Towards synthetic magnetic turbulence with coherent structures

Lübke, Jeremiah; Effenberger, Frederic; Wilbert, Mike; Fichtner, Horst; Grauer, Rainer

doi:10.1209/0295-5075/ad438f

Cited by 4 publications

(2 citation statements)

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“…With the aim of reducing the need for expensive numerical simulations, recent studies (Juneja et al 1994;Cametti et al 1998;Zimbardo et al 2000;Ruffolo et al 2006;Malara et al 2016;Lübke et al 2023Lübke et al , 2024 have been focusing on the development of software that can generate synthetic data of turbulent quantities, e.g., magnetic fields. The general approach used in synthetic turbulence is to avoid solving physical equations numerically, instead using simplified models and algorithms that are able to mimic properties that are characteristic of turbulence.…”

Section: Introductionmentioning

confidence: 99%

“…In this context, two classes of synthetic models have attracted widespread attention: wavelet-based algorithms (e.g., Juneja et al 1994;Cametti et al 1998;Malara et al 2016), which are very efficient in reproducing intermittency in the fields, and Fourier-based sampling procedures (e.g., Zimbardo et al 2000;Ruffolo et al 2006), in which the model can reproduce anisotropy but intermittency is not present. Recently, a combination of both approaches has also been proposed by Lübke et al (2023Lübke et al ( , 2024. BxC does not belong to either of these categories, but, as suggested by the name itself, it uses a completely alternative approach linked to the concept of multiplicative chaos (Kahane 2000a(Kahane , 2000bDurrive et al 2020), which is a wider research field on (hydrodynamic) turbulence (see Rhodes & Vargas 2014 for a review of the topic).…”

Section: Introductionmentioning

confidence: 99%

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BxC Toolkit: Generating Tailored Turbulent 3D Magnetic Fields

Maci,

Keppens,

Bacchini

2024

ApJS

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Turbulent states are ubiquitous in plasmas, and the understanding of turbulence is fundamental in modern astrophysics. Numerical simulations, which are the state-of-the-art approach to the study of turbulence, require substantial computing resources. Recently, attention shifted to methods for generating synthetic turbulent magnetic fields, affordably creating fields with parameter-controlled characteristic features of turbulence. In this context, the BxC toolkit was developed and validated against direct numerical simulations (DNSs) of isotropic turbulent magnetic fields. Here, we demonstrate novel extensions of BxC to generate realistic turbulent magnetic fields in a fast, controlled, geometric approach. First, we perform a parameter study to determine quantitative relations between the BxC input parameters and the desired characteristic features of the turbulent power spectrum, such as the extent of the inertial range, its spectral slope, and the injection and dissipation scale. Second, we introduce in the model a set of structured background magnetic fields, B 0, as a natural and more realistic extension to the purely isotropic turbulent fields. Third, we extend the model to include anisotropic turbulence properties in the generated fields. With all these extensions combined, our tool can quickly generate any desired structured magnetic field with controlled, anisotropic turbulent fluctuations, faster by orders of magnitude with respect to DNSs. These can be used, e.g., to provide initial conditions for DNSs or easily generate synthetic data for many astrophysical settings, all at otherwise unaffordable resolutions.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

BxC Toolkit: Generating Tailored Turbulent 3D Magnetic Fields

Maci,

Keppens,

Bacchini

2024

ApJS

View full text Add to dashboard Cite

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Strong turbulence and magnetic coherent structures in the interstellar medium

Ntormousi,

Vlahos,

Konstantinou

et al. 2024

A&A

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Magnetic turbulence is classified as weak or strong based on the relative amplitude of the magnetic field fluctuations compared to the mean field. These two classes have different energy transport properties. The purpose of this study is to analyze turbulence in the interstellar medium (ISM) based on this classification. Specifically, we examined the ISM of simulated galaxies to detect evidence of strong magnetic turbulence and provide statistics on the associated magnetic coherent structures (MCoSs), such as current sheets, that arise in this context. We analyzed magnetohydrodynamic galaxy simulations with different initial magnetic field structures (either completely ordered or completely random) and recorded statistics on the magnetic field fluctuations ($ B/B_0$) and the MCoSs, which are defined here as regions where the current density surpasses a certain threshold. We also studied the MCoS sizes and kinematics. The magnetic field disturbances in both models follow a log-normal distribution, peaking at values close to unity; this distribution turns into a power law at large values ($ B/B_0 > 1$), which is consistent with strong magnetic turbulence The current densities are widely distributed, with non-power-law deviations from a log-normal at the largest values. These deviating values of the current density define MCoSs. We find that, in both models, MCoSs are fractally distributed in space, with a typical volume-filling factor of about 10 percent, and tend to coincide with peaks of star formation density. Their fractal dimension is close to unity on sub-kiloparsec scales, and between 2 and 3 on larger scales. These values are consistent with MCoSs having a sheet-like or filament-like morphology. Our work challenges the prevailing paradigm of weak magnetic turbulence in the ISM by demonstrating that strong magnetic disturbances can occur even when the initial magnetic field is completely ordered. This strong magnetic turbulence arises self-consistently from differential rotation and supernova feedback. Our findings provide a foundation for a magnetic turbulence description of the galactic ISM that includes strong fluctuations of the magnetic field.

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Superdiffusion of energetic particles at shocks: A Lévy flight model for acceleration

Aerdker,

Merten,

Effenberger

et al. 2024

A&A

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In the heliosphere, power-law particle distributions are observed, for example, upstream of interplanetary shocks, which can result from superdiffusive transport. This non-Gaussian transport regime may be due to intermittent magnetic field structures. Recently, we have shown that a L\'evy flight model reproduces the observed features at shocks: power-law distributions upstream of the shock and enhanced intensities at the shock. In this work, we extend the L\'evy flight model to study the impact of superdiffusive transport on particle acceleration at shocks. We compared the acceleration timescale and spectral slope to Gaussian diffusion and a L\'evy walk model. We solved the fractional transport equation by sampling the number density with the corresponding stochastic differential equation that is driven by an alpha-stable L\'evy distribution. For both Gaussian and superdiffusive transport, we used a modified version of the cosmic ray propagation framework CRPropa 3.2. We obtained the number density and energy spectra for constant and energy-dependent anomalous diffusion, and we find, compared to the case of Gaussian diffusion, harder energy spectra at the shock as well as faster acceleration. The spectral slope is even harder than predicted for L\'evy walks. L\'evy flight models of superdiffusive transport lead to observed features in the heliosphere. We further show that superdiffusive transport impacts the acceleration process by changing the probability of escaping the shock. The flexibility of the L\'evy flight model allows for further studies in the future that can take the shock geometry and magnetic field structure into account.

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Towards synthetic magnetic turbulence with coherent structures

Cited by 4 publications

References 50 publications

BxC Toolkit: Generating Tailored Turbulent 3D Magnetic Fields

BxC Toolkit: Generating Tailored Turbulent 3D Magnetic Fields

Strong turbulence and magnetic coherent structures in the interstellar medium

Superdiffusion of energetic particles at shocks: A Lévy flight model for acceleration

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