Dipole Component Research Articles

Loading noise induced by cavitating flow and its simplified model prediction

This study employs a computational fluid dynamics approach in combination with acoustic analogy to investigate the loading noise characteristics induced by cavitating flow around a NACA66 (MOD) hydrofoil. The Large Eddy Simulation coupled with the Schnerr-Sauer cavitation model is used to capture the transient motion behaviour of the cavitating flow, which agrees well with experimental observations. The study covers three typical states of cavitation: sheet cavitation (σ = 1.44), sheet/cloud transition cavitation (σ = 1.29), and cloud cavitation (σ = 0.83). The study demonstrates that the energy of the loading noise source is concentrated in the middle-frequency band for sheet cavitation, while it is concentrated in the low-frequency band for cloud cavitation. The cavitation instability promotes loading noise to generate drastic fluctuation and even several extreme impulses, which are directly related to the processes of cloud formation and cavitation multistage shedding. Finally, a simplified model for calculating the loading noise is proposed, and its errors and equivalence are evaluated. The model can effectively predict the loading noise induced by cavitating flow and enable decoupling of data from the flow field to the sound field, which may facilitate the separation of the dipole component from the total noise in experimental measurements.

Broadband transverse unidirectional scattering and large range nanoscale displacement measuring based on the interaction between a tightly focused azimuthally polarized beam and a silicon hollow nanostructure.

We theoretically propose a broadband transverse unidirectional scattering scheme based on the interaction between a tightly focused azimuthally polarized beam (APB) and a silicon hollow nanostructure. When the nanostructure is located at a specific position in the focal plane of the APB, the transverse scattering fields can be decomposed into contributions from transverse components of the electric dipoles, longitudinal components of magnetic dipoles and magnetic quadrupole components. In order to satisfy the transverse Kerker conditions for these multipoles within a wide infrared spectrum, we design a novel nanostructure with hollow parallelepiped shape. Through numerical simulations and theoretical calculations, this scheme exhibits efficient transverse unidirectional scattering effects in the wavelength range of 1440 nm to 1820 nm (380 nm). In addition, by adjusting the position of the nanostructure on the x-axis, efficient nanoscale displacement sensing with large measuring ranges can be achieved. After analyses, the results prove that our research may have potential applications in the field of high-precision on-chip displacement sensors.

Three-dimensional dipole momentum analog based on L-shape metasurface

The achievement of a flat metasurface has realized extraordinary control over light–matter interaction at the nanoscale, enabling widespread use in imaging, holography, and biophotonics. However, three-dimensional metasurfaces with the potential to provide additional light–matter manipulation flexibility attract only little interest. Here, we demonstrate a three-dimensional metasurface scheme capable of providing dual phase control through out-of-plane plasmonic resonance of L-shape antennas. Under circularly polarized excitation at a specific wavelength, the L-shape antennas with rotating orientation angle act as spatially variant three-dimensional tilted dipoles and are able to generate desire phase delay for different polarization components. Generalized Snell's law is achieved for both in-plane and out-of-plane dipole components through arranging such L-shape antennas into arrays. These three-dimensional metasurfaces suggest a route for wavefront modulation and a variety of nanophotonic applications.

Surface structure of 45 Hercules: an otherwise unremarkable Ap star with a surprisingly weak magnetic field

ABSTRACTThe origin of magnetic fields and their role in chemical spot formation on magnetic Ap stars is currently not understood. Here, we contribute to solving this problem with a detailed observational characterization of the surface structure of 45 Her, a weak-field Ap star. We find this object to be a long-period, single-lined spectroscopic binary and determine the binary orbit as well as fundamental and atmospheric parameters of the primary. We study magnetic field topology and chemical spot distribution of 45 Her with the help of the Zeeman Doppler imaging technique. Magnetic mapping reveals the stellar surface field to have a distorted dipolar topology with a surface-averaged field strength of 77 G and a dipolar component strength of 119 G – confirming it as one of the weakest well-characterized Ap-star fields known. Despite its feeble magnetic field, 45 Her shows surface chemical inhomogeneities with abundance contrasts of up to 6 dex. Of the four chemical elements studied, O concentrates at the magnetic equator, whereas Ti, Cr, and Fe avoid this region. Apart from this trend, the positions of Fe-peak element spots show no apparent correlation with the magnetic field geometry. No signs of surface differential rotation or temporal evolution of chemical spots on the time-scale of several years were detected. Our findings demonstrate that chemical spot formation does not require strong magnetic fields to proceed and that both the stellar structure and the global field itself remain stable for sub-100 G field strengths contrary to theoretical predictions.

Magnetospheric accretion at the late phases of the pre-main-sequence evolution: the case of RZ Psc

ABSTRACT It has been shown that during the burst of accretion activity observed in UX Ori-type star RZ Psc in 2013, the accretion rate increased approximately by an order of magnitude. This means that the accretion process at the late stages of the pre-main sequence evolution is very unstable. Using the spectra obtained during this episode, we have studied the magnetospheric emission in the H α line. Models of magnetospheric accretion are calculated to obtain the parameters of the magnetosphere from this observation. In this work, we have taken into account the influence of the recombination delay effect during gas motion in the stellar magnetosphere. The accounting for this effect and the presence of the magnetospheric absorption in the IR Ca ii triplet lines and its absence in D Na i resonance lines allowed us to place a lower limit on the temperature in the magnetosphere at $\approx 10\, 000$ K, which significantly improved precision of our estimate of accretion rate. According to the best-fitting model, the logarithm of accretion rate is $\log \dot{M} = -10.1\pm 0.3$ ($\dot{M} \approx 7\times 10^{-11}\ \mathrm{M_\odot \, yr^{-1}}$) and the inclination angle of RZ Psc is 43° ± 3°. It is less than the inclination, typical for the UX Ori stars (about 70°), that explains the weak photometric variability of this star. Using the obtained accretion rate and magnetosphere radius, we estimate the strength of the dipole component of the magnetic field of RZ Psc ≈ 0.1 kGs.

Nuclear dipolar relaxation induced by interacting ground state electron spins

In samples used for dynamic nuclear polarization (DNP), spin–lattice relaxation times are usefully increased by going to high magnetic field and low temperature, typically several tesla and below 1 K. But the relaxation times for dipolar components of the nuclear spin energy remain stubbornly shorter than those for the Zeeman energy: dipolar order decays faster than the polarization itself by a huge factor—up to four orders of magnitude or more in the materials studied thus far. Such fast nuclear dipolar relaxation poses experimental challenges, for instance, when transferring polarization between different nuclear spin species via intermediate nuclear order: a proven technique to polarize rare nuclear spins. The origin of this fast nuclear dipolar relaxation remained a mystery for a long time—existing theories of nuclear spin–lattice relaxation could at best predict about one order of magnitude difference between the nuclear dipolar and nuclear Zeeman relaxation rates—until it was recently discovered to be due to conversion of nuclear dipolar energy into super-hyperfine energy induced by nuclear flip-flop transitions. A previous article showed that the inclusion of this relaxation path enables a quantitative explanation of nuclear dipolar relaxation induced by photo-excited triplet states. This article extends the theory to nuclear dipolar relaxation induced by ground state electron spins and demonstrates that this new mechanism enables a precise quantitative prediction from first principles of the nuclear dipolar relaxation rate for Ca(OH)2 doped with O2- centres—in which DNP is caused by the solid effect (SE)—and LiF doped with F-centres—in which DNP is caused by thermal mixing (TM)—both at 5.5 T and 0.4 K. It is noticed that the proposed mechanism extends to other spin systems which has implications for e.g. TM and spectral diffusion.

Magnetic Fields and Plasma Motions in a Hybrid Martian Magnetosphere

AbstractThe Martian magnetosphere contains elements of induced and intrinsic origin. To display them one must use different coordinate systems. Although the solar‐electric coordinate system (Mars Solar Electric [MSE]) adequately describes the main features of the induced magnetosphere, it removes/suppresses aspects caused by the crustal magnetic sources while rotating the spacecraft position to the MSE‐coordinate system and averaging over many orbits. On the other hand, to observe effects of the crustal field one should use the solar orbital coordinates (Mars Solar Orbital [MSO]). To find a compromise and keeping in mind that the most probable value of the clock angle of the interplanetary magnetic field (IMF) on the Mars orbit is ∼90° we can consider separately cases with positive and negative By components of the IMF. It is shown that dynamics of ion fluxes in the distant regions of the magnetosphere is mainly controlled by induced features. However, reconnection of the draping IMF with crustal field leads to a twisting of the classical draping configuration. Despite of the very intricate local geometry of the crustal field, the low‐order harmonics of the magnetic field and mainly the dipole component determine the reconnection sites, at least, statistically for many Mars rotations. For different signs of the By component of the IMF these sites occur either in the +Y‐MSO or −Y‐MSO hemispheres. As a result, statistically the magnetosphere of Mars looks like a hybrid magnetosphere formed during the solar wind interaction with the obstacle which simultaneously contains an extended ionosphere and a weak dipole magnetic field.

Quantum droplets with topological textures in a dipolar condensate

We investigate a spin–orbit coupled Bose–Einstein condensate containing both dipolar and non-dipolar components. Combined with Rashba spin–orbit coupling, the competition between contact and dipolar interaction reveals four distinctive exotic phases: droplet lattice phase, stripe phase, oblique stripe phase, and labyrinthine stripe phase. In particular, we adopt a condensate order parameter ansatz to characterize the superstripe phase in a harmonic trap through minimizing the analytical total energy. Furthermore, we find that the energy analysis of the dipolar condensate in the trap uncovers a higher order transition from lattice to stripe phases when contact interaction increases. Our results demonstrate the possibility for physicists to realize a wealth of controllable configurations in spin–orbit coupled dipolar condensates, including droplet lattice with topology and superstripe adjusted by changing the strength of contact and dipolar interactions. The topological phases we predict in this work can be realized by using state-of-the-art techniques in ultracold atomic experiments of chromium, erbium, or dysprosium condensates.

Sourceless LWD Borehole Acoustics: Field Testing the Concept

Seismic compressional and shear velocities are uniquely sensitive to the elasticity of the Earth and are used to estimate many properties of interest in oil exploration, reservoir development, and production efforts. Such properties include formation lithology, porosity, saturation, mechanical properties, presence of fractures, principal stresses, pore pressure, and formation damage. Logging-while-drilling (LWD) acoustic tools can be used when wireline acoustic logging is prohibitive and/or real-time decisions based on acoustics are needed. It has been shown that modern LWD acoustic tools with powerful, specialized acoustic transmitters can deliver most acoustic data and products with the same quality as their wireline counterparts. However, there is a lot of variation in the quality of traditional LWD acoustic data depending on the tool design. The operator’s decision to run an LWD acoustic tool is not an easy one, though, as the tool is often the most complicated and the longest part of the drillstring. In this paper, we present the field test results of a “sourceless” LWD acoustic tool that uses seismic energy generated by the drill bit to extract formation elastic properties instead of a powerful, specialized transmitter. Such a tool may consist of a receiver section of hydrophones only, which compared with traditional LWD tools, significantly simplifies the mechanical design, shortens the length of the tool, and reduces the tool cost. In the absence of an active transmitter, continuously recording the drill-bit energy as the drill bit rotates enables maximizing the signal-to-noise ratio (SNR) of the formation arrivals. In addition, the presence of azimuthally distributed receiver elements at each receiver axial position permits decomposition of the received wavefield into monopole, dipole, and quadrupole components. Processing results of the sourceless tool using time and frequency semblance are compared to those from a traditional LWD acoustic tool in the same well to demonstrate the viability of the concept. We also demonstrate that, in the case of “traditional” LWD acoustic tools, it is beneficial to place the tool transmitter below the receiver array. In this case, the drill-bit energy propagating in the formation along the borehole is “added” to the energy generated by the source and effectively increases the frequency bandwidth of the source signal and improves the SNR of the resulting formation arrivals.

Large-scale anisotropies of extragalactic cosmic rays below the ankle

We study the anisotropies on large angular scales which can be present in the flux of cosmic rays reaching the Earth from a population of extragalactic sources, focusing on the energy range between the second knee and the ankle. In this energy range the particles are significantly affected by the Galactic magnetic field, which then plays a relevant role in shaping the expected anisotropies. The Galactic magnetic field deflects the cosmic-ray trajectories and thus modifies the anisotropies present outside the halo of the Galaxy, in particular the dipolar one associated with the translational motion of the observer (Compton-Getting effect). Also, due to the Galactic rotation, in the reference frame of an observer at Earth there is an electric component of the Galactic field that produces a small change in the particles' momentum. This acceleration depends on the cosmic-ray arrival direction and it hence induces anisotropies in the flux observed in a given energy range. We analyse the expected amplitude and phase of the resulting dipolar component of the flux and discuss the possibility to explain via these effects the change in the phase of the right-ascension distribution which is observed at energies around 1 EeV.

Alternating-domain supersolids in binary dipolar condensates

Two-component dipolar condensates are now experimentally producible, and we theoretically investigate the nature of supersolidity in this system. We predict the existence of a binary supersolid state in which the two components form a series of alternating domains, producing an immiscible double supersolid. Remarkably, we find that a dipolar component can even induce supersolidity in a nondipolar component. In stark contrast to single-component dipolar supersolids, alternating-domain supersolids do not require quantum stabilization, and the number of crystal sites is not strictly limited by the condensate populations, with the density hence being substantially lower. Our results are applicable to a wide range of dipole moment combinations, marking an important step towards long-lived bulk-supersolidity.

High-Precision Characterization of Seismicity from the 2022 Hunga Tonga-Hunga Ha'apai Volcanic Eruption

AbstractThe earthquake swarm accompanying the January 2022 Hunga Tonga-Hunga Ha'apai (HTHH) volcanic eruption includes a large number of posteruptive moderate-magnitude seismic events and presents a unique opportunity to use remote monitoring methods to characterize and compare seismic activity with other historical caldera-forming eruptions. We compute improved epicentroid locations, magnitudes, and regional moment tensors of seismic events from this earthquake swarm using regional to teleseismic surface-wave cross correlation and waveform modeling. Precise relative locations of 91 seismic events derived from 59,047 intermediate-period Rayleigh- and Love-wave cross-correlation measurements collapse into a small area surrounding the volcano and exhibit a southeastern time-dependent migration. Regional moment tensors and observed waveforms indicate that these events have a similar mechanism and exhibit a strong positive compensated linear vector dipole component. Precise relative magnitudes agree with regional moment tensor moment magnitude (Mw) estimates while also showing that event sizes and frequency increase during the days after the eruption followed by a period of several weeks of less frequent seismicity of a similar size. The combined information from visual observation and early geologic models indicate that the observed seismicity may be the result of a complex series of events that occurred after the explosive eruption on 15 January, possibly involving rapid resupply of the magma chamber shortly after the eruption and additional faulting and instability in the following weeks. In addition, we identify and characterize an Mw 4.5 event five days before the paroxysmal explosion on 15 January, indicating that additional seismic events preceding the main eruption could have been identified with improved local monitoring. Our analysis of the HTHH eruption sequence demonstrates the value of potentially utilizing teleseismic surface-wave cross correlation and waveform modeling methods to assist in the detailed analysis of remote volcanic eruption sequences.

Sub Coulomb barrier d+ 208Pb scattering in the time-dependent basis function approach

We employ the non-perturbative time-dependent basis function (tBF) approach to study the scattering of the deuteron on 208Pb below the Coulomb barrier. We obtain the bound and discretized scattering states of the projectile, which form the basis representation of the tBF approach, by diagonalizing a realistic Hamiltonian in a large harmonic oscillator basis. We find that the higher-order inelastic scattering effects are noticeable for sub barrier scatterings with the tBF method. We have successfully reproduced experimental sub Coulomb barrier elastic cross section ratios with the tBF approach by considering only the electric dipole (E1) component of the Coulomb interaction between the projectile and the target during scatterings. We find that the correction of the polarization potential to the Rutherford trajectory is dominant in reproducing the data at very low bombarding energies, whereas the role of internal transitions of the deuteron projectile induced by the E1 interaction during the scattering becomes increasingly significant at higher bombarding energies.

Migrating Dynamo Waves and Consequences for Stellar Current Sheets

We study the relation between stellar dynamo-wave propagation and the structure of the stellar magnetic field. Modeling dynamo waves by the well-known Parker migratory dynamo, we vary the intensity of dynamo drivers in order to obtain activity-wave propagation toward the Equator (as in the solar-activity cycle) or towards the Poles. We match the magnetic field in the dynamo active shell with that in the surrounding stellar material, using a simple dissipativ magnetohydrodynamic system for the transition region. Introducing a weak asymmetry between the stellar hemispheres, we study phase shifts of the dipole, quadrupole, and octupole magnetic components at various distances from the star to demonstrate that several-percent asymmetry in dynamo drivers are sufficient to obtain a realistic relation between solar dipole and quadrupole moments. We study the behavior of the stellar current sheets and show that for the poleward propagating activity it is substantially different from solar ones. In particular, we demonstrate conditions in which the conical current sheets propagate opposite to the solar directions.

Vortex, stripe, Skyrmion lattice, and localized states in a spin–orbit coupled dipolar condensate

We investigate a spin-1/2 system containing both dipolar and non-dipolar components in a 52Cr Bose–Einstein condensate with spin–orbit coupling. Combined with energy analysis, phase diagram between contact interaction and Rashba spin–orbit coupling reveals four distinctive ground-state phases: vortex state phase, stripe phase, Skyrmion lattice phase, and localized Skyrmion phase. We adopt the condensate order parameter to characterize the supersolidity and topological charges of the quantum states of matter. In particular, we predict the emergence of a Skyrmion lattice phase with supersolidity which is justified by calculating the crystalline order. Moreover, we find that the critical value of the spin–orbit coupling from stripe to Skyrmion lattice increases with decreasing relative strength of the dipolar and contact interactions. Our results demonstrate the possibility for physicists to realize a wealth of controllable topological configurations in dipolar condensates, including Skyrmion lattices with both supersolidity and different topological charges adjusted by changing the strength of spin–orbit coupling. The topological phases we predict in this work can be realized by using state-of-the-art techniques in ultracold atomic experiments, such as using pulsed magnetic fields to induce and adjust spin–orbit couplings on atom chips of chromium, erbium, or dysprosium condensates.

Coseismic Faulting Complexity of the 2019 Mw 5.7 Silivri Earthquake in the Central Marmara Seismic Gap, Offshore Istanbul

Abstract The submarine Main Marmara fault is overdue for an M > 7 earthquake in direct vicinity to the Istanbul megacity and the only segment of the right-lateral North Anatolian fault zone that has not been activated since 1766. On 26 September 2019, an Mw 5.7 earthquake occurred offshore Silivri (west of Istanbul), stimulating the discussions on a future major earthquake. The predominant faulting style for this pending earthquake remains enigmatic. Here, we study the coseismic rupture evolution of the 2019 Silivri earthquake and decipher the multitype-faulting aspects by a nonstandard approach. The event was dominated by a large compensated linear vector dipole component, of about −50%, that we interpret as a consecutive strike-slip and thrust double-couple episode, closely collocated in space and time. Because of local variations of the fault geometry at the eastern boundary of the central basin, crustal shortening and related thrust faulting are expected. A striking observation is the almost synchronous occurrence of both faulting types during a single earthquake. The earthquake complexity here is reported for the first time in the Sea of Marmara region and has to be considered in future rupture scenarios of an expected M > 7 event, with subsequent consequences for tsunami hazard and risk.

Tunable interactions of quasibound states in the continuum with cavity mode in a metasurface-microcavity hybrid

All-dielectric photonic crystals or metasurfaces supporting optical bound states in the continuum (BIC) are emerging as a promising platform for the study of light-matter interactions in the strong coupling regime. However, coupling of BIC to other optical excitations is generally weak, with the coupling strength limited in the range of several tens of milli electronvolts with short decoherence time. This greatly hinders their applications in solid-base quantum information devices that require a fast information exchange rate and long coherence time. Herein, a cavity-coupling strategy is used to extend the BIC-based coupling platforms. We propose a hybrid system containing an all-dielectric metasurface embedded in an optical microcavity, demonstrating the formation of BIC-cavity polaritons in the strong coupling regime with a surprisingly large Rabi splitting over 220 meV, far beyond the strong coupling regime. A flexible tuning of BIC-cavity coupling from weak to strong coupling regime is realized by controlling the shape of dielectric element of the metasurface. Importantly, a full quantum model based on Heisenberg-Langevin formalism reveals the temporal population dynamics of the coupling system, demonstrating the ultrafast Rabi socillation within 19 fs with a long decoherence time over 200 fs. Utilizing the multipolar expansion approach combined with full wave simulations, we also investigate the coupling-induced polariton characteristics in both the weak and strong coupling regimes. We find that the electric dipole component of the BIC mode dominates in the polariton states in weak coupling regime, which dramatically changes as the system is tuned into strong coupling regime. Such a cavity-coupling strategy in the present hybrid system is expected to be of great use for enhancing BIC-based light-matter interactions for more complex hybrid nanostructures such as three-mode systems with mixing BIC-cavity-exciton interactions.

Magnetically confined mountains on accreting neutron stars with multipole magnetic fields

ABSTRACT Magnetically confined mountains on accreting neutron stars are candidates for producing continuous gravitational waves. We formulate a magnetically confined mountain on a neutron star with strong multipole magnetic fields and obtain some sequences of numerical solutions. We find that the mass ellipticity of the mountain increases by one order of magnitude if the neutron star has strong multipole magnetic fields. As matter accretes on to the magnetic pole, the size of the mountain increases and the magnetic fields are buried. If the neutron star has a dipole magnetic field, the dipole magnetic field is buried and transformed into multipole components. By contrast, if the neutron star has both dipole and strong multipole magnetic fields, the multipole magnetic fields are buried and transformed into a negative dipole component. We also calculate magnetically confined mountains with toroidal magnetic fields and find that the ellipticity becomes slightly smaller when the mountain has toroidal magnetic fields. If the multipole magnetic fields are buried, they sustain the intense toroidal magnetic field near the stellar surface, and the ratio of the toroidal magnetic field to the poloidal magnetic field is close to 100. The hidden strong toroidal magnetic fields are sustained by the buried multipole magnetic fields.

The north-south asymmetry of active regions of different magneto-morphological types in solar cycles 23 and 24

We used the elaborated earlier catalog of the magneto-morphological classes (MMC) of active regions (ARs) to study 2046 ARs of the solar cycle (SC) 23 and 1507 ARs of the SC24. According to empiric rules for sunspot groups (Hale’s polarity law, Joy’s law, etc.) and MMC, all ARs (except for unipolar spots) were sorted out between two categories: A-type – regular bipolar ARs; B-type – all the rest irregular ARs. We found that the number of both regular and irregular ARs follows the cycle with the Pearson’s correlation coefficient of 0.92 and 0.78, respectively. The regular ARs are distributed evenly between the two maxima of each cycle. The irregular ARs are also distributed evenly between the two maxima in the SC 23, however their number is enhanced in the second maximum of the SC 24. Both regular and irregular ARs exhibit strong north–south (N-S) asymmetry. The significance of asymmetry is confirmed using the Pearson’s χ-square test and one more test based on the normal approximation to a binomial distribution. During the two maxima of a cycle, the peaks in two hemispheres for both regular and irregular ARs number do not vary synchronously. This can be explained by the fluctuations in Babcock-Leighton mechanism. In general, there are more irregular ARs in the S-hemisphere in both cycles, which might be the result of an additional weakening of the toroidal field due to interplay between the dipole and quadrupole components of the global magnetic field.

ULX pulsar Swift J0243.6+6124 observations with NuSTAR: dominance of reflected emission in the super-Eddington state

ABSTRACT We report the discovery of the bright reflected emission component in the super-Eddington state of the ultraluminous X-ray pulsar Swift J0243.6+6124, based on the NuSTAR observations of the source during its 2017 outburst. The flux of the reflected emission is weakly variable over the pulsar phase while the direct emission shows significantly larger pulsation amplitude. We propose that in this system the neutron star finds itself in the centre of the well formed by the inner edge of the geometrically thick super-Eddington accretion disc truncated by the magnetic field of the pulsar. The aspect ratio of the well is H/R ∼ 1. The inner edge of the truncated disc is continuously illuminated by the emission of the accretion column giving rise to the weakly variable reflected emission. As the neutron star rotates, its emission sweeps through the line of sight, giving rise to the pulsating direct emission. From Doppler broadening of the iron line, we measure the truncation radius of the accretion disc ∼50 Rg. The inferred dipole component of the magnetic field is consistent with previous estimates favouring a not very strong field. The uniqueness of this system is determined by its moderately super-Eddington accretion rate and the moderate magnetic field so that the inner edge of the truncated geometrically thick accretion disc is seen from the neutron star at a large solid angle.