Article
Astronomy & Astrophysics
P. J. Kaepylae
Summary: Numerical simulations show that fully convective stars exhibit different patterns of differential rotation and magnetic fields at different rotation rates, similar to partially convective late-type stars. This suggests that the processes generating these phenomena are independent of the star's geometry.
ASTRONOMY & ASTROPHYSICS
(2021)
Article
Astronomy & Astrophysics
M. Viviani, M. J. Kaepylae
Summary: By using a more realistic treatment of heat conductivity, the discrepancies between observations and simulations in global magnetoconvection of solar-like stars have been alleviated to some extent. However, there are still differences between the models and observations in certain aspects.
ASTRONOMY & ASTROPHYSICS
(2021)
Article
Astronomy & Astrophysics
C. A. Ortiz-Rodriguez, P. J. Kaepylae, F. H. Navarrete, D. R. G. Schleicher, R. E. Mennickent, J. P. Hidalgo, B. Toro-Velasquez
Summary: This study explores the magnetic and flow properties of fully convective M dwarfs using a star-in-a-box set-up. The results show systematic differences in the behavior of the large-scale magnetic field as functions of rotation period and magnetic Prandtl number.
ASTRONOMY & ASTROPHYSICS
(2023)
Article
Astronomy & Astrophysics
Suoqing Ji, Jim Fuller, Daniel Lecoanet
Summary: The Tayler instability is a poorly studied magnetohydrodynamic instability that is likely to occur in stellar interiors. The non-linear saturation of the instability is not well understood, but it has crucial consequences for dynamo action and angular momentum transport in radiative regions of stars.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2023)
Article
Astronomy & Astrophysics
A. P. Prabhu, N. K. Singh, M. J. Kapyla, A. Lagg
Summary: This study presents a method to infer magnetic helicity spectra in spherical geometry directly from observations of the magnetic field, providing important information about the alpha effect. The formalism has many applications in solar and stellar observations, as well as in analyzing global magnetoconvection models of stars.
ASTRONOMY & ASTROPHYSICS
(2021)
Article
Astronomy & Astrophysics
Emma Kaufman, Daniel Lecoanet, Evan H. Anders, Benjamin P. Brown, Geoffrey M. Vasil, Jeffrey S. Oishi, Keaton J. Burns
Summary: This paper presents numerical simulatons showing a long-time scale, linear instability of the Prendergast magnetic field in massive main-sequence stars. The instability is robust to changes in boundary conditions and is not stabilized by strong stable stratification. The growth rate of the instability has a power-law dependence on resistivity, decreasing as resistivity decreases. The instability is sufficiently rapid to destabilize the magnetic field on time-scales shorter than the stellar evolution time-scale.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2022)
Article
Astronomy & Astrophysics
N. Kleeorin, I. Rogachevskii, N. Safiullin, R. Gershberg, S. Porshnev
Summary: Our research suggests that for low-mass, fast-rotating main sequence stars, the generated large-scale magnetic field is influenced by the mean-field alpha(2)Omega dynamo, modified by a weak differential rotation. The behavior of the magnetic activity changes significantly from aperiodic to nonlinear oscillations and even chaotic behavior with an increase in differential rotation. The periods of the magnetic cycles decrease as the differential rotation increases, ranging from tens to thousands of years. This long-term behavior may be related to the characteristic time of the evolution of the magnetic helicity density of the small-scale field.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2023)
Article
Astronomy & Astrophysics
D. Martinez-Gomez, B. Popescu Braileanu, E. Khomenko, P. Hunana
Summary: The ionisation degree and the interaction between charged and neutral species affect the generation and amplification of magnetic fields during instability. The collisions between fluids and the total plasma density also play a role in determining the behavior of magnetic field generation. Single-fluid models may overestimate the interaction between charged and neutral species compared to two-fluid models.
ASTRONOMY & ASTROPHYSICS
(2021)
Article
Astronomy & Astrophysics
Robert Wissing, Sijing Shen
Summary: Simulating and evolving magnetic fields within global galaxy simulations is complex due to the vast amount of physical processes involved. Understanding the numerical dependencies of the galactic dynamo is crucial in capturing the observed magnetic fields. In this study, we performed simulations on isolated spiral galaxies using smoothed particle magnetohydrodynamics to investigate these dependencies. The results show the occurrence of a mean-field dynamo in the spiral-arm region, with both destructive and positive effects of feedback. Galaxies with an effective dynamo saturate their magnetic energy density at levels between 10 and 30% of the thermal energy density. The numerical Prandtl number is below unity throughout the galaxy, with an increasing value with radius. The numerical magnetic Reynolds number indicates that some regions may not have the required levels for the small-scale dynamo to be active.
ASTRONOMY & ASTROPHYSICS
(2023)
Article
Astronomy & Astrophysics
Felipe H. Navarrete, Petri J. Kaepylae, Dominik R. G. Schleicher, Carolina A. Ortiz, Robi Banerjee
Summary: This study revisits the connection between stellar magnetic fields and the gravitational quadrupole moment Q(xx), and compares different dynamo-generated ETV models through simulations. The results show that the variations in the magnetic field structure are associated with changes in Q(xx), and different rotation periods of the simulations exhibit different magnetic field behaviors. If interpreted via the classical Applegate mechanism, the simulations show a difference of about two orders of magnitude compared to observations, but if tidal locking is not assumed, the results are compatible with the observed ETVs.
ASTRONOMY & ASTROPHYSICS
(2022)
Article
Astronomy & Astrophysics
David Martinez-Gomez, Roberto Soler, Jaume Terradas, Elena Khomenko
Summary: This study investigates the effects of density inhomogeneities on the periods and damping times of kink oscillations in flux tubes. The results show that the period ratio of the fundamental mode to the first harmonic increases with an increase in the central-to-footpoint density ratio or a decrease in the average density.
ASTRONOMY & ASTROPHYSICS
(2022)
Article
Astronomy & Astrophysics
J. R. Canivete Cuissa, R. Teyssier
Summary: This study conducted three-dimensional numerical simulations of magneto-hydrodynamics in stellar interiors and demonstrated the feasibility of the simulation method. The results showed an exponential growth of magnetic energy and a decrease in power of acoustic and internal gravity waves. The study highlights the importance of including magnetic fields in the study of pressure waves.
ASTRONOMY & ASTROPHYSICS
(2022)
Article
Astronomy & Astrophysics
Oscar M. Pimentel Diaz, P. Chris Fragile, F. D. Lora-Clavijo, Bridget Ierace, Deepika Bollimpalli
Summary: The study investigates the impact of magnetic susceptibility on magneto-rotational instability (MRI) within the context of black hole accretion, finding that paramagnetic discs exhibit shorter wavelength and faster growth rate for unstable modes compared to diamagnetic discs. Additionally, the magnetization parameter in the saturated state is predicted to be smaller in paramagnetic discs. Numerical simulations confirm these predictions, showing higher vertically integrated stress and mass accretion rate in paramagnetic discs compared to diamagnetic discs.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2021)
Article
Astronomy & Astrophysics
B. Zaire, L. Jouve, T. Gastine, J-F Donati, J. Morin, N. Landin, C. P. Folsom
Summary: This paper investigates the diversity of surface magnetic field configurations of cool stars using numerical simulations. The results show that dipolar solutions can exist in strongly stratified simulations, contrary to previous beliefs. The simulations resemble the outlier stars observed at Rossby numbers larger than 0.1, suggesting that the relative importance of inertial and Lorentz forces controls the dipolar to multipolar transition. Additionally, the ratio of kinetic to magnetic energies can also capture the transition in field morphology.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2022)
Article
Astronomy & Astrophysics
Craig D. Duguid, Paul J. Bushby, Toby S. Wood
Summary: The leading theoretical paradigm for the Sun's magnetic cycle is an alpha omega-dynamo process, in which a combination of differential rotation and turbulent, helical flows produces a large-scale magnetic field that reverses every 11 yr. Most alpha omega solar dynamo models rely on differential rotation in the solar tachocline to generate a strong toroidal field. The aim of this paper is to investigate an alternative mechanism for the poloidal field regeneration, namely the magnetic buoyancy instability in a shear-generated, rotating magnetic layer.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2023)
Article
Astronomy & Astrophysics
P. J. Kaepylae
Summary: This study investigates the effects of the effective thermal Prandtl number on the transition from anti-solar to solar-like differential rotation. The simulations show that the Prandtl number has little impact on the differential rotation. Magnetic fields and more turbulent regimes play a key role in achieving solar-like rotation.
ASTRONOMY & ASTROPHYSICS
(2023)
Article
Physics, Multidisciplinary
Axel Brandenburg
Summary: The early exponential growth phase of the 2019 novel coronavirus epidemic lasted for only six days and was followed by a subsequent slow-down. This can be explained by the merging of previously disconnected sites after the disease spread to a small number of separated sites. The subsequent variations in growth can be explained by reinfections and variations in their rate. A standard epidemiological model with spatial extent and reinfections can describe the observed behavior.
JOURNAL OF PHYSICS A-MATHEMATICAL AND THEORETICAL
(2023)
Article
Astronomy & Astrophysics
Axel Brandenburg, Igor Rogachevskii, Jennifer Schober
Summary: Small-scale dynamos are important in astrophysics and understanding their behavior is crucial for various observations and theoretical aspects. In this study, the authors show that during the kinematic phase of the dynamo, the cutoff wavenumber of the magnetic energy spectra scales differently than expected for small magnetic Prandtl numbers. However, in the non-linear saturated regime, the behavior changes and the cutoff scale has a different scaling with magnetic Prandtl number.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2023)
Article
Astronomy & Astrophysics
Frederick A. Gent, Mordecai-Mark Mac Low, Maarit J. Korpi-Lagg, Nishant K. Singh
Summary: Study finds that turbulent flow driven by supernova may cause rapid growth of magnetic fields in the interstellar medium, which is related to small-scale dynamo effect. The growth rate of magnetic fields is closely related to gas temperature, vorticity, and fluid Reynolds number. The magnetic energy density is highest in hot gas but saturates below 5% of the equipartition with kinetic energy in hot gas, while it reaches 100% in cold gas.
ASTROPHYSICAL JOURNAL
(2023)
Article
Astronomy & Astrophysics
Salome Mtchedlidze, Paola Dominguez-Fernandez, Xiaolong Du, Wolfram Schmidt, Axel Brandenburg, Jens Niemeyer, Tina Kahniashvili
Summary: In this study, the amplification mechanisms of large- and small-scale correlated magnetic fields in massive galaxy clusters were investigated using high-resolution magnetohydrodynamic cosmological zoom-in simulations. The results indicate that the amplification is more efficient for large-scale fields generated during inflation, while moderate growth is observed for seed fields generated during phase transitions. The differences between the models are imprinted on the spectral characteristics and final correlation lengths of the magnetic fields.
ASTROPHYSICAL JOURNAL
(2023)
Article
Mechanics
J. Labarbe, P. Le Gal, U. Harlander, S. Le Dizes, B. Favier
Summary: This paper presents a numerical analysis of the instability in horizontally sheared Poiseuille flow with vertical stratification. The study extends the previous work on the linear instability of this configuration and investigates nonlinear regimes through direct numerical simulations. The results show that the flow loses its vertical homogeneity through a secondary bifurcation and exhibits intermittent dynamics with bursting events and localized shear instabilities.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Physics, Fluids & Plasmas
Axel Brandenburg
Summary: The decay of the magnetic field in neutron star crusts, where the motion of electrons controls the evolution of the magnetic field, is still governed by the Hosking integral. However, the dimensions of the Hosking integral in this scenario are different from those in ordinary magnetohydrodynamic turbulence. This leads to a slower decay of the magnetic field and an increase in the correlation length, as compared to magnetohydrodynamic turbulence.
JOURNAL OF PLASMA PHYSICS
(2023)
Article
Astronomy & Astrophysics
M. Waidele, M. Roth, N. K. Singh, P. J. Kaepylae
Summary: Recent results by Singh, Raichur, and Brandenburg (2016) indicate that the emergence of an active region (AR) can be observed through an enhancement of f-mode power up to two days before the formation of the region. This study uses the Fourier-Hankel method as an independent approach to test the earlier findings. The data from SDO/HMI for ARs 11158, 11072, 11105, 11130, 11242, and 11768 show a similar behavior to the original work, with an increase in f-mode power one to three days prior to AR emergence, although AR 11105 and AR 11242 exhibit less significant enhancement compared to previous observations. The analysis of the absorption coefficient yielded by the Fourier-Hankel analysis shows no absorption or emission of power during the enhancement, indicating the absence of directional dependence in terms of inward and outward moving waves.
Article
Environmental Sciences
Axel Brandenburg, Gustav Larsson
Summary: Magnetic helicity has a great impact when it deviates from zero on average, leading to the phenomenon of inverse cascade. This study examines the decay of magnetohydrodynamic (MHD) turbulence and other examples of magnetic evolution under different effects. It introduces the notion of the Hosking integral as a new conserved quantity. Quantitative scaling results for magnetic integral scale and energy density are presented, and comparisons are made with cases where a magnetic version of the Saffman integral is initially finite. Rotation in MHD turbulence tends to suppress nonlinearity and inverse cascading. Finally, the role of the Hosking and magnetic Saffman integrals in shell models of turbulence is investigated.
Article
Astronomy & Astrophysics
Joern Warnecke, Maarit J. Korpi-Lagg, Frederick A. Gent, Matthias Rheinhardt
Summary: High-resolution simulations suggest that small-scale dynamos (SSDs) are possible in the Sun and other cool stars, contrary to previous theoretical expectations.
Article
Astronomy & Astrophysics
C. A. Ortiz-Rodriguez, P. J. Kaepylae, F. H. Navarrete, D. R. G. Schleicher, R. E. Mennickent, J. P. Hidalgo, B. Toro-Velasquez
Summary: This study explores the magnetic and flow properties of fully convective M dwarfs using a star-in-a-box set-up. The results show systematic differences in the behavior of the large-scale magnetic field as functions of rotation period and magnetic Prandtl number.
ASTRONOMY & ASTROPHYSICS
(2023)
Article
Astronomy & Astrophysics
Nikhil Sarin, Axel Brandenburg, Brynmor Haskell
Summary: Motivated by recent simulations of the Hall cascade, we propose a model where the largescale magnetic field of neutron stars is produced through an inverse cascade of small-scale turbulence. Applying this model to a simulated population of neutron stars, we demonstrate its ability to explain pulsar and magnetar observations in the PP diagram. Our findings suggest the presence of a highly resistive pasta layer in the neutron star crust and have implications for direct observables such as the nominal age and braking index of pulsars.
ASTROPHYSICAL JOURNAL LETTERS
(2023)
Article
Physics, Multidisciplinary
Axel Brandenburg, Nousaba Nasrin Protiti
Summary: The conversion of electromagnetic energy into magnetohydrodynamic energy is significant in the reheating period of the early universe, primarily through electrical energy dissipation, while magnetic energy plays a secondary role in this process.
Review
Astronomy & Astrophysics
Petri J. Kaepylae, Matthew K. Browning, Allan Sacha Brun, Gustavo Guerrero, Joern Warnecke
Summary: This paper reviews the state of the art in three-dimensional numerical simulations of solar and stellar dynamos. It summarizes the fundamental constraints and techniques to alleviate these restrictions in numerical modeling. A brief summary of the relevant observations that the simulations aim to capture is given. The current progress in simulations of solar convection and the resulting large-scale dynamo is surveyed. Studies of modeling the Sun at different ages and stars of different masses and evolutionary stages are also continued. Both simulations and observations indicate that rotation, measured by the Rossby number, is a key factor in determining the overall level and characteristics of magnetic activity. Finally, efforts to understand global 3D simulations in terms of mean-field dynamo theory are discussed.
SPACE SCIENCE REVIEWS
(2023)
Review
Astronomy & Astrophysics
Matthias Rempel, Tanayveer Bhatia, Luis Bellot Rubio, Maarit J. Korpi-Lagg
Summary: In this article, we review small-scale dynamo processes responsible for magnetic field generation on scales comparable to and smaller than the energy carrying scales of turbulence. The review focuses on critical observations of quiet Sun magnetism that support the operation of a small-scale dynamo in the solar photosphere and convection zone. Numerical studies of kinematic growth and non-linear saturation in idealized setups, as well as convective dynamo setups in the deep convection zone and photospheres of solar-like stars, are also discussed.
SPACE SCIENCE REVIEWS
(2023)
