Showing papers on "Magnetic flux published in 2018"

Experimental Observation of Aharonov-Bohm Cages in Photonic Lattices.

Abstract: We report on the experimental realization of a uniform synthetic magnetic flux and the observation of Aharonov-Bohm cages in photonic lattices. Considering a rhombic array of optical waveguides, we engineer modulation-assisted tunneling processes that effectively produce nonzero magnetic flux per plaquette. This synthetic magnetic field for light can be tuned at will by varying the phase of the modulation. In the regime where half a flux quantum is realized in each plaquette, all the energy bands dramatically collapse into nondispersive (flat) bands and all eigenstates are completely localized. We demonstrate this Aharonov-Bohm caging by studying the propagation of light in the bulk of the photonic lattice. Besides, we explore the dynamics on the edge of the lattice and discuss how the corresponding edge states can be continuously connected to the topological edge states of the Creutz ladder. Our photonic lattice constitutes an appealing platform where the interplay between engineered gauge fields, frustration, localization, and topological properties can be finely studied.

MPI-AMRVAC 2.0 for Solar and Astrophysical Applications

Abstract: We report on the development of MPI-AMRVAC version 2.0, which is an open-source framework for parallel, grid-adaptive simulations of hydrodynamic and magnetohydrodynamic (MHD) astrophysical applications. The framework now supports radial grid stretching in combination with adaptive mesh refinement (AMR). The advantages of this combined approach are demonstrated with one-dimensional, two-dimensional and three-dimensional examples of spherically symmetric Bondi accretion, steady planar Bondi-Hoyle-Lyttleton flows, and wind accretion in Supergiant X-ray binaries. Another improvement is support for the generic splitting of any background magnetic field. We present several tests relevant for solar physics applications to demonstrate the advantages of field splitting on accuracy and robustness in extremely low plasma $\beta$ environments: a static magnetic flux rope, a magnetic null-point, and magnetic reconnection in a current sheet with either uniform or anomalous resistivity. Our implementation for treating anisotropic thermal conduction in multi-dimensional MHD applications is also described, which generalizes the original slope limited symmetric scheme from 2D to 3D. We perform ring diffusion tests that demonstrate its accuracy and robustness, and show that it prevents the unphysical thermal flux present in traditional schemes. The improved parallel scaling of the code is demonstrated with 3D AMR simulations of solar coronal rain, which show satisfactory strong scaling up to 2000 cores. Other framework improvements are also reported: the modernization and reorganization into a library, the handling of automatic regression tests, the use of inline/online Doxygen documentation, and a new future-proof data format for input/output

Mode transition in electrical activities of neuron driven by high and low frequency stimulus in the presence of electromagnetic induction and radiation

Abstract: The Hindmarsh–Rose (HR) neuron model has been improved to investigate the complex electrophysiological and various physical phenomena at the level of single cell, for example, time-varying action potential can be induced by the exchange of ion currents and the fluctuation of ions concentration in the cell. When the magnetic flux is considered as a new variable associated to magnetic field, the improved HR neuron model can describe the effects of electromagnetic induction and radiation on membrane potential, where a memristor is used to bridge the membrane potential and the magnetic flux. In this paper, considering the magnetic flux driven, respectively, by the periodic high and low frequency electromagnetic radiation and the Gaussian white noise, the improved HR neuron model is employed to study the modes transition in electrical activities of neuron. The thought-provoking phenomena are detected and discussed by using bifurcation analysis on sampled time series of membrane potential. It is found that the electrical modes of HR neuron model under various parameters have different responses to the periodic high–low frequency electromagnetic radiation and the Gaussian white noise.

Record indoor magnetic field of 1200 T generated by electromagnetic flux-compression.

Abstract: A peak field of 1200 T was generated by the electromagnetic flux-compression (EMFC) technique with a newly developed megagauss generator system. Magnetic fields closely up to the turn-around peak were recorded by a reflection-type Faraday rotation magnetic-field optical-fiber probe. The performance was analyzed and compared with data obtained by the preceding EMFC experiments to show a significant increase in the liner imploding speed of up to 5 km/s.

RF coils: A practical guide for nonphysicists

Abstract: Radiofrequency (RF) coils are an essential MRI hardware component. They directly impact the spatial and temporal resolution, sensitivity, and uniformity in MRI. Advances in RF hardware have resulted in a variety of designs optimized for specific clinical applications. RF coils are the “antennas” of the MRI system and have two functions: first, to excite the magnetization by broadcasting the RF power (Tx‐Coil) and second to receive the signal from the excited spins (Rx‐Coil). Transmit RF Coils emit magnetic field pulses ( B1+) to rotate the net magnetization away from its alignment with the main magnetic field (B0), resulting in a transverse precessing magnetization. Due to the precession around the static main magnetic field, the magnetic flux in the receive RF Coil ( B1−) changes, which generates a current I. This signal is “picked‐up” by an antenna and preamplified, usually mixed down to a lower frequency, digitized, and processed by a computer to finally reconstruct an image or a spectrum. Transmit and receive functionality can be combined in one RF Coil (Tx/Rx Coils). This review looks at the fundamental principles of an MRI RF coil from the perspective of clinicians and MR technicians and summarizes the current advances and developments in technology. Level of Evidence: 1 Technical Efficacy: Stage 6 J. Magn. Reson. Imaging 2018;48:590–604.

Magnetic cage and rope as the key for solar eruptions

Abstract: Solar flares are spectacular coronal events that release large amounts of energy. They are classified as either eruptive or confined, depending on whether they are associated with a coronal mass ejection. Two types of model have been developed to identify the mechanism that triggers confined flares, although it has hitherto not been possible to decide between them because the magnetic field at the origin of the flares could not be determined with the required accuracy. In the first type of model, the triggering is related to the topological complexity of the flaring structure, which implies the presence of magnetically singular surfaces. This picture is observationally supported by the fact that radiative emission occurs near these features in many flaring regions. The second type of model attributes a key role to the formation of a twisted flux rope, which becomes unstable. Its plausibility is supported by simulations, by interpretations of some observations and by laboratory experiments. Here we report modelling of a confined event that uses the measured photospheric magnetic field as input. We first use a static model to compute the slowly evolving magnetic state of the corona before the eruption, and then use a dynamical model to determine the evolution during the eruption itself. We find that a magnetic flux rope must be present throughout the entire event to match the field measurements. This rope evolves slowly before saturating and suddenly erupting. Its energy is insufficient to break through the overlying field, whose lines form a confining cage, but its twist is large enough to trigger a kink instability, leading to the confined flare, as previously suggested. Topology is not the main cause of the flare, but it traces out the locations of the X-ray emission. We show that a weaker magnetic cage would have produced a more energetic eruption with a coronal mass ejection, associated with a predicted energy upper bound for a given region.

The formation of rings and gaps in magnetically coupled disc-wind systems: ambipolar diffusion and reconnection

Abstract: Radial substructures in circumstellar disks are now routinely observed by ALMA. There is also growing evidence that disk winds drive accretion in such disks. We show through 2D (axisymmetric) simulations that rings and gaps develop naturally in magnetically coupled disk-wind systems on the scale of tens of au, where ambipolar diffusion (AD) is the dominant non-ideal MHD effect. In simulations where the magnetic field and matter are moderately coupled, the disk remains relatively laminar with the radial electric current steepened by AD into a thin layer near the midplane. The toroidal magnetic field sharply reverses polarity in this layer, generating a large magnetic torque that drives fast accretion, which drags the poloidal field into a highly pinched radial configuration. The reconnection of this pinched field creates magnetic loops where the net poloidal magnetic flux (and thus the accretion rate) is reduced, yielding dense rings. Neighbouring regions with stronger poloidal magnetic fields accrete faster, forming gaps. In better magnetically coupled simulations, the so-called `avalanche accretion streams' develop continuously near the disk surface, rendering the disk-wind system more chaotic. Nevertheless, prominent rings and gaps are still produced, at least in part, by reconnection, which again enables the segregation of the poloidal field and the disk material similar to the more diffusive disks. However, the reconnection is now driven by the non-linear growth of MRI channel flows. The formation of rings and gaps in rapidly accreting yet laminar disks has interesting implications for dust settling and trapping, grain growth, and planet formation.

Saturn's magnetic field revealed by the Cassini Grand Finale.

Abstract: During 2017, the Cassini fluxgate magnetometer made in situ measurements of Saturn's magnetic field at distances ~2550 ± 1290 kilometers above the 1-bar surface during 22 highly inclined Grand Finale orbits. These observations refine the extreme axisymmetry of Saturn's internal magnetic field and show displacement of the magnetic equator northward from the planet's physical equator. Persistent small-scale magnetic structures, corresponding to high-degree (>3) axisymmetric magnetic moments, were observed. This suggests secondary shallow dynamo action in the semiconducting region of Saturn's interior. Some high-degree magnetic moments could arise from strong high-latitude concentrations of magnetic flux within the planet's deep dynamo. A strong field-aligned current (FAC) system is located between Saturn and the inner edge of its D-ring, with strength comparable to the high-latitude auroral FACs.

Strong dipole magnetic fields in fast rotating fully convective stars

Abstract: M dwarfs are the most numerous stars in our Galaxy with masses between approximately 0.5 and 0.1 solar mass. Many of them show surface activity qualitatively similar to our Sun and generate flares, high X-ray fluxes, and large-scale magnetic fields. Such activity is driven by a dynamo powered by the convective motions in their interiors. Understanding properties of stellar magnetic fields in these stars finds a broad application in astrophysics, including, e.g., theory of stellar dynamos and environment conditions around planets that may be orbiting these stars. Most stars with convective envelopes follow a rotation-activity relationship where various activity indicators saturate in stars with rotation periods shorter than a few days. The activity gradually declines with rotation rate in stars rotating more slowly. It is thought that due to a tight empirical correlation between X-ray and magnetic flux, the stellar magnetic fields will also saturate, to values around ~4kG. Here we report the detection of magnetic fields above the presumed saturation limit in four fully convective M-dwarfs. By combining results from spectroscopic and polarimetric studies we explain our findings in terms of bistable dynamo models: stars with the strongest magnetic fields are those in a dipole dynamo state, while stars in a multipole state cannot generate fields stronger than about four kilogauss. Our study provides observational evidence that dynamo in fully convective M dwarfs generates magnetic fields that can differ not only in the geometry of their large scale component, but also in the total magnetic energy.

A fast low-to-high confinement mode bifurcation dynamics in the boundary-plasma gyrokinetic code XGC1

Abstract: A fast edge turbulence suppression event has been simulated in the electrostatic version of the gyrokinetic particle-in-cell code XGC1 in a realistic diverted tokamak edge geometry under neutral particle recycling. The results show that the sequence of turbulent Reynolds stress followed by neoclassical ion orbit-loss driven together conspire to form the sustaining radial electric field shear and to quench turbulent transport just inside the last closed magnetic flux surface. The main suppression action is located in a thin radial layer around ψN≃0.96–0.98, where ψN is the normalized poloidal flux, with the time scale ∼0.1 ms.

The Revolution Revolution: Magnetic Morphology Driven Spin-down*

Abstract: Observations of young open clusters show a bimodal distribution of rotation periods that has been difficult to explain with existing stellar spin-down models. Detailed MHD stellar wind simulations have demonstrated that surface magnetic field morphology has a strong influence on wind-driven angular momentum loss. Observations suggest that faster rotating stars store a larger fraction of their magnetic flux in higher-order multipolar components of the magnetic field. In this work, we present a new model for stellar spin-down that, for the first time, accounts for the stellar surface magnetic field configuration. We show how a magnetic complexity that evolves from complex toward simple configurations as a star spins down can explain the salient features of stellar rotation evolution, including the bimodal distribution of both slow and fast rotators seen in young open clusters.

Magnetic Flux Cancelation as the Trigger of Solar Coronal-Hole Coronal Jets

Abstract: We investigate in detail the magnetic cause of minifilament eruptions that drive coronal-hole jets. We study 13 random on-disk coronal hole jet eruptions, using high resolution X-ray images from Hinode/XRT, EUV images from SDO/AIA, and magnetograms from SDO/HMI. For all 13 events, we track the evolution of the jet-base region and find that a minifilament of cool (transition-region-temperature) plasma is present prior to each jet eruption. HMI magnetograms show that the minifilaments reside along a magnetic neutral line between majority-polarity and minority-polarity magnetic flux patches. These patches converge and cancel with each other, with an average cancelation rate of ~0.6 X 10$^18 Mx hr^{-1} for all 13 jets. Persistent flux cancelation at the neutral line eventually destabilizes the minifilament field, which erupts outward and produces the jet spire. Thus, we find that all 13 coronal-hole-jet-driving minifilament eruptions are triggered by flux cancelation at the neutral line. These results are in agreement with our recent findings Panesar et al 2016b for quiet-region jets, where flux cancelation at the underlying neutral line triggers the minifilament eruption that drives each jet. Thus from that study of quiet-Sun jets and this study of coronal hole jets, we conclude that flux cancelation is the main candidate for triggering quiet region and coronal hole jets

Design and Adaptive Sliding-Mode Control of Hybrid Magnetic Bearings

Abstract: In this paper, a hybrid magnetic bearing (HMB) prototype system is designed and analyzed. Two compact bearings are used to suspend the rotor in five degrees of freedom. Electromagnets are used for axial suspension of the rotor, while permanent magnets are used for the passive radial stability. A brushless DC motor is designed in order to rotate the shaft around its axis. The 3-D finite-element model of the HMB system is established and distribution of magnetic fields in the air gaps and magnetic forces on the rotor under various control currents and displacements is calculated. A nonlinear adaptive sliding-mode controller is designed for the position control of the rotor in axial direction. Since the control characteristics of the active magnetic bearing system are highly nonlinear and time varying with external interference, a radial basis function compensator is designed first, and then, a sliding-mode control law is used to generate the control input. The stability analysis for the designed controller is given based on the Lyapunov theorem. Experimental setup is built to guide the design process. The performance of the HMB system based on the designed control algorithm is evaluated under different operating conditions.

Nature of the energy source powering solar coronal loops driven by nanoflares

Abstract: Magnetic energy is required to heat the corona, the outer atmosphere of the Sun, to millions of degrees. We study the nature of the magnetic energy source that is probably responsible for the brightening of coronal loops driven by nanoflares in the cores of solar active regions. We consider observations of two active regions (ARs), 11890 and 12234, in which nanoflares have been detected. To this end, we use ultraviolet (UV) and extreme ultraviolet (EUV) images from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) for coronal loop diagnostics. These images are combined with the co-temporal line-of-sight magnetic field maps from the Helioseismic and Magnetic Imager (HMI) onboard SDO to investigate the connection between coronal loops and their magnetic roots in the photosphere. The core of these ARs exhibit loop brightening in multiple EUV channels of AIA, particularly in its 9.4 nm filter. The HMI magnetic field maps reveal the presence of a complex mixed polarity magnetic field distribution at the base of these loops. We detect the cancellation of photospheric magnetic flux at these locations at a rate of about $10^{15}$ Mx s$^{-1}$. The associated compact coronal brightenings directly above the cancelling magnetic features are indicative of plasma heating due to chromospheric magnetic reconnection. We suggest that the complex magnetic topology and the evolution of magnetic field, such as flux cancellation in the photosphere and the resulting chromospheric reconnection, can play an important role in energizing active region coronal loops driven by nanoflares. Our estimate of magnetic energy release during flux cancellation in the quiet Sun suggests that chromospheric reconnection can also power the quiet corona.

A Simple Method for Performance Prediction of Permanent Magnet Eddy Current Couplings Using a New Magnetic Equivalent Circuit Model

Abstract: A simple and practical magnetic equivalent circuit (MEC) based analytical technique for calculating the performance parameters of the permanent magnet (PM) eddy current coupling is presented. In the proposed MEC model built with the lumped parameters, the eddy current effects are inherently taken into account by introducing a branch magnetic circuit allowing for the magnetomotive force and the reaction magnetic flux. A complete formulation for the reaction flux which is treated as a kind of leakage flux is derived. A verification process is conducted and it is shown that in a considerably wide range of slip speeds, the torques predicted by the presented method match well with those obtained by both the three-dimensional finite element analysis and experimental measurement. The new MEC-based method also proves to be effective in the performance simulation of the PM eddy current coupling with different design parameters. In addition, the limitation of the proposed approach is also discussed and the reasons are fully investigated.

Observational Evidence for Self-generation of Small-scale Magnetic Flux Ropes from Intermittent Solar Wind Turbulence

Abstract: We present unique {and additional} observational evidence for the self-generation of small-scale coherent magnetic flux rope structures in the solar wind. Such structures with durations between 9 and 361 minutes are identified from Wind in-situ spacecraft measurements through the Grad-Shafranov (GS) reconstruction approach. The event occurrence counts are on the order of 3,500 per year on average and have a clear solar cycle dependence. We build a database of small-scale magnetic flux ropes from twenty-year worth of Wind spacecraft data. We show a power-law distribution of the wall-to-wall time corresponding well to the inertial range turbulence, which agrees with relevant observations and numerical simulation results. We also provide the axial current density distribution from the GS-based observational analysis, which yields a non-Gaussian probability density function consistent with numerical simulation results.

Analysis of Consequent Pole Spoke Type Vernier Permanent Magnet Machine With Alternating Flux Barrier Design

Abstract: The mechanism of torque production in the Vernier machine is difficult to understand since the stator teeth originating flux modulation effects, upon which the torque is produced, are usually minor factors, which are ignored in normal synchronous machines. This paper begins with an analytical study of a spoke type Vernier permanent magnet machine (SVPM), which has a generic number of stator slots per pole per phase, upon which the key sizing equations are developed. An equivalent winding factor is defined to model stator teeth originating flux modulation phenomena and resulting multiharmonic field coupling effects. Performance predictions and design observations concerning SVPMs are then obtained. A new topology for an SVPM using ferrite magnets is proposed where a consequent pole design having an alternating leakage flux blocking ability is developed for the rotor. Overall, this improved SVPM design significantly boosts the back electromotive force compared to conventional SVPM, and the torque production even surpasses that of a benchmark rare earth interior permanent magnet machine (IPM) although with a slightly lower power factor.

Methods and apparatus for regulating a magnetic flux in an inductive power supply

Abstract: Aspects of the subject disclosure may include, supplying an alternating voltage waveform to a winding coupled to a magnetic core of an inductive power supply to regulate an alternating magnetic flux in the magnetic core. The alternating voltage waveform can be generated by selectively enabling one or more switches coupled to a storage device. The subject disclosure may further include configuring the one or more switches according to a configuration during a portion of a period of the alternating voltage waveform, and measuring a characteristic of an alternating current flowing in a conductor coupled to the magnetic core during the portion of the period of the alternating voltage waveform. Other embodiments are disclosed.

Investigation of Spoke Array Permanent Magnet Vernier Machine With Alternate Flux Bridges

Abstract: In recent years, permanent magnet vernier (PMV) machines are attracting more and more attentions owing to their high torque density feature, which is mainly benefited from its special operation principle called flux modulation effect. Nevertheless, It is found that its working flux density is low, viz., ∼0.3T for modulated flux density in the surface mounted PMV machines. Hence, there is a really large space to further improve the torque density of PMV machine. The spoke array magnet is widely used in the regular PM machine to improve working flux density. However, the flux barrier effect of spoke array magnet for the PMV machine has been proved, and a large torque reduction would be introduced in the high pole ratio spoke array PMV machine. In this paper, a novel spoke array PMV machine is proposed to improve working flux density and the torque density of PMV machines. The alternate magnetic bridges are added in the rotor core to connect iron pieces with same polarity and provide magnetic circuit for the low-pole number working magnetic field, hence the no-load back EMF and output torque of the proposed machine can be significantly improved. Moreover, analytical airgap flux density distribution based on equivalent magnetic circuit is presented to reveal the mechanism of working flux density improvement, and finite element analysis is used to investigate the performance features and structure parameter influence on the key performances, viz., back EMF and torque, of the proposed machine. Finally, a 25 N·m prototype has been built and tested to validate these analyses.

Field Orientation Based on Current Amplitude and Phase Angle Control for Wireless Power Transfer

Abstract: The low coupling coefficient between the transmitter and receiver is the major constraint of a wireless power transfer (WPT) system. Although some approaches, such as increasing the quality factor and achieving precise impedance matching, can reduce the adverse impacts of the low coupling coefficient and improve the system performance, high leakage magnetic flux between the transmitter and receiver remains a problem. With the increase in transfer distance and the misalignment between the transmitter and receiver, the increasing leakage magnetic flux of nondirectional fields degrades the WPT system performance. This paper proposes an active field orientation method to shape the magnetic flux so as to minimize the leakage flux. The amplitude and phase angle of the magnetizing current are controlled, and a coil structure for minimizing the coupling among the transmitters and generating a three-dimensional magnetic field is proposed. This method realizes three-dimension full-range field orientation with adjustable magnitude and direction of B-field at an arbitrary point, and as a result the B-field is concentrated with reduced leakage magnetic flux. The proposed field orientation shaping technique is verified by theoretical analysis, simulation, and experimental results.

Design-Oriented Modelling of Axial-Flux Variable-Reluctance Resolver Based on Magnetic Equivalent Circuits and Schwarz–Christoffel Mapping

Abstract: Axial flux variable reluctance (AFVR) resolvers have substantial benefits that make them suitable for motion control drives. However, they suffer from insufficient accuracy, especially in high-accuracy applications. Hence, optimizing the AFVR resolver structure is necessary for improving its commercial usage. However, its accurate modelling needs three-dimensional (3-D) time stepping finite element analysis (TSFEA) that is computationally expensive and unsuitable for co-usage with optimization algorithms. The aim of this paper is to establish an accurate, yet computationally fast, model suitable for optimal design of AFVR resolvers. The working of the proposed model is based on magnetic equivalent circuit (MEC) and conformal mapping, which are in turn based on Schwarz–Christoffel mapping. The model uses conformal mapping to calculate reluctances that are used in MEC for calculating magnetic fluxes linkages, inductances, and induced voltages. Then, the induced voltages are used for calculating angular position. The results of the proposed model are compared with those of 3-D TSFEA. Finally, the experimental prototype is used to evaluate the developed analytical model.

MTPV Flux-Weakening Strategy for PMSM High Speed Drive

Abstract: High speed permanent magnet synchronous motors (PMSMs) are used in electric vehicles because of their intense power density The high speed implies a significant electromotive force and requires flux weakening The usual control algorithms realize flux weakening by adding a negative $I_{d}$ current component when the voltage required by the current regulation exceeds the maximum voltage depending on the battery If the magnet can be totally defluxed, then it is better to use a maximum torque per volt strategy Furthermore, there is no speed regulation in the control and the driver gives a torque reference This reference value has to be limited by the attainable operating points; therefore, the battery power limit has to be taken into account in addition to the voltage and current limits The d–q current references are calculated to minimize the total current magnitude required to reach the reference torque This paper proposes a strategy to control a PMSM operating continuously since the speed zero up to the maximum speed without the switching algorithm, in order to take into account the different limitations (current, voltage, and power) and to expand the overspeed zone In order to validate the proposed strategy, experimental results are shown for a low power machine

Eruption of a multi-flux-rope system in solar active region 12673 leading to the two largest flares in Solar Cycle 24

Abstract: Context. Solar active region (AR) 12673 in 2017 September produced the two largest flares in Solar Cycle 24: the X9.3 flare on September 6 and the X8.2 flare on September 10.Aims. We attempt to investigate the evolutions of the two large flares and their associated complex magnetic system in detail.Methods. Combining observations from the Solar Dynamics Observatory and results of nonlinear force-free field (NLFFF) modeling, we identify various magnetic structures in the AR core region and examine the evolution of these structures during the flares.Results. Aided by the NLFFF modeling, we identify a double-decker flux rope configuration above the polarity inversion line (PIL) in the AR core region. The north ends of these two flux ropes were rooted in a negative- polarity magnetic patch, which began to move along the PIL and rotate anticlockwise before the X9.3 flare on September 6. The strong shearing motion and rotation contributed to the destabilization of the two magnetic flux ropes, of which the upper one subsequently erupted upward due to the kink-instability. Then another two sets of twisted loop bundles beside these ropes were disturbed and successively erupted within five minutes like a chain reaction. Similarly, multiple ejecta components were detected as consecutively erupting during the X8.2 flare occurring in the same AR on September 10. We examine the evolution of the AR magnetic fields from September 3 to 6 and find that five dipoles emerged successively at the east of the main sunspot. The interactions between these dipoles took place continuously, accompanied by magnetic flux cancellations and strong shearing motions.Conclusions. In AR 12673, significant flux emergence and successive interactions between the different emerging dipoles resulted in a complex magnetic system, accompanied by the formations of multiple flux ropes and twisted loop bundles. We propose that the eruptions of a multi-flux-rope system resulted in the two largest flares in Solar Cycle 24.

Physics and Applications of Metallic Magnetic Calorimeters

Abstract: Metallic magnetic calorimeters (MMCs) are calorimetric low-temperature particle detectors that are currently strongly advancing the state of the art in energy-dispersive single particle detection. They are typically operated at temperatures below $$100\,\mathrm {mK}$$ and make use of a metallic, paramagnetic temperature sensor to transduce the temperature rise of the detector upon the absorption of an energetic particle into a change of magnetic flux which is sensed by a superconducting quantum interference device. This outstanding interplay between a high-sensitivity thermometer and a near quantum-limited amplifier results in a very fast signal rise time, an excellent energy resolution, a large dynamic range, a quantum efficiency close to 100% as well as an almost ideal linear detector response. For this reason, a growing number of groups located all over the world is developing MMC arrays of various sizes which are routinely used in a variety of applications. Within this paper, we briefly review the state of the art of metallic magnetic calorimeters. This includes a discussion of the detection principle, sensor materials and detector geometries, readout concepts, the structure of modern detectors as well as the state-of-the-art detector performance.

Elliptic-cylindrical Analytical Flux Rope Model for Magnetic Clouds

Abstract: In this paper, we present the elliptic-cylindrical analytical flux rope model, which constitutes the first level of complexity above that of a circular-cylindrical geometry. The framework of this series of models was established by Nieves-Chinchilla et al. with the circular-cylindrical analytical flux rope model. The model describes the magnetic flux rope topology with distorted cross section as a possible consequence of the flux rope interaction with the solar wind. In this model, for the first time, a flux rope is completely described by a nonorthogonal geometry. The Maxwell equations can be consistently solved using tensorial analysis, and relevant physical quantities can be derived, such as magnetic fluxes, number of turns, or Lorentz force distribution. The model is generalized in terms of the radial dependence of the poloidal and axial current density components. The circular-cylindrical reconstruction technique has been adapted to the new geometry for a specific case of the model and tested against an interplanetary coronal mass ejection observed by the Wind spacecraft on 2005 June 12. In this specific case, from the comparative analysis between the circular-cylindrical and elliptic-cylindrical models, the inclusion of the cross-section distortion in the 3D reconstruction results in significant changes in the derived axis orientation, size, central magnetic field, magnetic fluxes, and force-freeness. The case studied in this paper exemplifies the use of the model and reconstruction technique developed. Furthermore, the novel mathematical formulation to model flux ropes in heliophysics paves the way to the inclusion of more complex magnetic field configurations.

Magnetic Reconnection Null Points as the Origin of Semirelativistic Electron Beams in a Solar Jet

Abstract: Magnetic reconnection, the central engine that powers explosive phenomena throughout the universe, is also perceived to be one of the principal mechanisms for accelerating particles to high energies. Although various signatures of magnetic reconnection have been frequently reported, observational evidence that links particle acceleration directly to the reconnection site has been rare, especially for space plasma environments currently inaccessible to in situ measurements. Here we utilize broadband radio dynamic imaging spectroscopy available from the Karl G. Jansky Very Large Array to observe decimetric type III radio bursts in a solar jet with high angular (∼20″), spectral (∼1%), and temporal resolution (50 ms). These observations allow us to derive detailed trajectories of semirelativistic (tens of keV) electron beams in the low solar corona with unprecedentedly high angular precision (<0.″65). We found that each group of electron beams, which corresponds to a cluster of type III bursts with 1–2 s duration, diverges from an extremely compact region (∼600 km2) in the low solar corona. The beam-diverging sites are located behind the erupting jet spire and above the closed arcades, coinciding with the presumed location of magnetic reconnection in the jet eruption picture supported by extreme ultraviolet/X-ray data and magnetic modeling. We interpret each beam-diverging site as a reconnection null point where multitudes of magnetic flux tubes join and reconnect. Our data suggest that the null points likely consist of a high level of density inhomogeneities possibly down to 10 km scales. These results, at least in the present case, strongly favor a reconnection-driven electron-acceleration scenario.

An Observationally Constrained Model of a Flux Rope that Formed in the Solar Corona

Abstract: Coronal mass ejections (CMEs) are large-scale eruptions of plasma from the coronae of stars. Understanding the plasma processes involved in CME initiation has applications for space weather forecasting and laboratory plasma experiments. James et al. used extreme-ultraviolet (EUV) observations to conclude that a magnetic flux rope formed in the solar corona above NOAA Active Region 11504 before it erupted on 2012 June 14 (SOL2012-06-14). In this work, we use data from the Solar Dynamics Observatory (SDO) to model the coronal magnetic field of the active region one hour prior to eruption using a nonlinear force-free field extrapolation, and find a flux rope reaching a maximum height of 150 Mm above the photosphere. Estimations of the average twist of the strongly asymmetric extrapolated flux rope are between 1.35 and 1.88 turns, depending on the choice of axis, although the erupting structure was not observed to kink. The decay index near the apex of the axis of the extrapolated flux rope is comparable to typical critical values required for the onset of the torus instability, so we suggest that the torus instability drove the eruption.

Automated Detection of Small-scale Magnetic Flux Ropes in the Solar Wind: First Results from the Wind Spacecraft Measurements

Abstract: We have developed a new automated small-scale magnetic flux rope (SSMFR) detection algorithm based on the Grad-Shafranov (GS) reconstruction technique. We have applied this detection algorithm to the Wind spacecraft in-situ measurements during 1996 - 2016, covering two solar cycles, and successfully detected a total number of 74,241 small-scale magnetic flux rope events with duration from 9 to 361 minutes. This large number of small-scale magnetic flux ropes has not been discovered by any other previous studies through this unique approach. We perform statistical analysis of the small-scale magnetic flux rope events based on our newly developed database, and summarize the main findings as follows. (1) The occurrence of small-scale flux ropes has strong solar cycle dependency with a rate of a few hundreds per month on average. (2) The small-scale magnetic flux ropes in the ecliptic plane tend to align along the Parker spiral. (3) In low speed ($<$ 400 km/s) solar wind, the flux ropes tend to have lower proton temperature and higher proton number density, while in high speed ($\ge$ 400 km/s) solar wind, they tend to have higher proton temperature and lower proton number density. (4) Both the duration and scale size distributions of the small-scale magnetic flux ropes obey a power law. (5) The waiting time distribution of small-scale magnetic flux ropes can be fitted by an exponential function (for shorter waiting times) and a power law function (for longer waiting times). (6) The wall-to-wall time distribution obeys double power laws with the break point at 60 minutes (corresponding to the correlation length). (7) The small-scale magnetic flux ropes tend to accumulate near the heliospheric current sheet (HCS). The entire database is available at \url{this http URL} and in machine readable format in this article.

Three-dimensional MHD Simulations of Solar Prominence Oscillations in a Magnetic Flux Rope

Abstract: Solar prominences are subject to all kinds of perturbations during their lifetime, and frequently demonstrate oscillations. The study of prominence oscillations provides an alternative way to investigate their internal magnetic and thermal structures as the oscillation characteristics depend on their interplay with the solar corona. Prominence oscillations can be classified into longitudinal and transverse types. We perform three-dimensional ideal magnetohydrodynamic simulations of prominence oscillations along a magnetic flux rope, with the aim to compare the oscillation periods with those predicted by various simplified models and to examine the restoring force. We find that the longitudinal oscillation has a period of about 49 minutes, which is in accordance with the pendulum model where the field-ligned component of gravity serves as the restoring force. In contrast, the horizontal transverse oscillation has a period of about 10 minutes and the vertical transverse oscillation has a period of about 14 minutes, and both of them can be nicely fitted with a two-dimensional slab model. We also find that the magnetic tension force dominates most of the time in transverse oscillations, except for the first minute when magnetic pressure overwhelms.

Experimental formation of a fractional vortex in a superconducting bi-layer

Abstract: We report the experimental formation of a fractional vortex generated by using a thin superconducting bi-layer in the form of a niobium bi-layer, observed as a magnetic flux distribution image taken by a scanning superconducting quantum interference device (SQUID) microscope. Thus, we demonstrated that multi-component superconductivity can be realized by an s-wave conventional superconductor, because, in these superconductors, the magnetic flux is no longer quantized as it is destroyed by the existence of an inter-component phase soliton (i-soliton).