Robert Bleckert Nils Håkan Wissing

• Wissing, Robert Bleckert Nils Håkan & Shen, Sijing (2023). Numerical dependencies of the galactic dynamo in isolated galaxies with SPH. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 673. doi: 10.1051/0004-6361/202244753. Full text in Research Archive Show summary

  Simulating and evolving magnetic fields within global galaxy simulations provides a large tangled web of numerical complexity due to the vast amount of physical processes involved. Understanding the numerical dependencies that act on the galactic dynamo is a crucial step in determining what resolution and conditions are required to properly capture the magnetic fields observed in galaxies. Here, we present an extensive study on the numerical dependencies of the galactic dynamo in isolated spiral galaxies using smoothed particle magnetohydrodynamics. We performed 53 isolated spiral galaxy simulations with different initial setups, feedback, resolution, Jeans floor, and dissipation parameters. The results show a strong mean-field dynamo occurring in the spiral-arm region of the disk, likely produced by the classical alpha-omega dynamo or the recently described gravitational instability dynamo. The inclusion of feedback is seen to work in both a destructive and positive fashion for the amplification process. Destructive interference for the amplification occurs due to the breakdown of filament structure in the disk, the increase of turbulent diffusion, and the ejection of magnetic flux from the central plane to the circumgalactic medium. The positive effect of feedback is the increase in vertical motions and the turbulent fountain flows that develop, showing a high dependence on the small-scale vertical structure and the numerical dissipation within the galaxy. Galaxies with an effective dynamo saturate their magnetic energy density at levels between 10 and 30% of the thermal energy density. The density-averaged numerical Prandtl number is found to be below unity throughout the galaxy for all our simulations, with an increasing value with radius. Assuming a turbulent injection length of 1 kpc, the numerical magnetic Reynolds number is within the range of Re mag  = 10 − 400, indicating that some regions are below the levels required for the small-scale dynamo (Re mag, crit  = 30 − 2700) to be active.

• García-Senz, Domingo; Wissing, Robert Bleckert Nils Håkan; Cabezón, R. M.; Vurgun, E. & Linares, Manuel (2022). Axisymmetric smoothed particle magnetohydrodynamics. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 518(3), p. 4115–4131. doi: 10.1093/mnras/stac3328. Full text in Research Archive Show summary

  Many astrophysical and terrestrial scenarios involving magnetic fields can be approached in axial geometry. Although the smoothed particle hydrodynamics (SPH) technique has been successfully extended to magnetohydrodynamics (MHD), a well-verified, axisymmetric MHD scheme based on such technique does not exist yet. In this work, we fill that gap in the scientific literature and propose and check a novel axisymmetric MHD hydrodynamic code, that can be applied to physical problems which display the adequate geometry. We show that the hydrodynamic code built following these axisymmetric hypothesis is able to produce similar results than standard 3D-SPMHD codes with equivalent resolution but with much lesser computational load.

• Wissing, Robert; Shen, Sijing; Wadsley, James & Quinn, Thomas (2022). Magnetorotational instability with smoothed particle hydrodynamics. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 659. doi: 10.1051/0004-6361/202141206. Full text in Research Archive Show summary

  The magnetorotational instability (MRI) is an important process in driving turbulence in sufficiently ionized accretion disks. It has been extensively studied using simulations with Eulerian grid codes, but remains fairly unexplored for meshless codes. Here, we present a thorough numerical study on the MRI using the smoothed particle magnetohydrodynamics method with the geometric density average force expression. We performed 37 shearing box simulations with different initial setups and a wide range of resolution and dissipation parameters. We show, for the first time, that MRI with sustained turbulence can be simulated successfully with smoothed-particle hydrodynamics (SPH), with results consistent with prior work with grid-based codes, including saturation properties such as magnetic and kinetic energies and their respective stresses. In particular, for the stratified boxes, our simulations reproduce the characteristic “butterfly” diagram of the MRI dynamo with saturated turbulence for at least 100 orbits. On the contrary, traditional SPH simulations suffer from runaway growth and develop unphysically large azimuthal fields, similar to the results from a recent study with meshless methods. We investigated the dependency of MRI turbulence on the numerical Prandtl number ( P m ) in SPH, focusing on the unstratified, zero net-flux case. We found that turbulence can only be sustained with a Prandtl number larger than ∼2.5, similar to the critical values for the physical Prandtl number found in grid-code simulations. However, unlike grid-based codes, the numerical Prandtl number in SPH increases with resolution, and for a fixed Prandtl number, the resulting magnetic energy and stresses are independent of resolution. Mean-field analyses were performed on all simulations, and the resulting transport coefficients indicate no α -effect in the unstratified cases, but an active αω dynamo and a diamagnetic pumping effect in the stratified medium, which are generally in agreement with previous studies. There is no clear indication of a shear-current dynamo in our simulation, which is likely to be responsible for a weaker mean-field growth in the tall, unstratified, zero net-flux simulation.

• Wissing, Robert & Shen, Sijing (2020). Smoothed particle magnetohydrodynamics with the geometric density average force expression. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 638. doi: 10.1051/0004-6361/201936739. Full text in Research Archive Show summary

  We present a novel method of magnetohydrodynamics (MHD) within the smoothed particle hydrodynamics scheme (SPMHD) using the geometric density average force expression. Geometric density average within smoothed particle hydrodynamics (GDSPH) has recently been shown to reduce the leading order errors and greatly improve the accuracy near density discontinuities, eliminating surface tension effects. Here, we extend the study to investigate how SPMHD benefits from this method. We implement ideal MHD in the GASOLINE2 and CHANGA codes with both GDSPH and traditional smoothed particle hydrodynamics (TSPH) schemes. A constrained hyperbolic divergence cleaning scheme was employed to control the divergence error and a switch for artificial resistivity with minimized dissipation was also used. We tested the codes with a large suite of MHD tests and showed that in all problems, the results are comparable or improved over previous SPMHD implementations. While both GDSPH and TSPH perform well with relatively smooth or highly supersonic flows, GDSPH shows significant improvements in the presence of strong discontinuities and large dynamic scales. In particular, when applied to the astrophysical problem of the collapse of a magnetized cloud, GDSPH realistically captures the development of a magnetic tower and jet launching in the weak-field regime, while exhibiting fast convergence with resolution, whereas TSPH failed to do so. Our new method shows qualitatively similar results to those of the meshless finite mass/volume schemes within the GIZMO code, while remaining computationally less expensive.

• Wissing, Robert & Hobbs, David (2020). A new equation of state applied to planetary impacts: I. Models of planetary interiors. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 635. doi: 10.1051/0004-6361/201935814. Full text in Research Archive Show summary

  We present a new analytical equation of state (EOS), which correctly models high pressure theory and fits well to the experimental data of ɛ-Fe, SiO2, Mg2, and the Earth. The cold part of the EOS is modeled after the Varpoly EOS. The thermal part is based on a new formalism of the Gruneisen parameter, which improves behavior from earlier models and bridges the gap between elasticity and thermoelasticity. The EOS includes an expanded state model, which allows for the accurate modeling of material vapor curves. The EOS is compared to both the Tillotson EOS and ANEOS model, which are both widely used in planetary impact simulations. The complexity and cost of the EOS is similar to that of the Tillotson EOS, while showing improved behavior in every aspect. The Hugoniot state of shocked silicate material is captured relatively well and our model reproduces vapor curves similar to that of the ANEOS model. To test its viability in hydrodynamical simulations, the EOS was applied to the lunar-forming impact scenario and the results are presented in Paper II and show good agreement with previous work.

• Wissing, Robert & Hobbs, David (2020). A new equation of state applied to planetary impacts: II. Lunar-forming impact simulations with a primordial magma ocean. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 643. doi: 10.1051/0004-6361/201936227. Full text in Research Archive Show summary

  Observed FeO/MgO ratios in the Moon and Earth are inconsistent with simulations done with a single homogeneous silicate layer. In this paper we use a newly developed equation of state to perform smoothed particle hydrodynamics simulations on the lunar-forming impact, testing the effect of a primordial magma ocean on Earth. This is investigated using the impact parameters of both the canonical case, in which a Mars-sized impactor hits a non-rotating Earth at an oblate angle, and the fast-rotating case, in which a half-sized Mars impactor hits a fast-spinning Earth head-on. We find that the inclusion of a magma ocean results in a less massive Moon and leads to slightly more mixing. Additionally, we test how an icy Theia would affect the results and find that this reduces the probability of a successful Moon formation. Simulations of the fast-spinning case are found to be unable to form a massive-enough Moon.

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• Wissing, Robert Bleckert Nils Håkan & Shen, Sijing (2022). Simulating Galactic Dynamo Processes with Smoothed Particle Magnetohydrodynamics. Universitetet i Oslo. Full text in Research Archive Show summary

  When we look out at the universe, we find that magnetic fields are everywhere. From solar flares to the launching of jets, magnetic fields play an important role for a wide range of systems. This includes galaxies, which have been found to host surprisingly strong magnetic fields compared to their predicted strength in the early universe. This means that there must have been significant amplification of the magnetic field during the evolution of these galaxies. But what kind of process could amplify the magnetic field this quickly? Enter magnetic dynamo processes, which describes the exponential growth of magnetic fields due to being stretched, twisted, and folded by the underlying fluid motions. Due to astronomy being an observational science, an important approach to testing astrophysical theories is through the use of numerical simulations. This gives us a ’virtual’ lab to understand how a system will evolve in time. This thesis goes through the development of a new numerical method and its application to the magnetized core-collapse, magneto-rotational instability and galaxy formation. In our simulations of galaxy formation, we find that our new method can capture many of the dynamo processes, recreating the amplification needed to explain our observations.

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