A5026643670- Astrophysics and Star Formation Studies
- Solar and Space Plasma Dynamics
- Stellar, planetary, and galactic studies
- Fluid Dynamics and Turbulent Flows
- Astro and Planetary Science
- Geomagnetism and Paleomagnetism Studies
- Cosmology and Gravitation Theories
- Astrophysical Phenomena and Observations
- Ionosphere and magnetosphere dynamics
- Advanced Combustion Engine Technologies
- Fluid Dynamics and Thin Films
- Theoretical and Computational Physics
- Meteorological Phenomena and Simulations
- Mechanics and Biomechanics Studies
- Chemical Thermodynamics and Molecular Structure
- Combustion and flame dynamics
- Advanced Thermodynamics and Statistical Mechanics
- Characterization and Applications of Magnetic Nanoparticles
- Advanced Battery Materials and Technologies
- Corporate Governance and Management
- Diverse Legal and Medical Studies
- High-pressure geophysics and materials
- Geotechnical and Geomechanical Engineering
- Computational Fluid Dynamics and Aerodynamics
- Copyright and Intellectual Property
Niels Brock
2017-2024
Humboldt-Universität zu Berlin
2022
NanoBioImaging (China)
2020
University of California, Berkeley
2014-2015
University of California, Santa Barbara
2014-2015
Institute for Advanced Study
2011
University of Cambridge
2007-2010
Nordic Institute for Theoretical Physics
2007
AlbaNova
2007
University of Copenhagen
2006
Aims.We study the influence of choice transport coefficients (viscosity and resistivity) on MHD turbulence driven by magnetorotational instability (MRI) in accretion disks.
We present fully three-dimensional local MRI simulations with the object of studying excitation non-axisymmetric spiral density waves that are observed to always be in such simulations. They potentially important for affecting protoplanetary migration through action associated stochastic gravitational forces and producing residual transport MHD inactive regions. The we perform zero net flux produce mean activity levels alpha ~ 0.005. reveal nature mechanism responsible these by determining...
We present the results of numerical 3D magnetohydrodynamic (MHD) simulations with radiative energy transfer fine structure in a small sunspot about 4 Mm width. The show development filamentary structures and flow patterns that are, except for lengths filaments, very similar to those observed. consist gaps reduced field strength relative their surroundings. Calculated synthetic images dark cores like seen observations; are result locally elevated τ = 1 surface. magnetic these is weaker more...
The feasibility of a mean-field dynamo in nonhelical turbulence with superimposed linear shear is studied numerically elongated shearing boxes. Exponential growth the magnetic field at scales much larger than outer scale found. characteristic lB proportional S(-1/2) and rate gamma S, where S rate. This newly discovered effect potentially represents very generic mechanism for generating large-scale fields broad class astrophysical systems spatially coherent mean flows.
Collaboration et al., (2021). The Pencil Code, a modular MPI code for partial differential equations and particles: multipurpose multiuser-maintained. Journal of Open Source Software, 6(58), 2807, https://doi.org/10.21105/joss.02807
In turbulent high-beta astrophysical plasmas (exemplified by the galaxy cluster plasmas), pressure-anisotropy-driven firehose and mirror fluctuations grow nonlinearly to large amplitudes, deltaB/B approximately 1, on a time scale comparable turnover of motions. The principle their nonlinear evolution is generate secularly growing small-scale magnetic that average cancel temporal change in large-scale field responsible for pressure anisotropies. presence may dramatically affect transport...
A rigorous theory for the generation of a large-scale magnetic field by random non-helically forced motions conducting fluid combined with linear shear is presented in analytically tractable limit low Rm and weak shear. The dynamo kinematic due to fluctuations net (volume-averaged) electromotive force. This minimal proof-of-concept quasilinear calculation aiming put dynamo, new effect recently found numerical experiments, on firm theoretical footing. Numerically observed scalings wavenumber...
We study and elucidate the mechanism of spiral density wave excitation in a differentially rotating turbulent flow. formulate set equations with sources that are only non-zero presence fluctuations. solve these shearing box domain using WKBJ method. It is found that, for particular azimuthal length, occurs through sequence regularly spaced swings during which changes from leading to trailing form. This generic process expected occur discs turbulence. Trailing waves equal amplitude...
ABSTRACT Mixtures of gas and dust are pervasive in the Universe, from active galactic nuclei (AGNs) molecular clouds to protoplanetary discs. When two species drift relative each other, a large class instabilities can arise, called ‘resonant drag instabilities’ (RDIs). The most famous RDI is streaming instability, which plays an important role planet formation. On other hand, acoustic RDIs, simplest kind, feature winds cool stars, AGNs, or starburst regions. Unfortunately, owing complicated...
We discuss a numerical 3D radiation-MHD simulation of penumbral fine structure in small sunspot. This shows the development short filamentary structures with horizontal flows, similar to observed Evershed and an inward propagation these at speed compatible observations. Although lengths filaments are much shorter than observed, we conjecture that this qualitatively reproduces mechanisms responsible for filament formation flows penumbrae. conclude flow represents horizontal-flow component...
Abstract Numerical simulations of forced turbulence in elongated shearing boxes are carried out to demonstrate that a nonhelical conjunction with linear shear can give rise mean‐field dynamo. Exponential growth magnetic field at scales larger than the outer (forcing) scale is found. Over range values rate S spanning approximately two orders magnitude, proportional imposed shear, γ ∝ , while characteristic spatial l B –1/2 . The effect quite general: earlier results for nonrotating case by...
Abstract Copper sulfide shows some unique physico‐chemical properties that make it appealing as a cathode active material (CAM) for solid‐state batteries (SSBs). The most peculiar feature of the electrode reaction is reversible formation µm‐sized Cu crystals during cycling, despite its large theoretical volume change (75%). Here, dynamic microstructural evolution CuS cathodes in SSBs studied using situ synchrotron X‐ray tomography. within CAM particles can be clearly followed. This process...
Aims. An efficient algorithm for calculating radiative transfer on massively parallel computers using domain decomposition is presented. Methods. The integral formulation of the equation used to divide problem into a local but compute-intensive part intensity and optical depth integrals, nonlocal communicating between adjacent processors. Results. waiting time idle processors during communication does not have severe impact scaling. wall clock thus scales nearly linearly with inverse number
ABSTRACT The main hurdle of planet formation theory is the metre-scale barrier. One most promising ways to overcome it via streaming instability (SI). Unfortunately, mechanism responsible for onset this remains mysterious. It has recently been shown that SI a Resonant Drag Instability (RDI) involving inertial waves. We build on insight and clarify physical picture how develops, while bolstering with transparent mathematics. Like all RDIs, built feedback loop: in ‘forward action’, an wave...
We study the evolution of a protoplanet few earth masses embedded in protoplanetary disc. If we assume that atmosphere protoplanet, i.e. volume gas hydrostatic equilibrium bound to core, has surface radius smaller than Roche lobe radius, show it expands as accretes both planetesimals and at fixed rate from nebula until fills lobe. The occurs on time-scale shorter formation or migration time-scales. Therefore, conclude protoplanets have an extends surface, where joins nebula. This is true...
We develop an analytic theory to describe spiral density waves propagating in a shearing disc the weakly non-linear regime. Such are generically found be excited simulations of turbulent accretion discs, particular if said turbulence arises from magneto-rotational instability (MRI). derive modified Burgers equation governing their dynamics, which includes effects steepening, dispersion and bulk viscosity support shocks. solve this approximately obtain sawtooth solutions that asymptotically...
ABSTRACT The linear response of a stellar system’s gravitational potential to perturbing mass comprises two distinct contributions. Most famously, the system will respond by forming polarization ‘wake’ around perturber. At same time, perturber may also excite one or more ‘Landau modes’, i.e. coherent oscillations entire which are either stable unstable depending on parameters. amplitude first (wake) contribution is known diverge as approaches marginal stability. In this paper, we consider...
Orbital advection is a significant bottleneck in disk simulations, and particularly tricky one when used connection with magnetohydrodynamics. We have developed an orbital algorithm suitable for the induction equation magnetic potential. The electromotive force split into shear term, we find that do not need advective gauge as solving implicitly precludes term from canceling term. prove demonstrate third order time accuracy of scheme. also suited non-magnetic problems. Benchmarked results...
We study the linear stability of weakly magnetized differentially rotating plasmas in both collisionless kinetic theory and Braginskii's collisional, plasmas. focus on very limit which β ≳ ωc/Ω, where is ratio thermal to magnetic energy ωc/Ω cyclotron frequency rotation frequency. This regime important for understanding how astrophysical fields originate are amplified at high redshift. show that single instability fluid – magnetorotational mediated by tension replaced two distinct...
We derive the conductivity tensor for axisymmetric perturbations of a hot, collisionless, and charge-neutral plasma in shearing sheet approximation. Our results generalize well-known linear Vlasov theory uniform plasmas to differentially rotating can be used wide range kinetic stability calculations. apply these magneto-rotational instability (MRI) collisionless plasmas. show analytically numerically how general derived here reduce appropriate limits previous literature, including...
The magnetorotational instability (MRI) is the most likely source of MHD turbulence in accretion disks. Recently, it has been realized that microscopic diffusion coefficients (viscosity and resistivity) are important determining saturated state thereby rate angular momentum transport. In this paper, we use a set numerical simulations performed with variety methods to investigate dependance α, transport, on these coefficients. We show α an increasing function magnetic Prandtl number Pm, ratio...
The traditional Chandrasekhar picture of the slow relaxation stellar systems assumes that stars' orbits are only modified by occasional, uncorrelated, two-body flyby encounters with other stars. However, long-range nature gravity means in reality large numbers stars can behave collectively. In stable this collective behaviour (i) amplifies noisy fluctuations system's gravitational potential, effectively 'dressing' (star-star) encounters, and (ii) allows system to support large-scale density...
The main hurdle of planet formation theory is the metre-scale barrier. One most promising ways to overcome it via streaming instability (SI). Unfortunately, mechanism responsible for onset this remains mysterious. It has recently been shown that SI a Resonant Drag Instability (RDI) involving inertial waves. We build on insight and clarify physical picture how develops, while bolstering with transparent mathematics. Like all RDIs, built feedback loop: in `forward action', an wave concentrates...