QCD θ-vacuum in a uniform magnetic field

Using a developing delta-class solar active region (NOAA 12673) as a case study, we analyzed the distribution of solar vector magnetic fields and derived the corresponding magnetic energy and helicity densities from vector magnetograms to characterize the relationship between free magnetic energy and potential energy. Our findings indicate that the accumulation of magnetic free energy density and the dynamic evolution of the current helicity primarily occur near the magnetic neutral line associated with the magnetic flux emergence. The free magnetic energy density is found to originate from two sources: the nonpotential energy density ( B n 2 / 8 π ) and the interactive magnetic energy density ( B n · B p /4 π ), where B n represents the nonpotential component and B p the potential component of the magnetic field. Notably, the interactive term B n · B p also provides a significant contribution to the free energy in the solar atmosphere. Furthermore, we examined correlations between physical parameters, such as nonpotential magnetic energy and current helicity, and flare activity during the energy accumulation phase over the long-term evolution to avoid limitations in studies of solar eruptive activity caused by the lack of magnetic field observations in the upper solar atmosphere. It is found that the energy derived from the photospheric transverse magnetic field persists in accumulating even as the apparent magnetic field gradually weakens, until the onset of the X-class flares. Our analysis established reference thresholds for predicting M-class and X-class flares in newly emerging magnetically active regions. Understanding how magnetic energy accumulates to trigger solar flares and coronal mass ejections is crucial for advancing our knowledge of solar activity and space weather forecasting.

We investigate the effect of a non-zero magnetic field on the chiral condensate using a holographic QCD approach. We extend the model proposed by Iatrakis, Kiritsis and Paredes in [1] that realises chiral symmetry breaking dynamically from 5d tachyon condensation. We calculate the chiral condensate, magnetisation and susceptibilities for the confined and deconfined phases. The model leads, in the probe approximation, to magnetic catalysis of chiral symmetry breaking in both confined and deconfined phases. In the chiral limit, mq = 0, we find that in the deconfined phase a sufficiently strong magnetic field leads to a second order phase transition from the chirally restored phase to a chirally broken phase. The transition becomes a crossover as the quark mass increases. Due to a scaling in the temperature, the chiral transition will also be interpreted as a transition in the temperature for fixed magnetic field. We elaborate on the relationship between the chiral condensate, magnetisation and the (magnetic) free energy density. We compare our results at low and moderate temperatures with lattice QCD results.

Effects of applying uniform and non-uniform external magnetic fields on the optimal design of microchannel heat sinks

The influence of the magnetic field on the convective heat transfer characteristics of Fe3O4/water nanofluids

view Abstract Citations (60) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Reflection and trapping of transient Alfven waves propagating in an isothermal atmosphere with constant gravity and uniform magnetic field An, C. -H. ; Musielak, Z. E. ; Moore, R. L. ; Suess, S. T. Abstract A time-dependent linear magnetohydrodynamic numerical model was used to investigate the propagation of Alfven waves in an isothermal and stratified atmosphere with constant gravity and uniform vertical magnetic field. Results show that the Alfven wave transit time for the wave source to infinity is finite and that the wave exhibits continuous partial reflection which becomes total reflection as the front approaches infinity. The total reflection causes the waves to be trapped in the cavity that extends from the wave source to infinity and in which the wave energy is stored. The results suggest that the reflection of Alfven waves (of sufficiently long period) from the outer corona is an intrinsic phenomenon for any stellar atmosphere stratified by gravity and an open magnetic field, and that, therefore, such waves may be trapped in the stellar atmosphere. Publication: The Astrophysical Journal Pub Date: October 1989 DOI: 10.1086/167933 Bibcode: 1989ApJ...345..597A Keywords: Computational Astrophysics; Magnetohydrodynamic Waves; Stellar Atmospheres; Stellar Magnetic Fields; Wave Propagation; Gravitational Effects; Kinetic Energy; Magnetic Field Configurations; Solar Wind; Stellar Coronas; Trapping; Wave Reflection; Astrophysics full text sources ADS |

The latest research on magnetically affected texture and grain structure evolution during recrystallization and grain growth in non-magnetic metals is reviewed. The microstructure development during grain growth in magnetically anisotropic zinc, titanium and zirconium can be effectively influenced by means of magnetic annealing. This is due to an additional driving force for grain boundary motion which arises from a difference in magnetic free energy density between differently oriented grains. Grain growth kinetics in anisotropic non-magnetic materials is substantially modified in the presence of a magnetic field. However, a magnetic field can affect the microstructure evolution not only at the stage of grain growth, but also at earlier stages, i.e. recovery and primary re-crystallization. Also, the impact of a magnetic field is not confined to magnetically anisotropic metals but can be observed in materials with magnetically isotropic properties.

Context. Weak magnetic field elements make a dominant contribution to the total magnetic field on the solar surface. Even so, little is known of their long-term occurrence. Aims. We study the long-term spatial–temporal evolution of the weak-field shift and skewness of the distribution of photospheric magnetic field values during solar cycles 21−24 in order to clarify the role and relation of the weak field values to the overall magnetic field evolution. Methods. We used Wilcox Solar Observatory (WSO) and the Synoptic Optical Long-term Investigations of the Sun Vector SpectroMagnetograph synoptic maps to calculate weak-field shifts for each latitude bin of each synoptic map, and thereby constructed a time–latitude butterfly diagram for shifts. We also calculated butterfly diagrams for skewness for all field values and for weak field values only. Results. The weak-field shifts and (full-field) skewness depict a similar spatial–temporal solar cycle evolution to that of the large-scale surface magnetic field. The field distribution has a systematic non-zero weak-field shift and a large skewness already at (and after) the emergence of the active region, even at the highest resolution. We find evidence for coalescence of opposite-polarity fields during the surge evolution. This is clearly more effective at the supergranulation scale. However, a similar dependence of magnetic field coalescence on spatial resolution was not found in the unipolar regions around the poles. Conclusions. Our results give evidence for the preference of even the weakest field elements toward the prevailing magnetic polarity since the emergence of an active region, and for a systematic coalescence of stronger magnetic fields of opposite polarities to produce weak fields during surge evolution and at the poles. We also find that the supergranulation process is reduced or turned off in the unipolar regions around the poles. These observations improve the understanding not only of the development of the weakest magnetic field elements, but also of the dynamics of magnetic fields at large, and even of processes below the solar surface.

The topological susceptibility is one of the quantities that has a large discretization error, and the error can be sensitive to the choice of fermion action. We report on our results from physical point simulations with 2+1 flavor Möbius domain wall fermion at finite temperature. We also present the chiral condensate and disconnected susceptibility. The temporal lattice size is Nt=12 and 16, and the temperature range is around 140 MeV to 250 MeV for the chiral condensate and susceptibility. A coarse lattice with Nt=10 covers up to 500 MeV to measure the topological susceptibility.

We determine the topological susceptibility of the gauge configurations generated by lattice simulations using 2 flavors of optimal domain-wall fermion on the 16 3 32 lattice with length 16 in the fifth dimension, at the inverse lattice spacing a -1 1.9 GeV. Using the adaptive thick-restart Lanczos algorithm, we project the low-lying eigenmodes of the overlap Dirac operator, and obtain the topological charge Q t of each gauge configuration in eight ensembles with pion masses in the range 230 -580 MeV. From our result of Q t , we compute the topological susceptibility and the second normalized cumulant. Our result of the topological susceptibility agrees with the sea-quark mass dependence predicted by the next-to-leading order chiral perturbation theory, and provides a determination of both the chiral condensate and the pion decay constant.

Numerical investigation of falling ferrofluid droplets under magnetic fields

We deduce the dissipative Alfven wave equation in a medium stratified in one direction, with a transverse magnetic field, in the presence of dissipation by fluid viscosity and electrical resistance; the dissipative Alfven wave equation generalizes earlier results for homogeneous (Cowling, 1960) and inhomogeneous (Campos, 1983a) media, and corrects an error in the literature (Heyvaerts and Priest, 1983). The wave equation is solved exactly in two cases: a uniform magnetic field, and a magnetic field decreasing with height. In both cases the mean state is assumed to be isothermal, with a constant rate of ionization, so that the magnetic diffusivity is constant, but the dynamic viscosity increases with height. There are therefore two regions, a low- (high-) altitude region where electrical resistance dominates fluid viscosity (or vice versa), and an asymptotic regime relevant to the uppermost (lowermost) layers. The two regions are separated by a transition layer, across which the wave field is cont...

Magneto-active elastomers (MAE) are a new branch of smart materials that consist of hard-magnetic particles such as barium ferrite in an elastomer matrix. Under the application of a uniform magnetic field, the MAE material undergoes large deformation as the material bends due to magnetic torques acting on the distribution of hard-magnetic particles. This behavior demonstrates the potential of MAEs to act as remote actuators. MAEs vary from magnetorheological elastomers (MRE) which use soft-magnetic iron particles in place of the hard-magnetic particles and they are driven by magnetic interactions between particles. In this work, MAEs were fabricated using 30% v/v 325 mesh M-type barium ferrite (BaM) particles in Dow Corning HS II silicone elastomers. Prior to curing, the samples were placed in a uniform (∼2 Tesla) magnetic field to align the magnetic particles and produce a magnetization oriented in the direction of the applied magnetic field. The specimens were bonded to a passive poldymethylsiloxane (PDMS) substrate to form a two-segment accordion structure where the MAEs with magnetization, M, were placed in opposing orientations a prescribed distance apart. The application of a uniform magnetic field perpendicular to the magnetization of the undeformed MAEs would result in a bend (on the PDMS) that is dependent upon the orientation of the magnetic particles and the direction of the applied field. This behavior of the composite structure highlights the ability of the MAE material to perform work. Experimental testing of the MAEs used a two-segment accordion structure with fixed boundary-conditions on both ends of the PDMS substrate and a uniform magnetic field was applied to the structure. The resulting deformation roughly represented either a mountain or valley fold (dependent upon the orientation of the applied field). The resulting axial force was observed and compared to computational simulations which utilized numerical techniques to develop approximate solutions. This procedure was repeated with a prescribed displacement on one of the two fixed boundary conditions to induce bending prior to the application of a uniform magnetic field. Results show a decrease in magnetic work potential with increases in the aforementioned prescribed displacement; results also show an increase in magnetic work potential with increases in the applied magnetic field.

Living organisms inspire the design of microrobots, buttheir functionalityis unmatched. Next-generation microrobots aim to leverage the sensingand communication abilities of organisms through magnetic hybridization,attaching magnetic particles to them for external control. However,the protocols used for magnetic hybridization are morphology specificand are not generalizable. We propose an alternative approach thatleverages the principles of negative magnetostatics and magnetophoresisto control nonmagnetic organisms with external magnetic fields. Todo this, we disperse model organisms in dispersions of Fe3O4 nanoparticles and expose them to either uniform orgradient magnetic fields. In uniform magnetic fields, living organismsalign with the field due to external torque, while gradient magneticfields generate a negative magnetophoretic force, pushing objectsaway from external magnets. The magnetic fields enable controllingthe position and orientation of Caenorhabditis elegans larvae and flagellated bacteria through directional interactionsand magnitude. This control is diminished in live spermatozoa andadult C. elegans due to stronger internalbiological activity, i.e., force/torque. Our study presents a methodfor spatiotemporal organization of living organisms without requiringmagnetic hybridization, opening the way for the development of controllableliving microbiorobots.

Chiral and topological properties of the QCD vacuum are studied on a lattice for the confinement and the quark‐gluon plasma phase. We observe in both phases a suppression of chiral and topological fluctuations in the vicinity of static mesons. The formation of a flux tube between the static quark‐antiquark pair is clearly visible in the confinement phase. In the plasma phase a global decrease of the quark condensate and the topological susceptibility takes place.

Magnetic fields play an important role in the dynamics of present-day molecular clouds. Recent work has shown that magnetic fields are equally important for primordial clouds, which form the first stars in the Universe. While the primordial magnetic field strength on cosmic scales is largely unconstrained, theoretical models strongly suggest that a weak seed field existed in the early Universe. We study how the amplification of such a weak field can influence the evolution of accretion discs around first stars, and thus affect the primordial initial mass function (IMF). We perform a suite of 3D ideal magneto-hydrodynamic simulations with different initial field strengths and numerical resolutions. We find that, in simulations with sufficient spatial resolution to resolve the Jeans scale during the collapse, even initially weak magnetic fields grow exponentially to become dynamically important due to both the so-called small-scale turbulent dynamo and the large-scale mean-field dynamo. Capturing the small-scale dynamo action depends primarily on how well we resolve the Jeans length, while capturing the large-scale dynamo depends on the Jeans resolution as well as the maximum absolute resolution. Provided enough resolution, we find that fragmentation does not depend strongly on the initial field strength, because even weak fields grow to become strong. However, fragmentation in runs with magnetic fields differs significantly from those without magnetic fields. We conclude that the development of dynamically strong magnetic fields during the formation of the first stars is likely inevitable, and that these fields had a significant impact on the primordial IMF.