly with the density gradient. Moreover, the scale lengths of plasma parameters, frequency spectrum, cross-correlation function, and fluctuation level of plasma densities have been measured in order to identify the instability. The cross field drift velocity due to fluctuation in plasma parameters has been measured from the wave number-frequency S (kz, ω) spectrum and verified with the theoretical values obtained from density scale length formula. Further, from the S (kz, ω) spectrum, it has been found that the drift velocity alternates the sign in the consecutive non-cusp regions.

Electrical conductivity and magnetic dynamos in magma oceans of Super-Earths

Super-Earths are extremely common among the numerous exoplanets that have been discovered. The high pressures and temperatures in their interiors are likely to lead to long-lived magma oceans. If their electrical conductivity is sufficiently high, the mantles of Super-Earth would generate their own magnetic fields. With ab initio simulations, we show that upon melting, the behavior of typical mantle silicates changes from semi-conducting to semi-metallic. The electrical conductivity increases and the optical properties are substantially modified. Melting could thus be detected with high-precision reflectivity measurements during the short time scales of shock experiments. We estimate the electrical conductivity of mantle silicates to be of the order of 100 Ω−1 cm−1, which implies that a magnetic dynamo process would develop in the magma oceans of Super-Earths if their convective velocities have typical values of 1 mm/s or higher. We predict exoplanets with rotation periods longer than 2 days to have multipolar magnetic fields.

An improved algorithm for crossing curved light surfaces: rapidly rotating pulsar magnetospheres in curved space–time

The structure of force-free, steady and axisymmetric magnetosphere of a neutron star (NS) is governed by the Grad-Shafranov (GS) equation, which is a second-order differential equation but degrades to first-order on the light surface (LS). The key to numerically solving the GS equation is to enable magnetic field lines smoothly cross the LS, and crossing a straight LS in flat spacetime has been a well-studied problem. But the numerical algorithm implementation becomes complicate in the presence of a bent LS, e.g. in curved spacetime, since there is no suitable computation grid adapted to it. We propose to circumvent this grid-LS mismatch problem by introducing a new coordinate frame designed such that the LS in it is a straight line. As an application, we investigate the general relativistic (GR) effect in magnetosphere structure of rapidly rotating pulsars in detail, where the LS is bent towards the central NS. We split the GR effect into two parts, curvature and frame-dragging; measure each of them and examine their dependence on the NS mass and the angular velocity for pulsars embedded in aligned dipole and multipole magnetic fields. Qualitatively speaking, we find that the curvature effect compactifies the magnetic field lines near the NS, therefore reduces the open magnetic flux and the Poynting luminosity, while the frame-dragging effect contributes a minor part in shaping the magnetosphere structure but plays a role in enhancing the spacelike current generation.

Magnetar Giant Flares in Multipolar Magnetic Fields. III. Multipolar Magnetic Field Structure Variations

Abstract We have analyzed the multipolar magnetic field structure variation at neutron star surface by means of the catastrophic eruption model and find that the variation of the geometry of multipolar fields on the magnetar surface could result in the catastrophic rearrangement of the magnetosphere, which provides certain physical mechanism for the outburst of giant flares. The magnetospheric model we adopted consists of two assumptions: (1) a helically twisted flux rope is suspended in an ideal force-free magnetosphere around the magnetar, and (2) a current sheet emerges during the flux rope evolution. Magnetic energy accumulates during the flux rope’s gradual evolution along with the variation of magnetar surface magnetic structure before the eruption. The two typical behaviors, either state transition or catastrophic escape, would take place once the flux rope loses equilibrium; thus, tremendous accumulated energy is radiated. We have investigated the equilibrium state of the flux rope and the energy release affected by different multipolar structures and find structures that could trigger violent eruption and provide the radiation approximately 0.5% of the total magnetic energy during the giant flare outburst. Our results provide certain multipolar structures of the neutron star’s magnetic field with an energy release percentage 0.42% in the state transition and 0.51% in the catastrophic escape case, which are sufficient for the previously reported energy release from SGR 1806–20 giant flares.

Could the low‐braking‐index pulsar PSR J1734–3333 evolve into a magnetar?

The low braking index pulsar PSR J1734−3333 could be born with a superhigh internal magnetic field of 1014–1016 G, and undergo a supercritical accretion soon after its formation in a supernova explosion. The buried multipole magnetic fields will merger into a dipole magnetic field. Since the magnetic flow transfers from the core to the crust of the pulsar, its surface dipole field grows quickly at a power‐law form assumed until it saturates at the level of internal dipole field. The increase in surface dipole magnetic field results in the small braking index of n = 0.9. Keeping the current field‐growth index ϵ = 1.34, this pulsar will become a magnetar with surface dipole magnetic field at the equator Bd ∼ 2.6(1) × 1014 G and ∼5.3(2) × 1014 G after next 50 kyrs and 100 kys, respectively.

The complex magnetic field topology of the cool Ap star 49 Cam

49 Cam is a cool magnetic chemically peculiar star which has been noted for showing strong, complex Zeeman linear polarisation signatures. This paper describes magnetic and chemical surface maps obtained for 49 Cam using the INVERS10 magnetic Doppler imaging code and high-resolution spectropolarimetric data in all four Stokes parameters collected with the ESPaDOnS and Narval spectropolarimeters at the Canada-France-Hawaii Telescope and Pic du Midi Observatory. The reconstructed magnetic field maps of 49 Cam show a relatively complex structure. Describing the magnetic field topology in terms of spherical harmonics, we find significant contributions of modes up to l=3, including toroidal components. Observations cannot be reproduced using a simple low-order multipolar magnetic field structure. 49 Cam exhibits a level of field complexity that has not been seen in magnetic maps of other cool Ap stars. Hence we concluded that relatively complex magnetic fields are observed in Ap stars at both low and high effective temperatures. In addition to mapping the magnetic field, we also derive surface abundance distributions of nine chemical elements, including Ca, Sc, Ti, Cr, Fe, Ce, Pr, Nd, Eu. Comparing these abundance maps with the reconstructed magnetic field geometry, we find no clear relationship of the abundance distributions with the magnetic field for some elements. However, for other elements some distinct patterns are found. We discuss these results in the context of other recent magnetic mapping studies and theoretical predictions of radiative diffusion.

Was the Devonian geomagnetic field dipolar or multipolar? Palaeointensity studies of Devonian igneous rocks from the Minusa Basin (Siberia) and the Kola Peninsula dykes, Russia

Abstract Defining variations in the behaviour of the geomagnetic field through geological time is critical to understanding the dynamics of Earth's core and its response to mantle convection and planetary evolution. Furthermore, the question of whether the axial dipole dominance of the recent palaeomagnetic field persists through the whole of Earth's history is fundamental to determining the reliability of palaeogeographic reconstructions and the efficacy of the magnetosphere in shielding Earth from solar wind radiation. Previous palaeomagnetic directional studies have suggested that the palaeofield had a complex configuration in the Devonian period (419–359 Ma). Here we present new high-quality palaeointensity determinations from rocks aged between 408 and 375 Ma from the Minusa Basin (southern Siberia), and the Kola Peninsula that enable the first reliable investigation of the strength of the field during this enigmatic period. Palaeointensity experiments were performed using the thermal Thellier, microwave Thellier and Wilson methods on 165 specimens from 25 sites. Six out of eight successful sites from the Minusa Basin and all four successful sites from the Kola Peninsula produced extremely low palaeointensities (<10 μT). These findings challenge the uniformitarian view of the palaeomagnetic field: field intensities of nearly an order of magnitude lower than Neogene values (except during relatively rare geomagnetic excursions and reversals) together with the widespread appearance of strange directions found in the Devonian suggest that the Earth's field during this time may have had a dominantly multipolar geometry. A persistent, low intensity multipolar magnetic field and associated diminished magnetosphere would increase the impact of solar particles on the Earth's magnetosphere, ionosphere and atmosphere with potential major implications for Earth's climate and biosphere.

An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907

Ultraluminous x-ray sources (ULXs) in nearby galaxies shine brighter than any x-ray source in our Galaxy. ULXs are usually modeled as stellar-mass black holes (BHs) accreting at very high rates or intermediate-mass BHs. We present observations showing that NGC 5907 ULX is instead an x-ray accreting neutron star (NS) with a spin period evolving from 1.43 seconds in 2003 to 1.13 seconds in 2014. It has an isotropic peak luminosity of [Formula: see text]1000 times the Eddington limit for a NS at 17.1 megaparsec. Standard accretion models fail to explain its luminosity, even assuming beamed emission, but a strong multipolar magnetic field can describe its properties. These findings suggest that other extreme ULXs (x-ray luminosity [Formula: see text] 1041 erg second[Formula: see text]) might harbor NSs.

Thermal protection performance of magnetohydrodynamic heat shield system based on multipolar magnetic field

In order to cover the shortage of dipole magnetic field in the magnetohydrodynamic(MHD) heat shield system, physical model of a multipolar magnetic field with central and peripheral solenoids is constructed. By employing the governing equations of three dimensional thermochemical nonequilibrium flow with electromagnetic source terms based on the low magneto-Reynolds assumption, the flow control performance of the dipole and multipolar magnetic fields are numerically simulated. To make the results comparable, two groups of cases are designed by first assuming equal stagnation magnetic induction strength and secondly assuming equal ampere-turns. Results show that, the five-magnet system, whose central polar orientation is the same with the peripheral ones, have stronger work capability and better shock control and thermal protection performance. Moreover, the five-solenoid systems are the best when the ampere-turns of the central solenoid are twice and fourth of the peripheral ones under those two circumstances respectively. Compared with the dipole magnetic field, the stagnation non-catalytic heat fluxes are decreased by a factor of 47.5% and 34.0% respectively.

Growth of the inner core in the mean-field dynamo model

Application of Parker's dynamo model to the geodynamo with the growing inner core is considered. It is shown that decrease of the inner core size, where intensive magnetic field generation takes place, leads to the multi-polar magnetic field in the past. This effect reflects the decrease of the region of the effective magnetic field generation. The process is accompanied by increase of the reversals number and decrease of intensity of the geomagnetic field. The constraints on the mechanisms of convection in the liquid core are discussed.

Discovery of a 0.42-s pulsar in the ultraluminous X-ray source NGC 7793 P13

Abstract NGC 7793 P13 is a variable (luminosity range ∼100) ultraluminous X-ray source proposed to host a stellar-mass black hole of less than 15 M⊙ in a binary system with orbital period of 64 d and a 18–23 M⊙ B9Ia companion. Within the EXTraS (Exploring the X-ray Transient and variable Sky) project, we discovered pulsations at a period of ∼0.42 s in two XMM–Newton observations of NGC 7793 P13, during which the source was detected at LX ∼ 2.1 × 1039 and 5 × 1039 erg s−1 (0.3–10 keV band). These findings unambiguously demonstrate that the compact object in NGC 7793 P13 is a neutron star accreting at super-Eddington rates. While standard accretion models face difficulties accounting for the pulsar X-ray luminosity, the presence of a multipolar magnetic field with B ∼ few × 1013 G close to the base of the accretion column appears to be in agreement with the properties of the system.

Post-fall-back evolution of multipolar magnetic fields and radio pulsar activation

It has long been unclear if the small-scale magnetic structures on the neutron star (NS) surface could survive the fall-back episode. The study of the Hall cascade (Cumming, Arras and Zweibel 2004; Wareing and Hollerbach 2009) hinted that energy in small scales structures should dissipate on short timescales. Our new 2D magneto-thermal simulations suggest the opposite. For the first $\sim$10 kyrs after the fall-back episode with accreted mass $10^{-3} M_\odot$, the observed NS magnetic field appears dipolar, which is insensitive to the initial magnetic topology. In framework of the Ruderman & Sutherland (1975) vacuum gap model during this interval, non-thermal radiation is strongly suppressed. After this time the initial (i.e. multipolar) structure begins to re-emerge through the NS crust. We distinguish three evolutionary epochs for the re-emergence process: the growth of internal toroidal field, the advection of buried poloidal field, and slow Ohmic diffusion. The efficiency of the first two stages can be enhanced when small-scale magnetic structure is present. The efficient re-emergence of high order harmonics might significantly affect the curvature of the magnetospheric field lines in the emission zone. So, only after few $10^4$ yrs would the NS starts shining as a pulsar again, which is in correspondence with radio silence of central compact objects (CCOs). In addition, these results can explain the absence of good candidates for thermally emitting NSs with freshly re-emerged field among radio pulsars (Bogdanov, Ng and Kaspi 2014), as NSs have time to cool down, and supernova remnants can already dissipate.

Non-Contact DC Electromagnetic Propulsion by Multipole Transversal Field: Numerical and Experimental Validation

This paper describes the numerical and experimental validation of a novel non-contact dc electromagnetic propulsion concept. In the proposed approach, a propulsion coil carrying a dc current is surrounded by a dc transversal multipole field created inside of a launch tube. The interaction between the transversal dc magnetic field and a dc propulsion coil results in the generation of axial (thrust) force with no need of sliding contacts. This paper describes a numerical assessment of the field-current-force interactions based on both analytical calculations and finite elements, and the corresponding validation experiments. The experiments demonstrate the generation of contactless axial (thrust) force in a dc launch coil using a dc transversal multipole field, and provide a benchmark for the model-based prediction of the field-current-force interactions. Partial magnetic shielding of the propulsion coil is used to selectively attenuate the magnetic field experienced by the thrust coil in a way that a net thrust force is achieved. Experimental measurements confirmed that it is indeed possible to generate contact-free thrust force on a dc propulsion coil using a dc multipole magnetic field, and the measured values of thrust force are in good agreement with the corresponding predictions.

Структура и принцип работы размагничивающей установки

А оperation new mode, a simplified schematic diagram, timing diagrams of electrical signals at the corresponding points, and the basic specifications of single-phase demagnetizer of magneto-el ectric devices permanent magnets developed and implemented in the production are introduced. The demagnetization principle is based on the method of impact of partial heating and fading of multi-polar magnetic field on the permanent magnets. The developed unit improves the efficiency of demagnetization, provides strong magnetic fields for demagnetization and versatility of time interval regulation of the demagnetization process, and eliminates the sudden movements and the occurrence of mechanical shocks on the demagnetizing part.

Low energy, high power hydrogen neutral beam for plasma heating.

A high power, relatively low energy neutral beam injector was developed to upgrade of the neutral beam system of the gas dynamic trap device and C2-U experiment. The ion source of the injector produces a proton beam with the particle energy of 15 keV, current of up to 175 A, and pulse duration of a few milliseconds. The plasma emitter of the ion source is produced by superimposing highly ionized plasma jets from an array of four arc-discharge plasma generators. A multipole magnetic field produced with permanent magnets at the periphery of the plasma box is used to increase the efficiency and improve the uniformity of the plasma emitter. Multi-slit grids with 48% transparency are fabricated from bronze plates, which are spherically shaped to provide geometrical beam focusing. The focal length of the Ion Optical System (IOS) is 3.5 m and the initial beam diameter is 34 cm. The IOS geometry and grid potentials were optimized numerically to ensure accurate beam formation. The measured angular divergences of the beam are ±0.01 rad parallel to the slits and ±0.03 rad in the transverse direction.

Optical force on toroidal nanostructures: Toroidal dipole versus renormalized electric dipole

We study the optical forces acting on toroidal nanostructures. A great enhancement of optical force is unambiguously identified as originating from the toroidal dipole resonance based on the source-representation, where the distribution of the induced charges and currents is characterized by the three families of electric, magnetic, and toroidal multipoles. On the other hand, the resonant optical force can also be completely attributed to an electric dipole resonance in the alternative field-representation, where the electromagnetic fields in the source-free region are expressed by two sets of electric and magnetic multipole fields based on symmetry. The confusion is resolved by conceptually introducing the irreducible electric dipole, toroidal dipole, and renormalized electric dipole. We demonstrate that the optical force is a powerful tool to identify toroidal response even when its scattering intensity is dwarfed by the conventional electric and magnetic multipoles.

MAGNETIC COMPLEXITY AS AN EXPLANATION FOR BIMODAL ROTATION POPULATIONS AMONG YOUNG STARS

Observations of young open clusters have revealed a bimodal distribution of fast and slower rotation rates that has proven difficult to explain with predictive models of spin down that depend on rotation rates alone. The Metastable Dynamo Model proposed recently by Brown, employing a stochastic transition probability from slow to more rapid spin down regimes, appears to be more successful but lacks a physical basis for such duality. Using detailed 3D MHD wind models computed for idealized multipole magnetic fields, we show that surface magnetic field complexity can provide this basis. Both mass and angular momentum losses decline sharply with increasing field complexity. Combined with observation evidence for complex field morphologies in magnetically active stars, our results support a picture in which young, rapid rotators lose angular momentum in an inefficient way because of field complexity. During this slow spin-down phase, magnetic complexity is eroded, precipitating a rapid transition from weak to strong wind coupling.

On the Origin of Water on Earth and Mars

The question regarding the origin of water on Earth is still to be answered scientifically and accurately. It is certain that water in liquid or gaseous form could not have existed on the young, red-hot Earth. It is therefore generally assumed that comets brought water to Earth. However, the isotope ratio (H/D) of water on comets is more than double that of water on Earth, which means that only part of Earth´s water could have come from comets. Intensive laboratory experiments at the Tokyo Institute of Technology in Japan shows that, the stable bipolar magnetic field that we have today, that protects the Earth from the solar wind – a plasma flow of protons and electrons – was actually formed just less than one billion years ago. Interestingly, recent laboratory tests at the University of Maryland, USA, have shown that during the Earth´s early period, when its core of iron and nickel was still molten, the young Earth must have had a multipolar magnetic field. However, this multipolar magnetic field could not have provided any protection from the solar wind. In fact, it would have acted as a huge collector of the solar wind and the same would have applied to our neighboring planet Mars. The result from these two research experiments have now led to a completely new hypothesis regarding the origin of water on both Earth and Mars. During the early period of the two planets, the solar wind would have been able to penetrate along the vertical magnetic field lines into the dense, Venus-like carbon dioxide atmospheres of Earth and Mars. Because of the lack of an ozone layer, the atmosphere would have been exposed to the powerful ultraviolet rays and strong solar wind of the young Sun, which would have split the carbon dioxide molecules in the atmosphere into their component parts of oxygen and carbon. The oxygen released would therefore have been able to combine with the hydrogen nuclei of the solar wind to form water molecules which then fell down as rain.

Photometry and Multipolar Magnetic Field Modeling of Polars: BY Camelopardalis and FL Ceti

We present new broad band optical photometry of two magnetic cataclysmic variable stars, the asynchronous polar BY Camelopardalis and the short period polar FL Ceti. Observations were obtained at the 2.1-m Otto Struve Telescope of McDonald Observatory with 3s and 1s integration times respectively. In an attempt to understand the observed complex changes in accretion flow geometry observed in BY Cam, we performed full 3D MHD simulations assuming a variety of white dwarf magnetic field structures. We investigate fields with increasing complexity including both aligned and non-aligned dipole plus quadrupole field components. We compare model predictions with photometry at various phases of the beat cycle and find that synthetic light curves derived from a multipolar field structure are broadly consistent with optical photometry. FL Ceti is observed to have two very small accretion regions at the foot-points of the white dwarf’s magnetic field. Both accretion regions are visible at the same time in the high state and are about 100 degrees apart. MHD modeling using a dipole plus quadrupole field structure yields quite similar accretion regions as those observed in FL Ceti. We conclude that accretion flows calculated from MHD modeling of multi-polar magnetic fields produce synthetic light curves consistent with photometry of these magnetic cataclysmic variables.