An MHD solution of zero magnetic field can be unstable to the dynamo instability. It plays the perhaps most important role in virtually all of astrophysics! But it requires kinetic energy which can be tapped and converted into magnetic energy. In a rotating body, shear may sometimes be important to produce an excess of toroidal field over poloidal. In the Sun, shear is the result of...
There are many observations of sunspots, but few attempts at sunspot simulations. Rempel (2012) presented realistic magneto-hydrodynamic (MHD) sunspot simulations. Jurčák et al. (2020) showed that the magnetic field of such simulations differs from observations; in particular, the $B_\mathrm{ver}$ at the umbral boundary is too low.
Using the MURaM MHD code and a potential field top boundary...
Early high-cadence chromospheric image sequences recorded with the recently commissioned Daniel K. Inouye Solar Telescope (DKIST) have revealed propagating arc-shaped bright fronts seen to originate from chromospheric bright grains. Prior to the appearance of the bright fronts, we observe vortical flows in the photosphere located underneath the chromospheric bright grains.
Corresponding image...
First, we discuss a model of the solar dynamo that explain its various periodicities on widely different time scales in a self-consistent manner. Starting with Rieger-type periodicities, we show that the two-planet spring tides of Venus, Earth and Jupiter are able to excite magneto-Rossby waves in the solar tachocline with periods between 100 and 300 days and amplitudes of m/s or even more. We...
The dynamo problem, in its simplest form, consists of identifying and quantifying material flows which lead to amplification of magnetic fields when inserted into the MHD induction equation [1]. In this connection, we focus on solar meridional flows and flow speeds, which may dictate the timing, amplitude, and shape of magnetic cycles in flux transport dynamo models [2]. While organized...
Solar magnetic activity is very crucial to understand as it mediates all aspects of space weather. The most successful dynamo model to explain various features of the solar magnetic cycle (e.g., equatorward migration of sunspots and 11-year periodicity) is the Babcock-Leighton Dynamo model. In this dynamo model, we solve kinematic MHD equations by providing the observed mean flows of the Sun...
In this study, we simulate 30000 years of solar activity using turbulent-alpha (TA) and Babcock-Leighton (BL) mechanisms in a non-kinematic nonlinear mean field flux-transport solar dynamo model. We evaluate their performances against observational data from proxies, like 14C, and direct solar observations. The TA and BL dynamos generate Schwabe-like variations, with the TA dynamo also...
Besides a dense coverage of their high latitudes by starspots, rapidly rotating cool stars also display low-latitude spots in Doppler images, although generally with a lower coverage. In contrast, flux emergence models of fast-rotating stars predict strong poleward deflection of radially rising magnetic flux as the Coriolis effect dominates over buoyancy, leaving a spot-free band around the...
Numerical models of the multiphase interstellar medium (ISM) in disk galaxies have recently underlined the importance of cosmic rays (CRs) and magnetic fields for the physics of the ISM. Thus, magnetic fields and CRs are important contributions in order to understand the large scale distribution of the ISM and its evolution. New observational evidence from radio-continuum studies of edge-on...
It is generally accepted that radio relics are the result of synchrotron emission from shock-accelerated electrons. Current models, however, are still unable to explain several aspects of their formation. In this paper, we focus on three outstanding problems: i) Mach number estimates derived from radio data do not agree with those derived from X-ray data, ii) cooling length arguments imply a...
Magnetic fields are observed on virtually all astrophysical scales of the modern Universe, from planets and stars to galaxies and galaxy clusters. Observations of blazars suggest that even the intergalactic medium is permeated by magnetic fields. Such large-scale fields were most likely generated very shortly after the Big Bang and therefore are a unique window into the physics of the very...
Observations of Faraday rotation and synchrotron emission suggest galaxy clusters harbor large-scale magnetic fields potentially extending to redshift $z=4$. Non-radiative cosmological simulations show slower magnetic growth, while our MHD simulations with galaxy formation physics reveal accelerated amplification. We identify three key phases: (1) High-redshift magnetization of the...
Astrophysical outflows are seen in objects ranging from compact binaries up to active galactic nuclei, and magnetized accretion disks are the central engines behind these phenomena. Disk turbulence has a profound effect on the evolution of the large-scale magnetic field and hence on the ability of the system to power its outflows. We aim to characterize the turbulence coefficients emerging in...
Transition disks (TDs) are a type of protoplanetary disk characterized by an inner dust and gas cavity. The processes behind how these cavities are formed and maintained, along with the observed high accretion rates, continue to be subjects of active research. In our work, we aim to investigate if and how the inclusion of the Hall Effect alongside Ohmic Resistivity and Ambipolar Diffusion...
Tayler instability of toroidal magnetic fields is broadly invoked as a trigger for turbulence and angular momentum transport in stars. I will discuss a recent systematic revision of the linear stability analysis. A new physical picture has merged where diffusive processes enable instability by causing the overstability of two classes of waves: inertial waves and magnetostrophic waves. The new...
The stability of toroidal magnetic fields in radiative stellar interiors is crucial for understanding the rotational and chemical evolution of low-mass stars. This study examines the roles of gravity and thermal conductivity on Tayler instabilities within stably stratified stellar interiors. Although it is often argued that the instability is most effective at very short radial length scales...
The stability of toroidal magnetic fields in radiative stellar interiors is a key unresolved problem in advancing our understanding of the rotational and chemical evolution of low-mass stars. We perform 3D direct numerical simulations in a spherical geometry to examine the Tayler instability, a kink-type instability of purely toroidal fields expected to occur in stably stratified stellar...
According to current findings, the self-organized creation and development of our solar system took place under the diverse influence of magnetic (e. g. dynamo, reconnection, accretion, acceleration) processes. In the context of magnetohydrodynamics, these will be discussed initially at a glance and exemplarily for protostellar systems (protostars, protostellar/protoplanetary disks and winds...
Liquid metals such as lead lithium (PbLi) are foreseen in future fusion reactors as coolant, heat transfer medium, and breeder material for generation of tritium, one of the plasma fuel components. The motion of an electrically conducting fluid in the plasma-confining magnetic field gives rise to induced currents and electromagnetic forces that significantly alter the flow behavior compared to...
A vertical magnetic field delays the onset of Rayleigh-Bénard convection in a conducting liquid in agreement with Lenz' law. An insulating sidewall impedes the spreading of induced eddy currents, whereby the suppression of convection becomes less effective there. This mechanism causes convection to appear near the walls of the container while it remains suppressed in the bulk. Busse developed...
In blankets for future fusion reactors, liquid metals have been chosen as breeding material, heat carrier and neutron multiplier. The use of electrically conducting fluids in a magnetic environment comes with the need of investigating their magnetohydrodynamic (MHD) interaction with the imposed magnetic field. The prediction of MHD phenomena for the development of liquid metal blankets...
Rayleigh-Bénard convection [1-2], i.e. heat and momentum transport driven by thermally induced buoyancy force, occurs between two horizontal parallel plates which are heated from the bottom and cooled from the top. It is deemed as a paradigmatic in the study of thermal fluid flow. In the past decades, magnetohydrodynamic RB convection [3] and radiative RB convection [4] have received a great...
Magnetohydrodynamic (MHD) convective flows have been well investigated theoretically and experimentally over the past few decades. However, studies on magneto-convection in closed geometries with heat transfer at internal obstacles has found less attention. In this work, we study the convective MHD flow in an engineering-relevant model geometry featuring a long rectangular cavity with a...
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Neutron stars have the strongest magnetic fields in the universe, with fields up to 10^15 G in so-called magnetars. At such field strengths the classical Hall effect becomes one of the dominant effects governing the evolution of the magnetic fields. One interesting aspect of the Hall effect is that it depends on the sign of the field, unlike virtually all of classical MHD, where the sign of...
A magnetohydrodynamic dynamo process is believed to occur in the
interior of the Sun or stars as well as in planets and smaller
celestial bodies like the ancient Moon or the asteroid Vesta,
motivating similar studies in laboratory settings. Currently, a new
dynamo experiment is under construction at Helmholtz-Zentrum
Dresden-Rossendorf (HZDR), in which liquid sodium will be forced by...
We carried out ultrasonic Doppler and power consumption measurements in a 1:6 downscaled water experiment to analyze the potential for dynamo action in a precession driven sodium experiment [1]. The experiments are embedded in the DRESDYN (DREsden Sodium facility for DYNamo and thermohydraulic studies) project devoted to a better understanding of the homogeneous dynamo effect in planets and...
Magnetorotational instability (MRI) is the most likely candidate driving angular momentum transport in astrophysical disks. Due to its great importance, there have been considerable efforts to capture MRI in the laboratory. Despite recent promising results, a definitive experimental evidence of MRI is, however, still elusive. I will present our preparatory theoretical study for upcoming...
The numerical representation of fully developed high-Reynolds-
number turbulence, particularly in the magnetohydrodynamic approximation,
represents a formidable challenge. We report on an interesting modelling
approach employing a network-based representation that shares fundamental
characteristics with port-Hamiltonian structures known from model reduction.
We show that this technique is...
We introduce a new method for exact decomposition of propagating, nonlinear magnetohydrodynamic (MHD) disturbances into their component eigenenergies associated with the familiar slow, Alfvén, and fast wave eigenmodes, and the entropy and field-divergence pseudoeigenmodes. First, the mathematical formalism is introduced, where it is illustrated how the ideal-MHD eigensystem can be used to...
Magnetized turbulence is ubiquitous in many astrophysical and terrestrial plasmas but no universal theory exists. Even in the simplest plasma approximation, magnetohydrodynamics (MHD), the detailed energy dynamics are still not well understood. In this talk, I present a suite of idealized MHD turbulence simulations that only vary in their dynamical range, i.e., in their separation between the...
Astrophysical systems, such as the solar atmosphere, can be modeled using various plasma descriptions. Magnetohydrodynamics (MHD) offers a less computationally expensive approach but may fail to capture small-scale phenomena, like magnetic reconnection, that require more sophisticated kinetic models. Since the regions requiring complex models are often much smaller than the rest of the domain,...
Plasma turbulence is a widespread phenomenon that is important in many astrophysical systems. It can be described as the superposition of Alfvén wave packets on various scales in space and time, which interact with each other non-linearly, giving rise to the direct energy cascade in 3D incompressible MHD turbulence. We study the temporal and spatial properties of the energy transfer process by...
Turbulent flows are believed to be present in the solar corona, especially in connection with solar flares and coronal mass ejections. They are supposed to be very effective processes in energy transportation and can contribute to the heating of the solar corona. We study turbulence in reconnection outflows associated with flares and CMEs. We simulate the generation and evolution of the...
During their early formation stages, massive stars are surrounded by accretion disks and launch powerful magnetically-driven jets and molecular outflows. Observing the innermost (embedded) material surrounding a forming massive star has only been possible recently, thanks to the use of techniques like very-long-baseline interferometry (VLBI). In this contribution, I will present a new...
Accretion disks and astrophysical jets are typically found in several sources, e.g., young stellar objects, X-ray binaries, gamma-ray bursts, or active galactic nuclei. The origin and impact of a large-scale magnetic field on the dynamical and radiative features of disks and jets is still unclear. First, I will briefly discuss dynamo processes within thin accretion disks, able to amplify the...
Stellar feedback shapes its environment from its local cloud to his own hosting galaxy. It combines several physical processes, such as the formation of magnetically-driven jets, irradiation and photoionization which restrain its growth and final mass. In this work, we include the MHD jet contribution self-consistently while adding radiative forces and photoionization. Our goal is to...
Magnetic reconnection is a fundamental process in astrophysical plasmas, influencing the dynamics, heating, and acceleration of particles in protostellar jets. This process often involves the conversion of magnetic energy into thermal and kinetic energy. In particular, the non-Maxwellian nature of astrophysical plasmas, necessitates the use of more accurate kinetic models to capture the plasma...
In this work we investigate the early stages of the formation of disks around protostars. We present 2D axially symmetric MHD simulations of the collapse of a rotating molecular cloud with an initial density distribution following a Bonnor-Ebert-sphere into the 1st and 2nd hydrostatic Larson core. We perform a parameter study for a range of initial cloud masses and aim to simulate the first...
Transition disks (TDs) are a type of protoplanetary disk characterized by an inner dust and gas cavity. The processes behind how these cavities are formed and maintained, along with the observed high accretion rates, continue to be subjects of active research. In our work, we aim to investigate if and how the inclusion of the Hall Effect alongside Ohmic Resistivity and Ambipolar Diffusion...
Transition disks (TDs) are a type of protoplanetary disk characterized by an inner dust and gas cavity. The processes behind how these cavities are formed and maintained, along with the observed high accretion rates, continue to be subjects of active research. In our work, we aim to investigate if and how the inclusion of the Hall Effect alongside Ohmic Resistivity and Ambipolar Diffusion...
Decaying magnetohydrodynamic (MHD) turbulence is important in various astrophysical contexts,
including early universe magnetic fields, star formation, turbulence in galaxy clusters,
magnetospheres and solar corona. I will talk about how decaying MHD turbulence displays the
phenomenon of inverse energy transfer. Our work has uncovered the important role of magnetic
reconnection in driving...
Plasma turbulence is a widespread phenomenon in astrophysical systems. However, three-dimensional simulations of these systems with realistic parameter values present a significant challenge due to the extensive spectral bandwidth of nonlinearly interacting fluctuations within turbulent flows.
To address this, model reduction techniques have been employed to facilitate a more cost-effective...
