scholarly journals Direct numerical simulations of helical dynamo action: MHD and beyond

2004 ◽  
Vol 11 (5/6) ◽  
pp. 619-629 ◽  
Author(s):  
D. O. Gómez ◽  
P. D. Mininni
Keyword(s):  
Magnetic Fields ◽  
Numerical Simulations ◽  
Periodic Boundary ◽  
Incompressible Flows ◽  
Magnetic Energy ◽  
Periodic Boundary Conditions ◽  
Direct Numerical Simulations ◽  
Dynamo Action ◽  
Spatial Fluctuations ◽  
Kinetic Effects

Abstract. Magnetohydrodynamic dynamo action is often invoked to explain the existence of magnetic fields in several astronomical objects. In this work, we present direct numerical simulations of MHD helical dynamos, to study the exponential growth and saturation of magnetic fields. Simulations are made within the framework of incompressible flows and using periodic boundary conditions. The statistical properties of the flow are studied, and it is found that its helicity displays strong spatial fluctuations. Regions with large kinetic helicity are also strongly concentrated in space, forming elongated structures. In dynamo simulations using these flows, we found that the growth rate and the saturation level of magnetic energy and magnetic helicity reach an asymptotic value as the Reynolds number is increased. Finally, extensions of the MHD theory to include kinetic effects relevant in astrophysical environments are discussed.

Magnetohydrodynamics: Overview

2020 ◽  
Author(s):  
E.R. Priest
Keyword(s):  
Magnetic Fields ◽  
Magnetic Energy ◽  
Liquid Metals ◽  
Plasma Heating ◽  
The Sun ◽  
Pressure Gradients ◽  
Dynamo Action ◽  
Electrically Conducting ◽  
Stellar Flares ◽  
Outer Atmosphere

Magnetohydrodynamics is sometimes called magneto-fluid dynamics or hydromagnetics and is referred to as MHD for short. It is the unification of two fields that were completely independent in the 19th, and first half of the 20th, century, namely, electromagnetism and fluid mechanics. It describes the subtle and complex nonlinear interaction between magnetic fields and electrically conducting fluids, which include liquid metals as well as the ionized gases or plasmas that comprise most of the universe. In places such as the Earth’s magnetosphere or the Sun’s outer atmosphere (the corona) where the magnetic field provides an important component of the free energy, MHD effects are responsible for much of the observed dynamic behavior, such as geomagnetic substorms, solar flares and huge eruptions from the Sun that dominate the Earth’s space weather. However, MHD is also of great importance in astrophysics, since many of the MHD processes that are observed in the laboratory or in the Sun and the magnetosphere also take place under different parameter regimes in more exotic cosmical objects such as active stars, accretion discs, and black holes. The different aspects of MHD include determining the nature of: magnetic equilibria under a balance between magnetic forces, pressure gradients and gravity; MHD wave motions; magnetic instabilities; and the important process of magnetic reconnection for converting magnetic energy into other forms. In turn, these aspects play key roles in the fundamental astrophysical processes of magnetoconvection, magnetic flux emergence, star spots, plasma heating, stellar wind acceleration, stellar flares and eruptions, and the generation of magnetic fields by dynamo action.

scholarly journals Magnetic fields in M dwarfs from the CARMENES survey

2019 ◽  
Vol 626 ◽  
pp. A86 ◽  
Author(s):  
D. Shulyak ◽  
A. Reiners ◽  
E. Nagel ◽  
L. Tal-Or ◽  
J. A. Caballero ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Near Infrared ◽  
Magnetic Energy ◽  
Spectral Lines ◽  
High Spectral Resolution ◽  
Stellar Rotation ◽  
M Dwarfs ◽  
Dynamo Action ◽  
Rotation Periods ◽  
Consistent Picture

Context. M dwarfs are known to generate the strongest magnetic fields among main-sequence stars with convective envelopes, but we are still lacking a consistent picture of the link between the magnetic fields and underlying dynamo mechanisms, rotation, and activity. Aims. In this work we aim to measure magnetic fields from the high-resolution near-infrared spectra taken with the CARMENES radial-velocity planet survey in a sample of 29 active M dwarfs and compare our results against stellar parameters. Methods. We used the state-of-the-art radiative transfer code to measure total magnetic flux densities from the Zeeman broadening of spectral lines and filling factors. Results. We detect strong kG magnetic fields in all our targets. In 16 stars the magnetic fields were measured for the first time. Our measurements are consistent with the magnetic field saturation in stars with rotation periods P < 4 d. The analysis of the magnetic filling factors reveal two different patterns of either very smooth distribution or a more patchy one, which can be connected to the dynamo state of the stars and/or stellar mass. Conclusions. Our measurements extend the list of M dwarfs with strong surface magnetic fields. They also allow us to better constrain the interplay between the magnetic energy, stellar rotation, and underlying dynamo action. The high spectral resolution and observations at near-infrared wavelengths are the beneficial capabilities of the CARMENES instrument that allow us to address important questions about the stellar magnetism.

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