Magnetic Field Topology Research Articles

The Equilibrium of Coronal Loops Near Separatrices

We present numerical models from the field-aligned HYDrodynamics and RADiation code (HYDRAD) of a highly asymmetric closed coronal loop with near-singular expansion factor. This loop was chosen to simulate a coronal magnetic flux tube that passes close to a null point, as in the last set of closed loops under the fan surface of a coronal jet or a pseudostreamer. The loop has a very large cross section localized near the coronal null. The coronal heating was assumed to be uniform and steady. A siphon flow establishes itself within 4 hr of simulation time, flowing from the smaller-area footpoint to the larger-area footpoint, with high initial speeds dropping rapidly as the plasma approaches the null region. Observationally, this would translate to strong upflows on the order of 10 km s−1 from the footpoint rooted in the localized minority polarity, and weak downflows from the fan-surface footpoint on the order of a few kilometers per second, along with near-stationary plasma near the null region. We present the model results for two heating rates. In addition, we analyzed analogous Hinode EUV Imaging Spectrometer observations of null-point topologies, which show associated Doppler shifts in the plasma that correlate well with the simulation results in both direction and magnitude of the bulk velocity. We discuss the implications of our results for determining observationally the topology of the coronal magnetic field.

Experimental Investigation of the CFHT-27 Cusped-Field Hall Thruster Performance

Cusped-field Hall thrusters (CFHT) have significant potential for use in many space missions due to their simplicity, long life and high efficiency. CFHT-27, designed and developed at the Bogazici University Space Technologies Laboratory (BUSTLab), is a prototype CFHT with a 27-mm diameter discharge channel. CFHT-27 utilizes samarium-cobalt (SmCo) permanent magnets. The thrust and efficiency of a cusped-field thruster is closely related to its size and design parameters. In this study, in order to understand the relationships between the design parameters and the thruster performance, the performance tests of the CFHT-27 are carried out for a given magnetic field topology. The thruster was operated at discharge voltages ranging from 250 to 500V with argon propellant with up to 3,000 s of specific impulse. The measurements show that CFHT-27 can achieve thrust values from 1 to 46 mN. Thus, this thruster can be used for a wide range of thrust values allowing throttle capabilities for different missions.

Topology of Magnetic and Velocity Fields at Kinetic Scales in Incompressible Plasma Turbulence

AbstractThe topology of the magnetic and velocity fields at the kinetic scales are investigated in the context of nearly incompressible magnetosheath plasma turbulence. Using the unprecedented high‐resolution data from the Magnetospheric MultiScale mission, the joint probability distribution functions of geometrical invariants characterizing the magnetic and velocity fields gradient tensor at the kinetic scales are computed. The topological features of the magnetic and velocity field gradient tensors and their symmetric component tensors present axisymmetric distribution patterns, indicating that the structure of the plasma turbulence at the kinetic scales are different from those in hydrodynamic and magnetohydrodynamic turbulence. Moreover, a strong correlation between the straining and rotational parts of the magnetic and velocity field gradient tensors was found, which manifests the dominance of sheet‐like structures at the kinetic‐scales dissipation in incompressible plasma turbulence.

Experimental characterization of thrust production mechanisms in a magnetic nozzle ECR thruster

Direct thrust measurements have been performed on a low-power (30 W) ECR plasma thruster operated with xenon. The total thrust, the force exerted on the walls of the source (thermal thrust), and the force exerted on the permanent magnet (magnetic thrust) have been measured separately for various operating conditions (power and flowrate). The results are compared for two magnetic field topologies (with a different magnetic field gradient and divergence). The magnetic thrust dominates thermal thrust for most operating conditions, and its contribution to total thrust can be as high as 80%. A saturation of magnetic thrust with increasing power has been observed and is correlated with a decrease in thrust efficiency. A high degree of electron anisotropy can be inferred from the thrust measurements.

Ganymede MHD Model: Magnetospheric Context for Juno's PJ34 Flyby

AbstractOn 7 June 2021 the Juno spacecraft visited Ganymede and provided the first in situ observations since Galileo's last flyby in 2000. The measurements obtained along a one‐dimensional trajectory can be brought into global context with the help of three‐dimensional magnetospheric models. Here we apply the magnetohydrodynamic model of Duling et al. (2014, https://doi.org/10.1002/2013ja019554) to conditions during the Juno flyby. In addition to the global distribution of plasma variables we provide mapping of Juno's position along magnetic field lines, Juno's distance from closed field lines and detailed information about the magnetic field's topology. We find that Juno did not enter the closed field line region and that the boundary between open and closed field lines on the surface matches the poleward edges of the observed auroral ovals. To estimate the sensitivity of the model results, we carry out a parameter study with different upstream plasma conditions and other model parameters.

3D PIC Simulations for relativistic jets with a toroidal magnetic field

ABSTRACT We have investigated how kinetic instabilities such as the Weibel instability (WI), the mushroom instability (MI), and the kinetic Kelvin–Helmholtz instability (kKHI) are excited in jets without and with a toroidal magnetic field, and how such instabilities contribute to particle acceleration. In this work, we use a new jet injection scheme, where an electric current is self-consistently generated at the jet orifice by the jet particles, which produce the toroidal magnetic field. We perform five different simulations for a sufficiently long time to examine the non-linear effects of the jet evolution. We inject unmagnetized e± and e−– p+ (mp/me = 1836), as well as magnetized e± and e−– i+ (mi/me = 4) jets with a top-hat jet density profile into an unmagnetized ambient plasmas of the same species. We show that WI, MI, and kKHI excited at the linear stage, generate a non-oscillatory x-component of the electric field accelerating, and decelerating electrons. We find that the two different jet compositions (e± and e−– i+) display different instability modes, respectively. Moreover, the magnetic field in the non-linear stage generated by different instabilities is dissipated and reorganized into new topologies. A 3D magnetic field topology depiction indicates possible reconnection sites in the non-linear stage, where the particles are significantly accelerated by the dissipation of the magnetic field associated to a possible reconnection event.

Total electron content depression in the nightside Martian ionosphere: statistical results from Mars Express MARSIS measurements and implications

ABSTRACT Thermal electron depression and energetic electron depletion are two well-known features of the nightside Martian ionosphere. While the latter has been extensively studied in the past, there is no report of systematic and firm identification of the former and hence the correlation between them is unknown. Applying an automatic procedure to the Mars Express total electron content (TEC) measurements, we identify a large number of TEC depression features that amount to one-third of the available nightside measurements. Similar to the observation of energetic electron depletion, large amplitude TEC depression is more frequently observed near strong magnetic fields, as an expected result of the established solar-wind-driven scenario that the shielding of precipitating electrons by closed magnetic loops is responsible for the observed reduction in thermal electron density. However, small amplitude depression appears to be uniformly distributed across the Martian surface, implying an alternative origin yet to be determined. Contrary to our expectation, TEC depression and energetic electron depletion are not usually collocated, but this dilemma may simply reflect the complexity of the magnetic field topology with respect to the spacecraft trajectory.

Polarimetric signatures of hot spots in black hole accretion flows

Context. The flaring events observed in the Sagittarius A* supermassive black hole system can be attributed to the nonhomogeneous nature of the near-horizon accretion flow. Bright regions in this flow may be associated with density or temperature anisotropies, corresponding to so-called “bright spots” or “hot spots.” Such orbiting features may explain observations at infrared wavelengths, as well as recent findings at millimeter wavelengths. Aims. In this work, we study the emission from an orbiting equatorial bright spot, imposed on a radiatively inefficient accretion flow background, to find polarimetric features indicative of the underlying magnetic field structure and other system variables, including inclination angle, spot size, black hole spin, and more. Specifically, we investigate the impact of these parameters on the Stokes 𝒬 − 𝒰 signatures that commonly exhibit a typical double loop (pretzel-like) structure. Methods. Our semi-analytical model, describing the underlying plasma conditions and the orbiting spot, is built within the framework of the numerical radiative transfer code ipole, which calculates synchroton emission at 230 GHz. Results. We showcase the wide variety of 𝒬 − 𝒰 loop signatures and the relation between inner and outer loops. For the vertical magnetic field topology, the inner 𝒬 − 𝒰 loop is explained by the suppression of the synchrotron emission as seen by the distant observer. For the radial and toroidal magnetic field topologies, the inner 𝒬 − 𝒰 loop corresponds to the part of the orbit where the spot is receding with respect to the observer. Conclusions. Based on our models, we conclude that it is possible to constrain the underlying magnetic field topology with an analysis of the 𝒬 − 𝒰 loop geometry, particularly in combination with circular polarization measurements.

An Improved Magnetohydrodynamic Simulation of the 2006 December 13 Coronal Mass Ejection of NOAA Active Region 10930

We present a magnetohydrodynamic simulation of the coronal mass ejection on 2006 December 13 in the emerging δ-sunspot NOAA Active Region 10930, improving upon a previous simulation by Fan as follows. (1) Incorporate an ambient solar wind instead of using a static potential magnetic field extrapolation as the initial state. (2) In addition to imposing the emergence of a twisted flux rope, also impose at the lower boundary a random electric field that represents the effect of turbulent convection, which drives field-line braiding and produces resistive and viscous heating in the corona. With the inclusion of this heating, which depends on the magnetic field topology, we are able to model the synthetic soft X-ray images that would be observed by the X-Ray Telescope (XRT) of the Hinode satellite, produced by the simulated coronal magnetic field. We find that the simulated preeruption magnetic field with the buildup of a twisted magnetic flux rope produces synthetic soft X-ray emission that shows qualitatively similar morphology as that observed by the Hinode/XRT for both the ambient coronal loops of the active region and the central inverse S-shaped “sigmoid“ that sharpens just before the onset of the eruption. The synthetic postflare loop brightening also shows similar morphology to that seen in the Hinode/XRT image during the impulsive phase of the eruption. It is found that the kinematics of the erupting flux rope is significantly affected by the open magnetic fields and fast solar wind streams adjacent to the active region.

Is the mm/submm dust polarization a robust tracer of the magnetic field topology in protostellar envelopes? A model exploration

Context. High-resolution millimeter and submillimeter (mm and submm) polarization observations have opened a new era in the understanding of how magnetic fields are organized in star forming regions, unveiling an intricate interplay between the magnetic fields and the gas in protostellar cores. However, to assess the role of the magnetic field in the process of solar-type star formation, it is important to understand to what extent the polarized dust emission is a good tracer of the magnetic field in the youngest protostellar objects. Aims. In this paper, we present a thorough investigation of the fidelity and limitations of using dust polarized emission to map the magnetic field topologies in low-mass protostars. Methods. To assess the importance of these effects, we performed an analysis of magnetic field properties in 27 realizations of magnetohydrodynamics (MHD) models following the evolution of physical properties in star-forming cores. Assuming a uniform population of dust grains the sizes of which follow the standard MRN size distribution, we analyzed the synthetic polarized dust emission maps produced when these grains align with the local B-field because of radiative torques (B-RATs). Results. We find that mm and submm polarized dust emission is a robust tracer of the magnetic field topologies in inner protostellar envelopes and is successful at capturing the details of the magnetic field spatial distribution down to radii ~100 au. Measurements of the line-of-sight-averaged magnetic field line orientation using the polarized dust emission are precise to <15° (typical of the error on polarization angles obtained with observations from large mm polarimetric facilities such as ALMA) in about 75%–95% of the independent lines of sight that pass through protostellar envelopes. Large discrepancies between the integrated B-field mean orientation and the orientation reconstructed from the polarized dust emission are mostly observed in (i) lines of sight where the magnetic field is highly disorganized and (ii) those that probe large column densities. Our analysis shows that the high opacity of the thermal dust emission and low polarization fractions could be used to avoid using the small fraction of measurements affected by large errors.

First results of a new quadrupole minimum-B permanent magnet electron cyclotron resonance ion source

The first experimental results of a novel permanent magnet electron cyclotron resonance ion source (ECRIS) with a minimum-B quadrupole magnetic field topology are presented. Bremsstrahlung measurements indicate that the electrons can reach energies of 150–200 keV. Ion charge states up to Ar11+ and Kr15+ were detected in the charge state distributions of the extracted beams. Measurements in pulsed operation mode reveal an afterglow peak, which implies that high charge state ions are confined electrostatically similar to a conventional ECRIS. The potential applications and future development of the ion source are discussed.

Accretion process, magnetic fields, and apsidal motion in the pre-main sequence binary DQ Tau

ABSTRACT Classical T Tauri stars (CTTSs) are young stellar objects that accrete materials from their accretion disc influenced by their strong magnetic field. The magnetic pressure truncates the disc at a few stellar radii and forces the material to leave the disc plane and fall onto the stellar surface by following the magnetic field lines. However, this global scheme may be disturbed by the presence of a companion interacting gravitationally with the accreting component. This work is aiming to study the accretion and the magnetic field of the tight eccentric binary DQ Tau, composed of two equal-mass (∼ 0.6 M⊙) CTTSs interacting at different orbital phases. We investigated the variability of the system using a high-resolution spectroscopic and spectropolarimetric monitoring performed with ESPaDOnS at the CFHT. We provide the first ever magnetic field analysis of this system, the Zeeman–Doppler imaging revealed a stronger magnetic field for the secondary than the primary (1.2 and 0.5 kG, respectively), but the small-scale fields analysed through Zeeman intensification yielded similar strengths (about 2.5 kG). The magnetic field topology and strengths are compatible with the accretion processes on CTTSs. Both components of this system are accreting, with a change of the main accretor during the orbital motion. In addition, the system displays a strong enhancement of the mass accretion rate at periastron and apastron. We also discovered, for the first time in this system, the apsidal motion of the orbital ellipse.

Classification of orbits in Poincaré maps using machine learning

Poincaré plots, also called Poincaré maps, are used by plasma physicists to understand the behavior of magnetically confined plasma in numerical simulations of a tokamak. These plots are created by the intersection of field lines with a two-dimensional poloidal plane that is perpendicular to the axis of the torus representing the tokamak. A plot is composed of multiple orbits, each created by a different field line as it goes around the torus. Each orbit can have one of four distinct shapes, or classes, that indicate changes in the topology of the magnetic fields confining the plasma. Given the (x, y) coordinates of the points that form an orbit, we want to assign a class to the orbit, a task that appears ideally suited for a machine learning approach. In this paper, we describe how we overcame two major challenges in solving this problem—creating a high-quality training set, with few mislabeled orbits, and converting the coordinates of the points into features that are discriminating, despite the variation within the orbits of a class and the apparent similarities between orbits of different classes. Our automated approach is not only more objective and accurate than visual classification, but is also less tedious, making it easier for plasma physicists to analyze the topology of magnetic fields from numerical simulations of the tokamak.

Optimization of Y3Fe5O12 based layered structures for quasi-optic spin wave elements

We present theoretical and experimental results on the losses and confinement of spin waves (SW) in a Ferromagnetic/Normal Metal/Air/Ferrite multilayer structure. The above structure can be used for the creation of a desired topology of static magnetic field in a thin ferrite film, used as SW waveguide. It is shown that a screening Normal Metal (NMe) layer, inserted between SW waveguide and ferromagnetic (FM) layer, significantly decreases the SW losses caused by a lossy FM. It is also shown that the SW amplitude increases within the air gap (d) between a Ferrite film and NMe layer, when the gap satisfies the relation kd = 1, where k is SW wavenumber. The power flow density at Ferrite/Air interface is maximal when d is equal to a quarter wavelength of SW.

Interplanetary mesoscale observatory (InterMeso): A mission to untangle dynamic mesoscale structures throughout the heliosphere

Mesoscale dynamics are a fundamental process in space physics, but fall within an observational gap of current and planned missions. Particularly in the solar wind, measurements at the mesoscales (100s RE to a few degrees heliographic longitude at 1 au) are crucial for understanding the connection between the corona and an observer anywhere within the heliosphere. Mesoscale dynamics may also be key to revealing the currently unresolved physics regulating particle acceleration and transport, magnetic field topology, and the causes of variability in the composition and acceleration of solar wind plasma. Studies using single-point observations do not allow for investigations into mesoscale solar wind dynamics and plasma variability, nor do they allow for the exploration of the sub-structuring of large-scale solar wind structures like coronal mass ejections (CMEs), co-rotating/stream interaction regions (CIR/SIRs), and the heliospheric plasma sheet. To address this fundamental gap in our knowledge of the heliosphere at these scales, the Interplanetary Mesoscale Observatory (InterMeso) concept employs a multi-point approach using four identical spacecraft in Earth-trailing orbits near 1 au. Varying drift speeds of the InterMeso spacecraft enable the mission to span a range of mesoscale separations in the solar wind, achieving significant and innovative science return. Simultaneous, longitudinally-separated measurements of structures co-rotating over the spacecraft also allow for disambiguation of spatiotemporal variability, tracking of the evolution of solar wind structures, and determination of how the transport of energetic particles is impacted by these variabilities.

Recent progress with observations and models to characterize the magnetic fields from star-forming cores to protostellar disks

In this review article, we aim at providing a global outlook on the progresses made in the recent years to characterize the role of magnetic fields during the embedded phases of the star formation process. Thanks to the development of observational capabilities and the parallel progress in numerical models, capturing most of the important physics at work during star formation; it has recently become possible to confront detailed predictions of magnetized models to observational properties of the youngest protostars. We provide an overview of the most important consequences when adding magnetic fields to state-of-the-art models of protostellar formation, emphasizing their role to shape the resulting star(s) and their disk(s). We discuss the importance of magnetic field coupling to set the efficiency of magnetic processes and provide a review of observational works putting constraints on the two main agents responsible for the coupling in star-forming cores: dust grains and ionized gas. We recall the physical processes and observational methods, which allow to trace the magnetic field topology and its intensity in embedded protostars and review the main steps, success, and limitations in comparing real observations to synthetic observations from the non-ideal MHD models. Finally, we discuss the main threads of observational evidence that suggest a key role of magnetic fields for star and disk formation, and propose a scenario solving the angular momentum for star formation, also highlighting the remaining tensions that exist between models and observations.

Laboratory observation of plasmoid-dominated magnetic reconnection in hybrid collisional-collisionless regime

Magnetic reconnection, breaking and reorganization of magnetic field topology, is a fundamental process for rapid release of magnetic energy into plasmas that occurs pervasively throughout the universe. In natural circumstances, the plasma properties on either side of the reconnection layer are almost asymmetric, in particular for the collision rates that critically determine the underlying reconnection mechanism. To date, all laboratory experiments on magnetic reconnections have been limited to purely collisional or collisionless regimes. Here, we report a well-designed experimental investigation on magnetic reconnections in a hybrid collisional-collisionless regime by interactions between laser-ablated copper and plastic plasmas. We directly observe the topology evolutions of the whole process of this asymmetric magnetic reconnection by highly-resolved proton radiography. Through this, we show that the growth rate of tearing instability in such a hybrid regime is still extremely large, resulting in rapid formation of multiple plasmoids and generation of plasmoid-dominated current sheet.

The Observed Polarization Direction Depending on Geometrical and Kinematic Parameters of Relativistic Jets

The study of the polarization direction is crucial in the issue of restoring the spatial structure of the magnetic field in the active galaxy parsec-scale jets. But, due to relativistic effects, the magnetic field projected onto the celestial sphere in the source reference frame cannot be assumed to be orthogonal to the observed direction of the electric vector in the wave. Moreover, the local axis of the jet component may not coincide with its motion direction, which affects the observed polarization direction. In this article, we analyze the transverse to jet distributions of the electric vector in the wave, obtained as a result of modeling with different jet kinematic and geometrical parameters for a helical magnetic field with a different twist angle and for a toroidal magnetic field in the center, surrounded by a varying thickness sheath, penetrated by a poloidal field. We obtained: 1) the shape of the electric vector transverse distribution depends in a complex way on the angles of the jet axis and the velocity vector with the line of sight; 2) ambiguity in determining the twist direction of the helical magnetic field under using only the distributions of the electric vector in the wave; 3) both considered magnetic field topologies can reproduce both the ``spine-sheath'' polarization structure and individual bright details with the longitudinal to the jet axis polarization direction.

Magnetic field properties in star formation: A review of their analysis methods and interpretation

Linearly polarized emission from dust grains and molecular spectroscopy is an effective probe of the magnetic field topology in the interstellar medium and molecular clouds. The longstanding Davis-Chandrasekhar-Fermi (DCF) method and the recently developed Histogram of Relative Orientations (HRO) analysis and the polarization-intensity gradient (KTH) method are widely used to assess the dynamic role of magnetic fields in star formation based on the plane-of-sky component of field orientations inferred from the observations. We review the advances and limitations of these methods and summarize their applications to observations. Numerical tests of the DCF method, including its various variants, indicate that its largest uncertainty may come from the assumption of energy equipartition, which should be further calibrated with simulations and observations. We suggest that the ordered and turbulent magnetic fields of particular observations are local properties of the considered region. An analysis of the polarization observations using DCF estimations suggests that magnetically trans-to-super-critical and averagely trans-to-super-Alfvénic clumps/cores form in sub-critical clouds. High-mass star-forming regions may be more gravity-dominant than their low-mass counterparts due to higher column density. The observational HRO studies clearly reveal that the preferential relative orientation between the magnetic field and density structures changes from parallel to perpendicular with increasing column densities, which, in conjunction with simulations, suggests that star formation is ongoing in trans-to-sub-Alfvénic clouds. There is a possible transition back from perpendicular to random alignment at higher column densities. Results from observational studies using the KTH method broadly agree with those of the HRO and DCF studies.

A denser and elevated Martian nightside ionosphere during Mars season Ls 180–360°

Using the Martian ionospheric electron density data obtained by the LPW instrument onboard MAVEN spacecraft from Oct. 2014 to Oct. 2018, we analyzed the seasonal behaviors of the Martian nightside ionosphere. It is found that from Mars season Ls 0–180° to Mas season Ls 180–360°, 1) at 200–300 km, the nightside ionospheric electron densities uniformly increased; 2) below 200 km, the electron density profiles rose in altitude, and peak electron densities increased. It is also found that the seasonal behaviors of the Martian nightside ionosphere are related to the crustal magnetic field topology. Larger seasonal variations of nightside ionospheric electron density can be observed in the regions where the crustal magnetic field is weak or quasi-vertical. We found that our current understandings of the Martian nightside ionosphere can only explain part of its seasonal behaviors, while further observations and simulations are needed to explain the remaining part.