Magnetohydrodynamic Simulations Research Articles

Measurement of spherical tokamak plasma compression in the PCS-16 magnetized target fusion experiment

Abstract A sequence of magnetized target fusion (MTF) devices built by General Fusion have compressed magnetically confined deuterium plasmas inside imploding aluminum liners. Here we describe the best-performing compression experiment, PCS-16, which was the fifth of the most recent experiments that compressed a spherical tokamak plasma configuration. In PCS-16 the plasma remained axisymmetric with δBpol/Bpol < 20% to a high radial compression factor (CR > 8) with significant poloidal flux conservation (77% up to CR = 1.7, and ≈30% up to CR = 8.65) and a total compression time of 167 μs. Magnetic energy of the plasma increased from 0.96 kJ poloidal and 17 kJ toroidal to a peak of 1.14 kJ poloidal and 29.9 kJ toroidal during the compression, while thermal energy was in the range of 350 ± 25 J. Plasma equilibrium was a low-β state with βtor ≈ 4% and βpol ≈ 15%. Ingress of impurities from the lithium coated aluminum wall was not a dominant effect. Neutron yield from D-D fusion increased significantly during compression. Thermodynamics during the early phase of compression (CR < 1.7) were consistent with increasing Ohmic heating of the electrons due to a geometric rise in the current density at near-constant resistivity, and with increasing ion cooling that approximately matched ion compressional heating power. Ion cooling by electrons was significant because the electrons were cooler than the ions (Te = 200 eV, Ti = 600 eV). Magnetohydrodynamic simulation was used to model the emergence of instabilities that increased electron thermal transport in the final phase of compression. Conditions for ideal stability were actively maintained during compression through a current ramp applied to the central shaft, and after this current ramp reached its peak two-thirds of the way through compression we measured a transition in plasma behavior across multiple diagnostics.

Extension of the multilevel radiative transfer code PyRaTE to model linear polarization of molecular lines

Linear polarization of spectral lines, commonly known as the Goldreich-Kylafis effect within the star formation community, is one of the most underutilized techniques for probing magnetic fields in the dense and cold interstellar medium. In this study, we implement linear polarization of molecular spectral lines into the multilevel, non-local thermodynamic equilibrium radiative transfer code PyRaTE Different modes of polarized radiation are treated individually, with separate optical depths computed for each polarization direction. Our implementation is valid in the so-called strong magnetic field limit and is exact for either a system satisfying the large-velocity-gradient approximation, and/or for any system with a uniform magnetic field. We benchmark our implementation against analytical results and provide tests for various limiting cases. In agreement with previous theoretical results, we find that in the multilevel case the amount of fractional polarization decreases when compared to the two-level approximation, but this result is subject to the relative importance between radiative and collisional processes. Finally, we post-process an axially symmetric, nonideal magnetohydrodynamic chemo-dynamical simulation of a collapsing prestellar core and provide theoretical predictions regarding the shape (as a function of velocity) of the polarization fraction of $ CO $ during the early stages in the evolution of molecular clouds.

Transport coefficient sensitivities in a semi-analytic model for magnetized liner inertial fusion

Performance of magnetized liner inertial fusion (MagLIF) experiments is highly dependent on transport processes including magnetized heat flows and magnetic flux losses. Magnetohydrodynamic simulations used to model these experiments require a choice of model for the transport coefficients, which are the constants of proportionality relating driving terms, such as temperature gradients and currents, to the associated heat and magnetic field transport. The coefficients have been the subject of repeated recalculation using various methods throughout the years. Using a semi-analytic MagLIF model [McBride and Slutz, Phys. Plasmas 22, 052708 (2015)], we compare models for the transport coefficients provided by Braginskii [Reviews of Plasma Physics, edited by M. A. Leontovich (Consultants Bureau, New York, 1965), Vol. 1, p. 205], Epperlein and Haines [Phys. Fluids 29, 1029 (1986)], Ji and Held [Phys. Plasmas 20, 042114 (2013)], Davies et al. [Phys. Plasmas 28, 012305 (2021)], and Sadler et al. [Phys. Rev. Lett. 126, 075001 (2021)]. The choice of model modifies magnetic-flux losses caused by the Nernst thermoelectric effect and thermal conduction losses. We present simulated results from parameter scans conducted in order to compare the effects of the different models on parameters of interest in MagLIF. In some regions of parameter space, discrepancies of up to 38% are found in integrated quantities like the fusion yield. These results may serve as a guide for experimental validation of the various models, particularly as laser preheat energies and initial axial field strengths are increased on MagLIF experiments.

A Data-driven Magnetohydrodynamic Simulation of the 2011 February 15 Coronal Mass Ejection from Active Region NOAA 11158

We present a boundary data-driven magnetohydrodynamic (MHD) simulation of the 2011 February 15 coronal mass ejection (CME) event of Active Region NOAA 11158. The simulation is driven at the lower boundary with an electric field derived from the normal magnetic field and the vertical electric current measured from the Solar Dynamics Observatory (SDO) Helioseismic Magnetic Imager vector magnetograms. The simulation shows the buildup of a pre-eruption coronal magnetic field that is close to the nonlinear force-free field extrapolation, and it subsequently develops multiple eruptions. The sheared/twisted field lines of the pre-eruption magnetic field show qualitative agreement with the brightening loops in the SDO Atmospheric Imaging Assembly (AIA) hot passband images. We find that the eruption is initiated by the tether-cutting reconnection in a highly sheared field above the central polarity inversion line and a magnetic flux rope with dipped field lines forms during the eruption. The modeled erupting magnetic field evolves to develop a complex structure containing two distinct flux ropes and produces an outgoing double-shell feature consistent with the Solar Terrestrial Relations Observatory B/Extreme UltraViolet Imager observation of the CME. The foot points of the erupting field lines are found to correspond well with the dimming regions seen in the SDO/AIA observation of the event. These agreements suggest that the derived electric field is a promising way to drive MHD simulations to establish the realistic pre-eruption coronal field based on the observed vertical electric current and model its subsequent dynamic eruption.

The Stellar Bar–Dark Matter Halo Connection in the TNG50 Simulations

Stellar bars in disk galaxies grow as stars in near-circular orbits lose angular momentum to their environments, including their dark matter (DM) halo, and transform into elongated bar orbits. This angular momentum exchange during galaxy evolution hints at a connection between bar properties and the DM halo spin λ, the dimensionless form of DM angular momentum. We investigate the connection between halo spin λ and galaxy properties in the presence/absence of stellar bars, using the cosmological magnetohydrodynamic TNG50 simulations at multiple redshifts (0 < z r < 1). We determine the bar strength (or bar amplitude, A 2/A 0), using Fourier decomposition of the face-on stellar density distribution. We determine the halo spin for barred and unbarred galaxies (0 < A 2/A 0 < 0.7) in the center of the DM halo, close to the galaxy’s stellar disk. At z r = 0, there is an anticorrelation between halo spin and bar strength. Strongly barred galaxies (A 2/A 0 > 0.4) reside in DM halos with low spin and low specific angular momentum at their centers. In contrast, unbarred/weakly barred galaxies (A 2/A 0 < 0.2) exist in halos with higher central spin and higher specific angular momentum. The anticorrelation is due to the barred galaxies’ higher DM mass and lower angular momentum than the unbarred galaxies at z r = 0, as a result of galaxy evolution. At high redshifts (z r = 1), all galaxies have higher halo spin compared to those at lower redshifts (z r = 0), with a weak anticorrelation for galaxies having A 2/A 0 > 0.2. The formation of DM bars in strongly barred systems highlights how angular momentum transfer to the halo can influence its central spin.

Thermodynamics of Giant Molecular Clouds: The Effects of Dust Grain Size

The dust grain size distribution (GSD) likely varies significantly across star-forming environments in the Universe, but its impact on star formation remains unclear. This ambiguity arises because the GSD interacts nonlinearly with processes like heating, cooling, radiation, and chemistry, which have competing effects and varying environmental dependencies. Processes such as grain coagulation, expected to be efficient in dense star-forming regions, reduce the abundance of small grains and increase that of larger grains. Motivated by this, we investigate the effects of similar GSD variations on the thermochemistry and evolution of giant molecular clouds (GMCs) using magnetohydrodynamic simulations spanning a range of cloud masses and grain sizes, which explicitly incorporate the dynamics of dust grains within the full-physics framework of the STARFORGE project. We find that grain size variations significantly alter GMC thermochemistry: the leading-order effect is that larger grains, under fixed dust mass, GSD dynamic range, and dust-to-gas ratio, result in lower dust opacities. This reduced opacity permits interstellar radiation field and internal radiation photons to penetrate more deeply. This leads to rapid gas heating and inhibited star formation. Star formation efficiency is highly sensitive to grain size, with an order-of-magnitude reduction when grain size dynamic range increases from 10−3–0.1 μm to 0.1–10 μm. Additionally, warmer gas suppresses low-mass star formation, and decreased opacities result in a greater proportion of gas in diffuse ionized structures.

Measuring Black Hole Light Echoes with Very Long Baseline Interferometry

Light passing near a black hole can follow multiple paths from an emission source to an observer due to strong gravitational lensing. Photons following different paths take different amounts of time to reach the observer, which produces an echo signature in the image. The characteristic echo delay is determined primarily by the mass of the black hole, but it is also influenced by the black hole spin and inclination to the observer. In the Kerr geometry, echo images are demagnified, rotated, and sheared copies of the direct image and lie within a restricted region of the image. Echo images have exponentially suppressed flux, and temporal correlations within the flow make it challenging to directly detect light echoes from the total light curve. In this Letter, we propose a novel method to search for light echoes by correlating the total light curve with the interferometric signal at high spatial frequencies, which is a proxy for indirect emission. We explore the viability of our method using numerical general relativistic magnetohydrodynamic simulations of a near-face-on accretion system scaled to M87-like parameters. We demonstrate that our method can be used to directly infer the echo delay period in simulated data. An echo detection would be clear evidence that we have captured photons that have circled the black hole, and a high-fidelity echo measurement would provide an independent measure of fundamental black hole parameters. Our results suggest that detecting echoes may be achievable through interferometric observations with a modest space-based very long baseline interferometry mission.

Magnetic dissipation in short gamma-ray-burst jets

Aims. Short gamma-ray bursts originate when relativistic jets emerge from the remnants of binary neutron star (BNS) mergers, as observed in the first multi-messenger event GW170817–GRB 170817A, which coincided with a gravitational wave signal. Both the jet and the remnant are believed to be magnetized, and the presence of magnetic fields is known to influence the jet propagation across the surrounding post-merger environment. In the magnetic interplay between the jet and the environment itself, effects due to a finite plasma conductivity may be important, especially in the first phases of jet propagation. We aim to investigate these effects, from jet launching to its final breakout from the post-merger environment. Methods. We used the PLUTO numerical code to perform 2D axisymmetric and full 3D resistive relativistic magnetohydrodynamic (MHD) simulations, employing spherical coordinates with spatial radial stretching. We considered and compared different models for physical resistivity, which must be small but still dominating over the intrinsic numerical dissipation (which yields unwanted smearing of structures in any ideal MHD code). Stiff terms in the current density are treated with IMplicit-EXplicit Runge Kutta algorithms for time-stepping. A Synge-like gas (Taub equation of state) is also considered. All simulations were performed using an axisymmetric analytical model for both the jet propagation environment and the jet injection; we leave the case of jet propagation in a realistic environment (i.e., imported from an actual BNS merger simulation) to a future study. Results. As expected, no qualitative differences are detected due to the effect of a finite conductivity, but significant quantitative differences in the jet structure and induced turbulence are clearly seen in 2D axisymmetric simulations, and we also compare different resistivity models. We see the formation of regions with a resistive electric field parallel to the magnetic field, and nonthermal particle acceleration may be enhanced there. The level of dissipated Ohmic power is also dependent on the various recipes for resistivity. Most of the differences arise before the breakout from the inner environment, whereas once the jet enters the external weakly magnetized environment region, these differences are preserved during further propagation despite the lower grid refinement. Finally, we show and discuss the 3D evolution of the jet within the same environment in order to highlight the emergence of nonaxisymmetric features.

Chaotic magnetic disconnections trigger flux eruptions in accretion flows channeled onto magnetically saturated Kerr black holes

Magnetized accretion flow onto a black hole (BH) may lead to the accumulation of poloidal magnetic flux across its horizon, which for high BH spin can power far-reaching relativistic jets. The BH magnetic flux is subject to a saturation mechanism by means of magnetic flux eruptions involving relativistic magnetic reconnection. Such accretion flows have been described as magnetically arrested disks (MAD) or magnetically choked accretion flows (MCAF). The main goal of this work is to describe the onset of relativistic reconnection and initial development of magnetic flux eruption in accretion flow onto magnetically saturated BHs. We analyzed the results of 3D general relativistic ideal magnetohydrodynamic (GRMHD) numerical simulations in the Kerr metric, starting from weakly magnetized geometrically thick tori rotating either prograde or retrograde. We integrate d large samples of magnetic field lines in order to probe magnetic connectivity with the BH horizon. The boundary between magnetically connected and disconnected domains coincides roughly with enthalpy equipartition. The geometrically constricted innermost part of the disconnected domain develops a rigid structure of magnetic field lines -- rotating slowly and insensitive to the BH spin orientation. The typical shape of innermost disconnected lines is a double spiral converging to a sharp inner tip anchored at the single equatorial current layer. The foot points of magnetic flux eruptions are found to zip around the BH along with other azimuthal patterns. Magnetic flux eruptions from magnetically saturated accreting BHs can be triggered by minor density gaps in the disconnected domain, resulting from the chaotic disconnection of plasma-depleted magnetospheric lines. Accretion flow is effectively channeled along the disconnected lines toward the current layer, and further toward the BH by turbulent cross-field diffusion. Rotation of flux eruption foot points may contribute to the variability of BH crescent images.

A Model for Flux Rope Formation and Disconnection in Pseudostreamer Coronal Mass Ejections

Coronal mass ejections (CMEs) from pseudostreamers represent a significant fraction of large-scale eruptions from the Sun. In some cases, these CMEs take a narrow jet-like form reminiscent of coronal jets; in others, they have a much broader fan-shaped morphology like CMEs from helmet streamers. We present results from a magnetohydrodynamic simulation of a broad pseudostreamer CME. The early evolution of the eruption is initiated through a combination of breakout interchange reconnection at the overlying null point and ideal instability of the flux rope that forms within the pseudostreamer. This stage is characterized by a rolling motion and deflection of the flux rope toward the breakout current layer. The stretching out of the strapping field forms a flare current sheet below the flux rope; reconnection onset there forms low-lying flare arcade loops and the two-ribbon flare footprint. Once the CME flux rope breaches the rising breakout current layer, interchange reconnection with the external open field disconnects one leg from the Sun. This induces a whip-like rotation of the flux rope, generating the unstructured fan shape characteristic of pseudostreamer CMEs. Interchange reconnection behind the CME releases torsional Alfvén waves and bursty dense outflows into the solar wind. Our results demonstrate that pseudostreamer CMEs follow the same overall magnetic evolution as coronal jets, although they present different morphologies of their ejecta. We conclude that pseudostreamer CMEs should be considered a class of eruptions that are distinct from helmet-streamer CMEs, in agreement with previous observational studies.

Structure Formation Through Magnetohydrodynamical Instabilities in Primordial Disks

The shear flow instabilities under the presence of magnetic fields in the primordial disk can greatly facilitate the formation of density structures that serve as seeds prior to the onset of the gravitational Jeans instability. We evaluate the effects of the Parker, magnetorotational and kinematic dynamo instabilities by comparing the properties of these instabilities. We calculate the mass spectra of coagulated density structures by the above mechanism in the radial direction for an axisymmetric magnetohydrodynamic (MHD) torus equilibrium and power density profile models. Our local three-dimensional MHD simulation indicates that the coupling of the Parker and magnetorotational instability creates spiral arms and gas blobs in an accretion disk, reinforcing the theory and model. Such a mechanism for the early structure formation may be tested in a laboratory. The recent progress in experiments involving shear flows in rotating tokamak, field reversed configuration (FRC) and laser plasmas may become a key element to advance in nonlinear studies.

Exploring the Magnetic and Thermal Evolution of a Coronal Jet

Coronal jets are the captivating eruptions that are often found in the solar atmosphere and primarily formed due to magnetic reconnection. Despite their short-lived nature and lower energy compared to many other eruptive events, e.g., flares and coronal mass ejections, they play an important role in heating the corona and accelerating charged particles. However, their generation in the ambience of nonstandard flare regime is not fully understood, and warrant a deeper investigation, in terms of their onset, growth, eruption processes, and thermodynamic evolution. Toward this goal, this paper reports the results of a data-constrained three-dimensional magnetohydrodynamics (MHD) simulation of an eruptive jet; initialized with a non-force-free-field (NFFF) extrapolation and carried out in the spirit of implicit large eddy simulation (ILES). The simulation focuses on the magnetic and dynamical properties of the jet during its onset, and eruption phases, that occurred on 2015 February 5 in an active region NOAA AR12280, associated with a seemingly three-ribbon structure. In order to correlate its thermal evolution with computed energetics, the simulation results are compared with differential emission measurement analysis in the vicinity of the jet. Importantly, this combined approach provides an insight to the onset of reconnection in transients in terms of emission and the corresponding electric current profiles from MHD evolutions. The presented study captures the intricate topological dynamics, finds a close correspondence between the magnetic and thermal evolution in and around the jet location. Overall, it enriches the understanding of the thermal evolution due to MHD processes, which is one of the broader aspects to reveal the coronal heating problem.

Constraints on the accretion properties of quasi-periodic erupters from GRMHD simulations

Some apparently quiescent supermassive black holes (BHs) at centers of galaxies show quasi-periodic eruptions (QPEs) in the X-ray band, the nature of which is still unknown. A possible origin for the eruptions is an accretion disk. However, the properties of such disks are restricted by the timescales of recurrence and the duration of the flares. In this work, we test the possibility that the temporal properties of known QPEs can be explained by accretion from a compact accretion disk with an outer radius out g and we focus on a particular object, GSN 069. We ran several 3D general relativistic magnetohydrodynamic (GRMHD) simulations with the H-AMR code of thin and thick disks and studied how the initial disk parameters such as thickness, magnetic field configuration, magnetization, and Kerr parameter affect the observational properties of QPEs. We show that accretion onto a slowly rotating BH through a small, moderately thin accretion disk with an initially low plasma beta can explain the observed time between outbursts and the lack of evidence for a variable jet emission. In order to form such a disk, the accreting matter should have a low net angular momentum. A potential source for such low angular momentum matter with a quasi-periodic feeding mechanism might be a tight binary of wind-launching stars. Apart from their primary application, our results can also be useful for general studies of systems with small accretion disks, in which evolution occurs very rapidly so that the disks cannot be considered stationary. For such systems, it is important to understand how the initial conditions affect the results.

Connectivity between the solar photosphere and chromosphere in a vortical structure

Context. High-resolution solar observations have revealed the existence of small-scale vortices, as seen in chromospheric intensity maps and velocity diagnostics. Frequently, these vortices have been observed near magnetic flux concentrations, indicating a link between swirls and the evolution of the small-scale magnetic fields. Vortices have also been studied with magneto-hydrodynamic (MHD) numerical simulations of the solar atmosphere, revealing their complexity, dynamics, and magnetic nature. In particular, it has been proposed that a rotating magnetic field structure driven by a photospheric vortex flow at its footprint produces the chromospheric swirling plasma motion. Aims. We present a complete and comprehensive description of the time evolution of a small-scale magnetic flux concentration interacting with the intergranular vortex flow and affected by processes of intensification and weakening of its magnetic field. In addition, we study the chromospheric dynamics associated with the interaction, including the analysis of a chromospheric swirl and an impulsive chromospheric jet. Methods. We studied observations taken with the CRisp Imaging SpectroPolarimeter (CRISP) instrument and the CHROMospheric Imaging Spectrometer (CHROMIS) at the Swedish Solar Telescope (SST) in April 2019. The data were recorded at quiet-Sun disc centre, consisting of full Stokes maps in the Fe I line at 6173 Å and in the Ca II infrared triplet line at 8542 Å, as well as spectroscopic maps in the lines of Hα 6563 Å and Ca II K 3934 Å. Utilising the multi-wavelength data and performing height-dependent Stokes inversion, based on methods of local correlation tracking and wavelet analysis, we studied several atmospheric properties during the event lifetime. This approach allowed us to interpret the spatial and temporal connectivity between the photosphere and the chromosphere. Results. We identified the convective collapse process as the initial mechanism of magnetic field intensification, generating a re-bound flow moving upwards within the magnetic flux concentration. This disturbance eventually steepens into an acoustic shock wave that dissipates in the lower chromosphere, heating it locally. We observed prolonged magnetic field amplification when the vortex flow disappears during the propagation of the upward velocity disturbance. We propose that this type of magnetic field amplification could be attributed to changes in the local vorticity. Our analysis indicates the rotation of a magnetic structure that extends from the photosphere to the chromosphere, anchored to a photospheric magnetic flux concentration. It appears to be affected by a propagating shock wave and its subsequent dissipation process could be related to the release of the jet.

Energy-dependent and Energy-integrated Two-moment General-relativistic Neutrino Transport Simulations of a Hypermassive Neutron Star

We compare two-moment-based energy-dependent and three variants of energy-integrated neutrino transport general-relativistic magnetohydrodynamics simulations of a hypermassive neutron star. To study the impacts due to the choice of the neutrino transport schemes, we perform simulations with the same setups and input neutrino microphysics. We show that the main differences between energy-dependent and energy-integrated neutrino transport are found in the disk and ejecta properties, as well as in the neutrino signals. The properties of the disk surrounding the neutron star and the ejecta in energy-dependent transport are very different from the ones obtained using energy-integrated schemes. Specifically, in the energy-dependent case, the disk is more neutron-rich at early times and becomes geometrically thicker at later times. In addition, the ejecta is more massive and, on average, more neutron-rich in the energy-dependent simulations. Moreover, the average neutrino energies and luminosities are about 30% higher. Energy-dependent neutrino transport is necessary if one wants to better model the neutrino signals and matter outflows from neutron star merger remnants via numerical simulations.

Unveiling the Connection between the Alfvénic Oval and the Open–Closed Field Line Boundary at Ganymede

Alfvén waves generated by the interactions between the Ganymede magnetosphere and the fast-corotating Jovian plasma carry a significant amount of energy. This energy is partially transferred to precipitating particles, which correspond to a feature called the Alfvénic oval. However, the spatial distribution of the Alfvénic oval in the Ganymede system is unclear, and the physical correlation between the Alfvénic oval and the open–closed field line boundary (OCFB) has not been studied quantitatively. Using a high-resolution, three-dimensional global magnetohydrodynamics simulation of Ganymede's magnetosphere, we investigate the spatial distribution of the Alfvénic oval mapped on the ionosphere surface of Ganymede based on upstream measurements from Juno's PJ34 flyby. Our results show that (1) the simulated Alfvénic oval of Ganymede closely resembles the observed aurora oval, and although without kinetic physics, the magnitude of the simulated Afvénic Poynting flux is significantly lower than the estimation based on auroral luminosity; (2) the OCFB aligns closely with the poleward (equatorial) boundary of the enhanced Alfvénic power on the downstream (upstream) side; and (3) the magnetospheric flux tube convection pattern can guide the Alfvénic wave packet to propagate toward latitudes either equatorward or poleward of the OCFB on the downstream/upstream side. These results, presented for the first time in this study, provide testable predictions for both the ongoing Juno mission and future Jovian missions such as JUICE probing the minimagnetosphere system.

Pulsar-wind nebulae meeting the circumstellar media of their progenitors

A significative fraction of high-mass stars sail away through the interstellar medium of the galaxies. Once they evolved and died via a core-collapse supernova, a magnetised, rotating neutron star (a pulsar) is usually left over. The immediate surroundings of the pulsar is the pulsar wind, which forms a nebula whose morphology is shaped by the supernova ejecta and channelled into the circumstellar medium of the progenitor star in the pre-supernova time. Irregular pulsar-wind nebulae display a large variety of radio appearances, screened by their interacting supernova blast wave, or harbour asymmetric up–down emission. Here, we present a series of 2.5-dimensional (2 dimensions for the scalar quantities plus a toroidal component for the vectors) black non-relativistic magneto-hydrodynamical simulations exploring the evolution of the pulsar-wind nebulae generated by a red supergiant and a Wolf-Rayet massive supernova progenitor, moving with Mach number $M=1$ and $M=2$ into the warm phase of the Galactic plane. black In such a simplified approach, the progenitor's direction of motion, the local ambient medium magnetic field, and the progenitor and pulsar axis of rotation, are all aligned; this restricted our study to peculiar pulsar-wind nebula of high-equatorial-energy flux. We find that the reverberation of the termination shock of the pulsar-wind nebulae, when sufficiently embedded into its dead stellar surroundings and interacting with the supernova ejecta, is asymmetric and differs greatly as a function of the past circumstellar evolution of its progenitor, which reflects into their projected radio synchrotron emission. This mechanism is particularly at work in the context of remnants involving slowly moving or very massive stars. We find that the mixing of material in plerionic core-collapse supernova remnants is strongly affected by the asymmetric reverberation in their pulsar-wind nebulae.

What Controls the Dawn‐Dusk Asymmetry of Dayside Alfvénic Power: Interplanetary Magnetic Field or Ionospheric Conductance?

AbstractAlfvén waves are important carriers of solar wind energy into the Earth's magnetosphere and upper atmosphere. The observed distribution of Alfvénic power at low altitude exhibits significant dawn‐dusk asymmetry with different causes and impacts on the dayside and nighside. The nightside asymmetry has been shown to be controlled by the meridional gradient in the ionospheric Hall conductance. It is not clear what controls the dayside asymmetry. Both the ionospheric conductance gradient and orientation of the y‐component of the interplanetary magnetic field (IMF) are possible factors. We examined both possibilities using global magnetohydrodynamic (MHD) simulations. Our results show: (a) The dayside distribution of Alfvénic power is very sensitive to the orientation of IMF By, with enhanced power in the dawn (dusk) sector of the northern hemisphere for positive (negative) By (and opposite in the southern hemisphere); and (b) gradients in ionospheric conductance exert a weaker influence on the dayside asymmetry.

Accreting Neutron Stars in 3D General-relativistic Magnetohydrodynamic Simulations: Jets, Magnetic Polarity, and the Interchange Slingshot

Abstract Accreting neutron stars differ from black holes by the presence of the star’s own magnetic field, whose interaction with the accretion flow is a central component in understanding these systems’ disk structure, outflows, jets, and spin evolution. It also introduces an additional degree of freedom, as the stellar dipole can have any orientation relative to the inner disk’s magnetic field. We present a suite of 3D general-relativistic magnetohydrodynamic simulations in which we investigate the two extreme polarities, with the dipole field being either parallel or antiparallel to the initial disk field, in both the accreting and propeller states. When the magnetosphere truncates the disk near or beyond the corotation radius, most of the system’s properties, including the relativistic jet power, are independent of the star–disk relative polarity. However, when the disk extends well inside the corotation radius, in the parallel orientation the jet power is suppressed and the inner disk is less dense and more strongly magnetized. We suggest a physical mechanism that may account for this behavior—the interchange slingshot—and discuss its astrophysical implications. When the star is in the rapidly accreting regime, which in most cases will be associated with strong spin-up, we expect large observational differences between the two magnetic orientations. This may be reflected in increased variability as the accretion flow drags in successive magnetic structures of varying polarity.

Comparing the interstellar and circumgalactic origin of gas in the tails of jellyfish galaxies

Simulations and observations have found long tails in `jellyfish galaxies', which are commonly thought to originate from ram-pressure stripped gas of the interstellar medium (ISM) in the immediate galactic wake. At larger distances from the galaxy, the long tails have been claimed to form in situ, owing to thermal instability and fast radiative cooling of mixed ISM and intracluster medium (ICM). In this paper, we use magnetohydrodynamical wind tunnel simulations of a galaxy with the arepo code to study the origin of gas in the tails of jellyfish galaxies. To this end, we modelled the galaxy orbit in a cluster by accounting for a time-varying galaxy velocity, ICM density, and the turbulent magnetic field. By tracking gas flows between the ISM, the circumgalactic medium (CGM), and the ICM, we find -- contrary to popular opinion -- that the majority of the gas in the tail originates in the CGM. Prior to the central passage of the jellyfish galaxy in the cluster, the CGM is directly transported to the clumpy jellyfish tail that has been shattered into small cloudlets. After the central cluster passage, gas in the tail originates both from the initial ISM and the CGM, but that from the latter is accreted onto the galactic ISM before being ram-pressure stripped to form filamentary tentacles in the tail. Our simulation shows a declining gas metallicity in the tail as a function of downstream distance from the galaxy. We conclude that the CGM plays an important role in shaping the tails of jellyfish galaxies.