Twisted Field Research Articles

Lorentzian manifolds with shearfree congruences and Kähler-Sasaki geometry

We study Lorentzian manifolds (M,g) of dimension n≥4, equipped with a maximally twisting shearfree null vector field p, for which the leaf space S=M/{exp⁡tp} is a smooth manifold. If n=2k, the quotient S=M/{exp⁡tp} is naturally equipped with a subconformal structure of contact type and, in the most interesting cases, it is a regular Sasaki manifold projecting onto a quantisable Kähler manifold of real dimension 2k−2. Going backwards through this line of ideas, for any quantisable Kähler manifold with associated Sasaki manifold S, we give the local description of all Lorentzian metrics g on the total spaces M of A-bundles π:M→S, A=S1,R, such that the generator of the group action is a maximally twisting shearfree g-null vector field p. We also prove that on any such Lorentzian manifold (M,g) there exists a non-trivial generalised electromagnetic plane wave having p as propagating direction field, a result that can be considered as a generalisation of the classical 4-dimensional Robinson Theorem. We finally construct a 2-parametric family of Einstein metrics on a trivial bundle M=R×S for any prescribed value of the Einstein constant. If dim⁡M=4, the Ricci flat metrics obtained in this way are the well-known Taub-NUT metrics.

Enhanced second-order Stark effect in twisted bilayer graphene quantum dots

External electric field and interlayer twist introduce diverse changes in their confined electronic states of bilayer graphene quantum dots. Using a quantum-dot model, the band gaps of twisted bilayer graphene in finite sizes of about 1.4–2.4 nm with varying twist angles are studied in the presence of an electrostatic field perpendicular to the flakes by means of first-principles calculations. The size-dependent gaps are widened by the interlayer twist, but narrowed by the applied field. Their coupling, however, results in an enhanced Stark response in the twisted structures of which the field-induced band-gap variations are about 3–4 times as large as that of the corresponding untwisted structures under the same field strength. The exceptional Stark shifts come from the field-induced asynchronous shifts in their occupied and virtual energy levels, which are further enhanced by strong interlayer coupling at specific twist angles. Moreover, the shift of band gaps with the field strength follows the quadratic Stark response with large second-order shifting coefficients. The enhanced and tunable Stark shift suggests a gateway to the band engineering of bilayer graphene quantum dots by tuning their sizes, twist angles and their coupling with applied field.

Rotating vector model for magnetars

ABSTRACT The modification of the rotating vector model in the case of magnetars are calculated. Magnetars may have twisted magnetic field compared with normal pulsars. The polarization position angle of magnetars will change in the case of a twisted magnetic field. For a twisted dipole field, we found that the position angle will change both vertically and horizontally. During the untwisting process of the magnetar magnetosphere, the modifications of the position angle will evolve with time monotonously. This may explain the evolution of the position angle in magnetar PSR J1622-4950 and XTE J1810-197. The relation between the emission point and the line of sight will also change. We suggest every magnetospheric models of magnetars also calculate the corresponding changes of position angle in their models. Order of magnitude estimation formula for doing this is given. This opens the possibility to extract the magnetic field geometry of magnetars from their radio polarization observations.

Simulations of Switchback, Fragmentation and Sunspot Pair in δ-Sunspots during Magnetic Flux Emergence.

Strong flares and coronal mass ejections (CMEs), launched from -sunspots, are the most catastrophic energy-releasing events in the solar system. The formations of -sunspots and relevant polarity inversion lines (PILs) are crucial for the understanding of flare eruptions and CMEs. In this work, the kink-stable, spot-spot-type -sunspots induced by flux emergence are simulated, under different subphotospheric initial conditions of magnetic field strength, radius, twist, and depth. The time evolution of various plasma variables of the -sunspots are simulated and compared with the observation data, including magnetic bipolar structures, relevant PILs, and temperature. The simulation results show that magnetic polarities display switchbacks at a certain stage and then split into numerous fragments. The simulated fragmentation phenomenon in some -sunspots may provide leads for future observations in the field.

Effects of magnetized coaxial plasma gun operation on spheromak formation and plasma characteristics

Spheromak plasma formed by a magnetized coaxial plasma gun possesses high propagation velocity and electron density, which has been extensively investigated, for it has a variety of applications, such as fueling of fusion reactor, magnetized target fusion, and labratory simulations of astrophysical phenomena. Formation and optimization of the gun-type spheromak are studied by investigating the discharge characteristics of the gun and the scaling of plasma parameters with various operation conditions. Based on the spheromak formation mechanism, several significant operation parameters are identified, including peak value of gun current, bias flux, gas-puffed mass and the length of neutral gas distribution inside the gun channel: this length can be controlled by adjusting the time delay between gas injection and discharge of the capacitor bank to initiate gas breakdown and for a long time delay the current path distribution inside the gun channel can be characterized by a moving plasma ring which carries almost all of the gun current. Under a sufficient pressure of the self-generated field, the moving plasma ring with freezed toroidal field pushes the bias field into the vacuum chamber, the twisted field lines are then broken, reconnected, and thus forming a free spheromak. The injected gas is desired to exist only in the gun channel: if downstream region of the gun is filled with neutral gas, a weakly ionized and cold spheromak will be formed, which is not beneficial to practical applications. The multiple current path phenomenon is observed using two spatially separated magnetic coils inside the gun channel, excepting for the plasma ring, there are a stagnant current path and a reversed current path separately located in upstream and middle region of the gun channel. Development of the upstream current path is due to the residual charged particles deteached from the tail of accelerated plasma ring and the unswept netural particles, which reduces the energy injected into the plasma ring from capacitor bank, and thus having a negative effect on the performance of spheormak. The axial propagation velocity of spheromak, electron temperature and density are shown to increase with the capacitor bank voltage rising, which can be attributed to the elevation in energy injected into the plasma ring. Only higher electron density is obatined by increasing the gas-puffed mass, and the propagation velocity and electron temperature are observed to decrease. The energy injected into the plasma ring is independent of the gas-puffed mass, and electron density is elevated with gas-puffed mass increasing. Since the frequency of electron impact ionization increases, electrons undergo more collisions and transfer more energy to other particle species, thus the thermal energy of electrons decreases.

Correlation functions of composite Ramond fields in deformed D1-D5 orbifold SCFT2

We study two families of composite twisted Ramond fields (made by products of two operators) in the $\cal {N}=(4,4)$ supersymmetric D1-D5 SCFT$_2$ deformed by a marginal modulus operator away from its $(T^4)^N/ S_N$ free orbifold point. We construct the large-$N$ contributions to the four-point functions with two composite operators and two deformation fields. These functions allow us to derive short-distance OPE limits and to calculate the anomalous dimensions of the composite operators. We demonstrate that one can distinguish two sets of composite Ramond states with twists $m_1$ and $m_2$: protected states, for which $m_1+m_2=N$, and "lifted" states for which $m_1+m_2<N$. The latter require an appropriate renormalisation. We also derive the leading order corrections to their two-point functions, and to their three-point functions with the deformation operator.

Evolution of the Toroidal Flux of CME Flux Ropes during Eruption

SummaryCoronal mass ejections (CMEs) are large-scale explosions of the coronal magnetic field. It is believed that magnetic reconnection significantly builds up the core structure of CMEs, a magnetic flux rope, during the eruption. However, the quantitative evolution of the flux rope, particularly its toroidal flux, is still unclear. In this paper, we study the evolution of the toroidal flux of the CME flux rope for four events. The toroidal flux is estimated as the magnetic flux in the footpoint region of the flux rope, which is identified by a method that simultaneously takes the coronal dimming and the hook of the flare ribbon into account. We find that the toroidal flux of the CME flux rope for all four events shows a two-phase evolution: a rapid increasing phase followed by a decreasing phase. We further compare the evolution of the toroidal flux with that of the Geostationary Operational Environmental Satellites soft X-ray flux and find that they are basically synchronous in time, except that the peak of the former is somewhat delayed. The results suggest that the toroidal flux of the CME flux rope may be first quickly built up by the reconnection mainly taking place in the sheared overlying field and then reduced by the reconnection among the twisted field lines within the flux rope, as enlightened by a recent 3D magnetohydrodynamic simulation of CMEs.

Symmetry resolved entanglement in integrable field theories via form factor bootstrap

We consider the form factor bootstrap approach of integrable field theories to derive matrix elements of composite branch-point twist fields associated with symmetry resolved entanglement entropies. The bootstrap equations are determined in an intuitive way and their solution is presented for the massive Ising field theory and for the genuinely interacting sinh-Gordon model, both possessing a ℤ2 symmetry. The solutions are carefully cross-checked by performing various limits and by the application of the ∆-theorem. The issue of symmetry resolution for discrete symmetries is also discussed. We show that entanglement equipartition is generically expected and we identify the first subleading term (in the UV cutoff) breaking it. We also present the complete computation of the symmetry resolved von Neumann entropy for an interval in the ground state of the paramagnetic phase of the Ising model. In particular, we compute the universal functions entering in the charged and symmetry resolved entanglement.

Polymerization of furan by a sequential liquid crystal W-STEP method

Chemical oxidative oligomerization of furan as a monomer in helical twisted liquid crystal field and subsequently electrochemical polymerization were carried out with convenient one-pot process, referred to as double step (W-STEP) polymerization in liquid crystal. The pre-alignment of linear formed oligo-furan in the liquid crystal matrix and electrochemical polymerization affords to yield electo-optically active π-conjugate polymer with beautiful finger print pattern.

Induced fermionic current in AdS spacetime in the presence of a cosmic string and a compactified dimension

In this paper, we consider a massive charged fermionic quantum field and investigate the current densities induced by a magnetic flux running along the core of an idealized cosmic string in the background geometry of a 5-dimensional anti-de Sitter spacetime, assuming that an extra dimension is compactified. Along the compact dimension quasi-periodicity condition is imposed on the field with a general phase. Moreover, we admit the presence of a magnetic flux enclosed by the compactified axis. The latter gives rise to Ahanorov–Bohm-like effect on the vacuum expectation value of the currents. In this setup, only azimuthal and axial current densities take place. The former presents two contributions, with the first one due to the cosmic string in a 5-dimensional AdS spacetime without compact dimension, and the second one being induced by the compactification itself. The latter is an odd function of the magnetic flux along the cosmic string and an even function of the magnetic flux enclosed by the compactified axis with period equal to the quantum flux. As to the induced axial current, it is an even function of the magnetic flux along the string’s core and an odd function of the magnetic flux enclosed by the compactification perimeter. For untwisted and twisted field along compact dimension, the axial current vanishes. The massless field case is presented as well some asymptotic limits for the parameters of the model.

Amplification of magnetic field twisting by a stagnation point flow

Context.Flows are a common feature of many processes occurring in the solar atmosphere, such as the formation of prominences where evaporated plasma from the chromosphere condensates along thin prominence threads that are seen to twist and oscillate.Aim.We aim to investigate the twisting of these threads by plasma condensation during their formation.Methods.We introduce a simple model with fixed critical points where the flow speed matches the Alfvén speed. This allows us to study the problem separately in the sub-Alfvénic and super-Alfvénic regimes. The temporal and spatial evolution of small amplitude initial twists along a thread is investigated analytically and numerically.Results.Analytical solutions are constructed in terms of the generalised hypergeometric functions. The solutions grow in time, despite the absence of any influxes of energy or magnetic fields. These results are confirmed numerically: We find oscillations with an amplifying amplitude and increasing period in the sub-Alfvénic regime. In the super-Alfvénic regime, we find twist amplification without any accompanying oscillations. An interesting result is the convergence of the twists at the critical points that leads to the formation of steep gradients and small scales. Energy is transferred from the flow to the amplifying twists.Conclusions.Magnetic field lines may be twisted by a stagnation point flow without the influx of any azimuthal field or energy. This twisting could assist in the formation of topology that is able to support the growth of prominences. The formation of steep gradients and small scales at the critical point is a new phenomenon which requires further investigation in the non-linear regime with the inclusion of magnetic diffusion.

Cross Helicity of the 2018 November Magnetic Cloud Observed by the Parker Solar Probe

Abstract Magnetic clouds are large-scale transient structures in the solar wind with low plasma-β, low-amplitude magnetic field fluctuations, and twisted field lines with both ends often connected to the Sun. Their inertial-range turbulent properties have not been examined in detail. In this Letter, we analyze the normalized cross helicity, σ c , and residual energy, σ r , of plasma fluctuations in the 2018 November magnetic cloud observed at 0.25 au by the Parker Solar Probe. A low value of was present in the cloud core, indicating that wave power parallel and antiparallel to the mean field was approximately balanced, while the cloud’s outer layers displayed larger amplitude Alfvénic fluctuations with high values and σ r ∼ 0. These properties are discussed in terms of the cloud’s solar connectivity and local interaction with the solar wind. We suggest that low is likely a common feature of magnetic clouds given their typically closed field structure. Antisunward fluctuations propagating immediately upstream of the cloud had strongly negative σ r values.

Studies of stationary features in jets: BL Lacertae

Context.Monitoring of BL Lacertae at 15 GHz with the Very Long Baseline Array (VLBA) has revealed a quasi-stationary radio feature in the innermost part of the jet, at 0.26 mas from the radio core. Stationary features are found in many blazars, but they have rarely been explored in detail.Aims.We aim to study the kinematics, dynamics, and brightness of the quasi-stationary feature of the jet in BL Lacertae based on VLBA monitoring with submilliarcsecond resolution (subparsec-scales) over 17 years.Methods.We analysed position uncertainties and flux leakage effects of the innermost quasi-stationary feature and developed statistical tools to distinguish the motions of the stationary feature and the radio core. We constructed a toy model to simulate the observed emission of the quasi-stationary component.Results.We find that trajectories of the quasi-stationary component are aligned along the jet axis, which can be interpreted as evidence of the displacements of the radio core. The intrinsic motions of the core and quasi-stationary component have a commensurate contribution to the apparent motion of the stationary component. During the jet-stable state, the core shift significantly influences the apparent displacements of the stationary component, which shows orbiting motion with reversals. The quasi-stationary component has low superluminal speeds on time scales of months. On time-scales of few years, the apparent mean speeds are subrelativistic, of about 0.15 the speed of light. We find that the brightness profile of the quasi-stationary component is asymmetric along and transverse to the jet axis, and this effect remains unchanged regardless of epoch.Conclusions.Accurate positional determination, a high cadence of observations, and a proper accounting for the core shift are crucial for the measurement of the trajectories and speeds of the quasi-stationary component. Its motion is similar to the behaviour of the jet nozzle, which drags the outflow in a swinging motion and excites transverse waves of different amplitudes travelling downstream. A simple modelling of the brightness distribution shows that the configuration of twisted velocity field formed at the nozzle of the jet in combination with small jet viewing angle can account for the observed brightness asymmetry.

Entanglement and symmetry resolution in two dimensional free quantum field theories

We present a thorough analysis of the entanglement entropies related to different symmetry sectors of free quantum field theories (QFT) with an internal U(1) symmetry. We provide explicit analytic computations for the charged moments of Dirac and complex scalar fields in two spacetime dimensions, both in the massive and massless cases, using two different approaches. The first one is based on the replica trick, the computation of the partition function on Riemann surfaces with the insertion of a flux α, and the introduction of properly modified twist fields, whose two-point function directly gives the scaling limit of the charged moments. With the second method, the diagonalisation in replica space maps the problem to the computation of a partition function on a cut plane, that can be written exactly in terms of the solutions of non-linear differential equations of the Painlevé V type. Within this approach, we also derive an asymptotic expansion for the short and long distance behaviour of the charged moments. Finally, the Fourier transform provides the desired symmetry resolved entropies: at the leading order, they satisfy entanglement equipartition and we identify the subleading terms that break it. Our analytical findings are tested against exact numerical calculations in lattice models.

Magnetar outburst and spin-down glitch

ABSTRACT The outburst and spin-down glitch of magnetars are modelled from the magnetospheric point of view. We try to discuss the following four questions: (1) Which pulsar on the period and period-derivative diagram are more likely to show magnetar outburst? (2) Which outburst will make the glitch that triggered the outburst to become a spin-down glitch? (3) Can we model the outburst and spin-down glitch in PSR J1119−6127 simultaneously? (4) Why the torque variation is delayed compared with the peak of the X-ray luminosity in 1E 1048.1−5937 and PSR J1119−6127? It is found that both the global and local twisted magnetic field will affect the radiation and timing behaviours of magnetars. Especially, the delay of torque variations may due to the combined effect of increasing twist in the j-bundle and untwisting of the global magnetosphere. A toy model is built for magnetar outburst and torque variations. It can catch the general trend of magnetar outburst: decaying flux, shrinking hotspot, and torque variations.

Studying Magnetic Field Variations Accompanying the 2011 June 7 Eruptive Event, by Using Nonlinear Force-Free Field Modeling

We study the features of the magnetic field variations within the 2011 June 7 eruptive event that includes a large filament eruption, a flare, and a CME formation. The magnetic field characteristics were obtained by using vector measurements of the magnetic field with the SDO/HMI and 3D magnetic field calculations based on nonlinear force-free field (NLFFF) modeling. Strong and relatively fast variations in the photospheric field characteristics after the flare onset are shown to be observed only within a small site ($20''\times 20''$) of the eruption region in the neighborhood of the polarity inversion line (PIL). We found that the magnetic field strength, the electric current density, current helicity density and free magnetic energy density above this region are growing with height reaching their maximums at the level of ∼15 Mm. After 2011 July 7 00:00 UT, this height started gradually reducing. The NLFFF extrapolation revealed the presence of a magnetic flux rope elongated approximately along the main PIL and an arcade of magnetic field lines over it. The flux-rope axis is located at height of ∼15 Mm. The flux-rope footpoints approximately coincide with the eruptive filament footpoints. Thus, we concluded that the detected flux rope is associated with the magnetic structure of the observed filament. The detected strong variation of the magnetic field within the eruption region are most probably associated with the magnetic field reconfiguration after the filament eruption. The $T_{n}$ parameter, which is the average magnetic field twist within the flux rope, was found to increase up to 2.5 rotations before the flare onset, and to dramatically decrease afterward. This may reflect the developing of kink instability that presumably triggered this eruption.

Electrons in twisted fields and ponderomotive effects

In a first stage we revisit the description of Bessel (B) and Laguerre–Gauss (LG) beams, two frequently used instances of beams carrying orbital angular momentum on their propagation direction. The expansion of a linearly polarized LG beam in terms of Bessel modes is derived starting from the scalar solutions of the Helmholtz equation. Then we explore by numerical calculations the motions of electrons interacting with pulses of twisted radiation. In order to discuss the numerical results, in particular those displaying the effect of dynamic trapping, we use the approximation based on the ponderomotive (PM) potential.

Yang–Baxter deformation as an transformation

The rules for Yang–Baxter (YB) deformation for a generic Green–Schwarz string sigma model has been obtained recently. We show that the deformation can be described through the action of a coordinate dependent matrix on the target space fields both in the NS–NS and the RR sectors, generalizing previous results. This enables us to show that the YB deformed fields can be regarded as duality twisted fields in the context of gauged double field theory (GDFT). We compute the fluxes associated with the twist and show that the conditions on the R-matrix determining the YB deformation give rise to conditions for the fluxes on the GDFT side. More precisely, we show that YB deformation is a process which takes a solution of DFT with geometric flux associated with the isometry group G and deforms it to another solution of DFT with vanishing R-flux and non-vanishing Q-flux given by the structure constants of the dual Lie algebra determined by the R-matrix. We also show that the non-unimodularity of the R-matrix forces the generalized dilaton field to pick up a linear dependence on the winding type coordinates of DFT, implying that the corresponding target space fields satisfy the field equations of DFT in the generalized supergravity frame. This provides a new perspective on the relation between the non-unimodularity of the R-matrix, the trace of the Q-flux and the generalized supergravity equations.

Conformal perturbation theory for twisted fields

We investigate second order conformal perturbation theory for orbifolds of conformal field theories in two dimensions. To evaluate the necessary twisted sector correlation functions and their integrals, we map them from the sphere to its torus double cover. We discuss how this relates crossing symmetry to the modular group, and introduce a regularization scheme on the cover that allows to evaluate the integrals numerically. These methods do not require supersymmetry. As an application, we show that in the torus orbifold of 8 and 16 free bosons, twist fields are marginal at first order, but stop being marginal at second order.

Simultaneous Magnetic Polar Cap Heating during a Flaring Episode from the Magnetar 1RXS J170849.0–400910

Abstract During a pointed 2018 NuSTAR observation, we detected a flare with a 2.2 hr duration from the magnetar 1RXS J170849.0−400910. The flare, which rose in ∼25 s to a maximum flux 6 times larger than the persistent emission, is highly pulsed with an rms pulsed fraction of 53%. The pulse profile shape consists of two peaks separated by half a rotational cycle, with a peak flux ratio of ∼2. The flare spectrum is thermal with an average temperature of 2.1 keV. Phase-resolved spectroscopy shows that the two peaks possess the same temperature, but differ in size. These observational results, along with simple light curve modeling, indicate that two identical antipodal spots, likely the magnetic poles, are heated simultaneously at the onset of the flare and for its full duration. Hence, the origin of the flare has to be connected to the global dipolar structure of the magnetar. This might best be achieved externally, via twists to closed magnetospheric dipolar field lines seeding bombardment of polar footpoint locales with energetic pairs. Approximately 1.86 hr following the onset of the flare, a short burst with its own 3 minutes thermal tail occurred. The burst tail is also pulsating at the spin period of the source and phase-aligned with the flare profile, implying an intimate connection between the two phenomena. The burst may have been caused by a magnetic reconnection event in the same twisted dipolar field lines anchored to the surface hot spots, with subsequent return currents supplying extra heat to these polar caps.